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Groundwater-Evaporation and Recharge for a floodplain in a Sub-Humid Monsoon Climate in Ethiopia: Groundwater-Evaporation and Recharge in a Sub-Humid Monsoon Climate
Abstract
Groundwater levels are declining at unsustainable levels in many areas of the world. To secure this resource for future generation, well-documented groundwater level observations are needed to derive recharge rates and sustainable withdrawal amounts. Although groundwater is routinely monitored in most of the world, relatively little information is available in sub-Saharan Africa including the Ethiopian highlands where groundwater irrigation could greatly increase food security. To obtain sustainable withdrawal rates for the wet Ethiopian highlands, we selected the Fogera plain in the Lake Tana basin with a sub-humid climate. Weekly groundwater levels were measured in 32 abandoned shallow wells over a two-year period in 2014 and 2015. Our observations showed that in the two years, the groundwater level reached the surface during the rainy monsoon phase around first week of August. At the end of the dry monsoon phase in May, the average groundwater table was at 4 m depth. The amount of water removed by evaporation during the dry phase was 850 mm/year. This is replenished through recharge during the wet phase. Rain in excess of the recharge and evaporation is stored on the surface or overland flow. The groundwater flowed slowly towards the lake but water passing the lake shore was not a significant term in the water balance of the plain. Irrigation from groundwater seems to be sustainable especially when rivers flood periodically.
... Similar to Kebede et al. [28], SMEC [26] assumed insignificant lake water-groundwater interaction with lake bottom as well as surrounding aquifers, owing to the thick clay and silt deposit on most of the lake floor, and the very low hydraulic gradient between the lake and the groundwater aquifers near to the lake. This low hydraulic gradient, and hence low groundwater inflow to the lake from the shallow lacustrine aquifer was also supported by Enku et al. [54] through their study conducted on the Gumara floodplain ( Figure 6). Chebud and Melesse [42] estimated the groundwater inflow using a simple MODFLOW model at 3% of the total river water discharge. ...
... Similarly, Getenet [25] calculated the recharge in the floodplain part of Rib catchment using the WTF and SMB methods and found annual values of 98 mm and 124 mm, respectively. These estimates of around 105 mm were in contradiction with the one determined by Enku et al. [54] using a similar method (Table 5). Enku et al. [54] estimated a groundwater recharge rate of 850 mm Yenehun et al. [65] applied different methods to estimate recharge on the upper part of Gilgel Abay catchment. ...
... These estimates of around 105 mm were in contradiction with the one determined by Enku et al. [54] using a similar method (Table 5). Enku et al. [54] estimated a groundwater recharge rate of 850 mm Yenehun et al. [65] applied different methods to estimate recharge on the upper part of Gilgel Abay catchment. They estimated 431 mm, 477 mm, and 462 mm, as mean annual recharge values using the SMB, the CMB, and the WetSpass (spatially distributed water balance model), respectively. ...
The Lake Tana Basin, comprising the largest natural lake in Ethiopia, is the source and the uppermost part of the Upper Blue Nile Basin. In this review paper, research papers, mainly on the rainfall-runoff modeling and lake water balance, and on the hydrogeology, have been reviewed. The earlier water balance estimation attempts used simple conceptual and statistical approaches and calculate on a monthly timescale. More recent research has been using advanced semi-physically or physically based distributed hydrological models. Accordingly, mean annual precipitation over the lake was estimated in the range 36.1–53.1%; lake evaporation at 45.3–57.5%; river inflow (all gauged and estimated ungauged) at 43.6–63.9%; and river (lake) water outflow at 0–9.2%. With the few isotope studies, groundwater inflow and outflow are found insignificant. Different studies had estimated groundwater recharge, ranging from 57 mm to 850 mm. The basin has a heterogenous aquifer system consisting of different volcanic rocks and alluvio-lacustrine sediments. Generally, groundwater with low TDS, Ca–Mg–HCO3 type, isotopically relatively enriched, and high TDS, Na–HCO3 type, isotopically relatively depleted, water types have been identified. In this paper, major research gaps such as aquifer hydraulic characterization, surface-groundwater interaction, groundwater flow and groundwater balance have been identified. Hence, future research shall focus on the groundwater resources, so that existing surface water studies are updated and future water usage options are explored.
... In addition, eucalyptus plantation acreage has increased in the Lake Tana basin over the last 10 years [39][40][41]. Eucalyptus patches in the landscape can evaporate up to twice the rate of potential evaporation during the dry season [42] and either draw down the water table farther than without the trees in the Fogera Plain east of Lake Tana [43] or decrease the amount of direct runoff by 50% or more in the uplands [41]. Of interest is that wells in the area with eucalyptus had a greater concentration of dissolved solids than other wells [32] indicating enhanced evaporation by the trees. ...
... The groundwater inflow to the lake was neglected based on the chemical and isotope studies by Kebede, et al. [32] and Enku, et al. [43] also reported that groundwater inflow is low because the hydraulic gradient in the plains is less than 0.01 and another study by SMEC [8] notes that the 80 m thick clay layer at the bottom of the lake prevents groundwater inflow into the lake. ...
... Kebede, et al. [32] and Dessie, et al. [31] argued the unaccounted evaporation loss of the plains around Lake Tana equal to 0.5 km 3 a −1 are responsible for the unaccounted losses. However, this would not explain the negative monthly values during the low flow period, because although the additional evaporation occurs during the dry period, the plains are not hydrologically connected with the lake [43]. During the rain phase, the plains are re-saturated, and thus the evaporation during the dry phase decrease the flow at that time. ...
Lakes hold most of the freshwater resources in the world. Safeguarding these in a changing environment is a major challenge. The 3000 km 2 Lake Tana in the headwaters of the Blue Nile in Ethiopia is one of these lakes. It is situated in a zone destined for rapid development including hydropower and irrigation. Future lake management requires detailed knowledge of the water balance of Lake Tana. Since previous water balances varied greatly this paper takes a fresh look by calculating the inflow and losses of the lake. To improve the accuracy of the amount of precipitation falling on the lake, two new rainfall stations were installed in 2013. The Climate Hazards Group Infrared Precipitation Version two (CHIRPS-v2) dataset was used to extend the data. After reviewing all the previous studies and together with our measurements, it was found that the period of 1990-1995 likely had the most accurate gauged discharge data. During some months in this period, the lake water balance was negative. Since the river inflow to the lake cannot be negative, water was either lost from the lake via the subsurface through faults, or the outflow measurements were systematically underestimated. Based on the evaporation rate of 1650 mm, we found that unaccounted loss was 0.6 km 3 a −1 , equivalent to 20 cm of water over the lake area each year. This implies the need for reliable rainfall data and improved river discharge measurements over a greater portion of the basin both entering and exiting the lake. Also, integrated hydrological and geologic investigations are needed for a better understanding of the unaccounted water losses and quantifying the amount of subsurface flow leaving the lake.
... In semi-arid Tigray, Walraevens et al. (2009Walraevens et al. ( , 2015 estimated the groundwater storage to vary between 18 to 335 mm y -1 using MODFLOW. Enku et al. (2016) measured a recharge of 800 mm y -1 in the sub-humid Fogera plain. Walker et al. (2018) estimated groundwater recharge in the humid highlands from 45 mm y -1 to 814 mm y -1 depending on the type of method used. ...
... In this method the specific yield or drainable porosity ( is a critical variable to convert water table height in amount of groundwater available for irrigation (Delleur, 2010). Enku et al. (2016) found that in the nearby Fogera Plain the soil moisture content decreased linearly with height above the water table and therefore expressed the drainable porosity as: ...
... 15 value is at the high end of this range. The only recharge value that is much greater than the others is by Enku et al (2016) who measured a recharge and storage of 800 mm y -1 in the nearby Fogera Plain which is entirely underlain by an aquifer and has a slope less than 0.1%. Thus, the recharge value in Enku et al. (2016) agree with the average recharge value on the aquifer in Robit Bata of 800 mm y -1 on the average over the two years. ...
Feeding nine billion people in 2050 will require sustainable development of all water resources, both surface and subsurface. Yet, little is known about the irrigation potential of hillside shallow aquifers in many highland settings in sub‐Saharan Africa that are being considered for providing irrigation water during the dry monsoon phase for smallholder farmers. Information on the shallow groundwater being available in space and time on sloping lands might aid in increasing food production in the dry monsoon phase. Therefore, the research objective of this work is to estimate potential groundwater storage as a potential source of irrigation water for hillside aquifers where lateral subsurface flow is dominant. The research was carried out in the Robit‐Bata experimental watershed in the Lake Tana basin which is typical of many undulating watersheds in the Ethiopian highlands. Farmers have excavated more than 300 hand dug wells for irrigation. We used 42 of these wells, to monitor water table fluctuation from April 16, 2014 to December 2015. Precipitation, and runoff data were recorded for the same period. The temporal groundwater storage was estimated using two methods: one based on the water balance with rainfall as input and baseflow and evaporative losses leaving the watershed as outputs; the second based on the observed rise and fall of water levels in wells. We found that maximum groundwater storage was at the end of the rain phase in September after which it decreased linearly until the middle of December due to short groundwater retention times. In the remaining part of the dry season period, only wells located close to faults contained water. Thus, without additional water sources, sloping lands can only be used for significant irrigation inputs during the first three months out of the eight month long dry season. This article is protected by copyright. All rights reserved.
... In this method, the specific yield or drainable porosity (η) is a critical variable to convert water table height in amount of groundwater available for irrigation (Delleur, 2010). Enku et al. (2016) found that in the nearby Fogera Plain the soil moisture content decreased linearly with height above the water table and therefore expressed the drainable porosity as: ...
... Our watershed recharge value is at the high end of this range. The only recharge value that is much greater than the others is byEnku et al. (2016) who measured a recharge and storage of 800 mm/year in the nearby Fogera Plain which is entirely underlain by an aquifer and has a slope less than 0.1%. Thus, the recharge value inEnku et al. (2016) agree with the average recharge value on the aquifer in Robit Bata of 800 mm/year on the average over the 2 years. ...
... The only recharge value that is much greater than the others is byEnku et al. (2016) who measured a recharge and storage of 800 mm/year in the nearby Fogera Plain which is entirely underlain by an aquifer and has a slope less than 0.1%. Thus, the recharge value inEnku et al. (2016) agree with the average recharge value on the aquifer in Robit Bata of 800 mm/year on the average over the 2 years. The storage in the Fogera Plain is much greater because the interflow is negligible, and the water can be stored in the entire watershed. ...
The Ethiopian government has declared the Lake Tana Beles region a growth corridor Irrigation development is one of the priority. Since baseflow is limited, groundwater has the greatest potential for increasing irrigation in the near future. The main drawback is a lack of information on sustainable groundwater use and specifically the ground water recharge. Therefore the objective of this research is to calculate the annual recharge to the ground water. The study is conducted in Robit-Bata, an experimental watershed of 911 ha, located at the south-eastern edge of Lake Tana. Farmers have excavated more than 300 hand dug wells for irrigation and we used 50 shallow groundwater wells and observed water table fluctuation and precipitation for the past one year. The annual recharge was estimated using water – level fluctuation method. Specific yield was taken to be the difference of porosity and field capacity of the subsurface formation. The annual average areal groundwater recharge of the watershed was 640 mm/year, which is 41% of the rainfall and ranged from 50mm to 390mm for the various locations in the watershed. The greatest recharge amounts were found in the plains at the foot of the hills in river course plain areas consisting mostly weathered basalt rock. Here the groundwater rose steadily during the rainy monsoon phase. Smaller amount of recharge occurred both near the top of the hills with tough rock formation and in the, flat areas near to stream sandy and clay deposits and groundwater at, shallow well depth. Our study indicates that the current use of the groundwater seems sustainable, but will become unsustainable if more wells are constructed.
... Due to rapid population growth, expansion and effective use of shallow groundwater supply for agriculture and domestic services are mandatory [53][54][55]. Despite the high groundwater potential and opportunity to overcome drought in Ethiopia, no significant attention has been given to using groundwater for agriculture [56]. ...
... The erratic variability in rainfall distribution ( Figure 3) and shortage of perennial streams have augmented the dependability of groundwater for irrigation [32,[54][55][56][57]. However, people have been using groundwater from wells without understanding the consequence of over-pumping. ...
Groundwater is a strategic resource in all climatic regions of Ethiopia, contributing about 80% of the domestic supply of urban and rural populations. However, little research has been available compared with extensive geographical coverage and increasing population growth rates. Hence, the present study aimed to review published groundwater research of Ethiopian aquifers to realize potential research challenges and suggest future research directions. We focused on groundwater potential, recharge process, and qualities. The total potential groundwater of the country ranges from 2.5 to 47 billion cubic meters. The study depicted that the mean annual recharge estimate varies from 24.9 mm to 457 mm at catchments scales. However, the overall country was about 39.1 mm. The study found a need for a detailed investigation of different factors susceptible to groundwater pollution, as some of the evaluations indicated exceeding acceptable standards. This study observed that the main challenge was the lack of data and convergence research trends. Henceforth, future research in different climate regions should focus on multifaceted technical and stakeholder settings. This study gives the insight to integrate palatable research findings with the national policy and decision-making process to enhance the sustainability of groundwater resources significantly.
... In Ethiopia, where the moisture content in the soil during the rainy phase was the same for Eucalyptus and native trees, a faster decline of the moisture content under the Eucalyptus was observed during the dry phase (Chanie et al. 2013). According to Enku et al. (2017) and Mhiret et al. (2019) the decrease in discharge in watersheds south of Lake Tana in Ethiopia were caused by the expansion of Eucalyptus. In addition, in the last 20 years in the semi-arid northern Ethiopian highlands riverbeds have stabilized indicating as stated by Nyssen et al. (2009) that the runoff was decreased as a consequence of the implementation of conservation practices and planting of in Eucalyptus trees. ...
... Field measurements of specific yield Figure 4 shows the relative saturation for three well locations in the Fogera plain at the end of 2015 dry season and reported in Enku et al. (2017). The relative saturation is defined as the ratio of the observed moisture content to the saturated moisture content in Fig. 4. The relative saturation decreases with height above a water table. ...
Given the critical role of water in rural development, it is essential to quantify water use by various sectors. Reportedly, a significant water loss during the dry monsoon phase is by Eucalyptus trees in the Ethiopian highlands but few quantitative studies have been carried out. In our study in the Fogera plain near Lake Tana, consumptive water use of a small Eucalyptus patch was calculated from the fluctuating diurnal water table observations for two dry monsoon phases. During the dry monsoon phase in 2014, the groundwater table declined linearly at a rate of 2.6 cm d−1 and in 2015 the decline was 2.5 cm d−1. Daily fluctuations in groundwater were around 20 cm. Translating changing groundwater levels in precise evapotranspiration rates was cumbersome because of uncertainty in the value of drainable porosity. We concluded that the evapotranspiration over the small Eucalyptus patch was at a minimum 3.4 mm d−1 but more likely around 10 mm d−1 at the end of the dry phase. The 10 mm d−1 is twice the reference evaporation rate. This is in accordance with advection aridity approach where the actual evaporation of isolated patches with access to groundwater in dry environments can evaporate at twice the reference rate. The current and future extent of Eucalyptus patches and forests need, therefore, be considered in the management of the groundwater resource.
... The shallow aquifer has primary porosity with textures between fine silts and clays and fine sands with some sand and coarse gravels zones (Duque, 2017), its hydraulic behavior is predominant unconfined, with water table near to surface terrain. The geology of this aquifer is conformed for quaternary deposits, mainly alluvial deposits (Qal), with some alluvial terrace (QT) extensions (mainly around river axis), overlaid on a sedimentary sequence of the Neogene, corresponding to Pavo Formation (Ngpv, Early Miocene) -Arenas Monas Formation (Ngam, Late Miocene to Early Pliocene) and Corpa Formation (Ngco, Pliocene), its extension is limited to east with Abibe mountains, to southern and southwest with León river, and to northwest with Gulf of Urabá (Caribbean Sea) (Ingeominas, 1995;Villegas et al., 2018;Ossa & Betancur, 2018). ...
... Finally the total recharge and storage variation results are summarized in Table 6. The monthly mean recharge varies between 16.6 mm and 86.9 mm, representing between 6 % and 39 % of monthly mean precipitation, this indicates the important spatial variability of total recharge in the study area, as shown in Figure 13, where it is observed that the highest recharge rates occur in the south-central area of the study area and in the proximity of the indirect recharge zones, towards the foothills of Abibe mountains, but there is also a high recharge concentrated in the 41 Casanova station, perhaps influenced by a specific area of cover land and soils textures that favor infiltration, while the lowest recharge rates occur towards the west of the study area, close to the coastline in the Gulf of Urabá and close to the León River, which according to the conceptual hydrogeological model are the main discharge zones of the multilayer aquifer (Duque, 2017;Villegas et al., 2018). The spatial behavior of the recharge is too consistent with the potential recharge zones defined by Betancur et al., (2018), as shown in Figure 13. ...
This paper presents the quantitative and qualitative analysis of water table fluctuations of shallow aquifer of Gulf of Urabá in Antioquia-Colombia, using the hourly water table records and daily aggregated records from December 2016 to May 2018 in 14 monitoring points spatially distributed in a 1,206 km 2 extension of the aquifer system. The obtained data are graphically and statistically related with other meteorological variables such as precipitation, evaporation and tides on the coast of Gulf of Urabá. Using the soils characterization, pedotransfer functions and a water balance approach, specific yield scenarios are determined and together with daily water table fluctuations series, detailed estimates of groundwater total recharge are obtained applying the Water Table Fluctuation method (WTF). The most influential meteorological variable in the water table fluctuations is the precipitation, showing high statistical correlations with a lag of less than 5 days on the daily scale. Evaporation and atmospheric pressure also influence fluctuations, but mainly in the diurnal cycle with small level variations compared to those caused by precipitation. The tides in the Gulf of Urabá show a strong influence on the water table fluctuations in the portion of the aquifer near the coastline. Total recharge rates in the area show spatio-temporal variability, influenced by the cited meteorological variables and soils properties (texture and specific yield), showing greater magnitudes in the south-central zone of the aquifer and lower ones in the east zone and close to the coastline of the Gulf of Urabá in Turbo municipality.
... In Robit Bata, Ca 2+ and Mg 2+ were high from January to June, when the groundwater was only available near the faults and evaporation was much greater than the rainfall. The cation concentrations were low from July to September when groundwater was recharged [47]. ...
Nitrate is globally the most widespread and widely studied groundwater contaminant. However, few studies have been conducted in sub-Saharan Africa, where the leaching potential is enhanced during the rainy monsoon phase. The few monitoring studies found concentrations over drinking water standards of 10 mg N-NO3− L−1 in the groundwater, the primary water supply in rural communities. Studies on nitrate movement are limited to the volcanic Ethiopian highlands. Therefore, this study aimed to evaluate the transport and fate of nitrate in groundwater and identify processes that control the concentrations. Water table height, nitrate, chloride, ammonium, reduced iron, and three other groundwater constituents were determined monthly in the groundwater in over 30 wells in two contrasting volcanic watersheds over two years in the Ethiopian highlands. The first watershed was Dangishta, with lava intrusion dikes that blocked the subsurface flow in the valley bottom. The water table remained within 3 m of the surface. The second watershed without volcanic barriers was Robit Bata. The water table dropped rapidly within three months of the end of the rain phase and disappeared except near faults. The average nitrate concentration in both watersheds was between 4 and 5 mg N-NO3− L−1. Hydrogeology influenced the transport and fate of nitrogen. In Dangishta, water was blocked by volcanic lava intrusion dikes, and residence time in the aquifer was larger than in Robit Bata. Consequently, nitrate remained high (in several wells, 10 mg N-NO3− L−1) and decreased slowly due to denitrification. In Robit Bata, the water residence time was lower, and peak concentrations were only observed in the month after fertilizer application; otherwise, it was near an average of 4 mg N-NO3− L−1. Nitrate concentrations were predicted using a multiple linear regression model. Hydrology explained the nitrate concentrations in Robit Bata. In Dangishta, biogeochemistry was also significant.
... In Robit Bata, the agreement is better because a too small recharge value offsets the too large watershed area. Higher groundwater recharge of about 800 mm a − 1 was reported in the alluvial deposited aquifer with a very flat (slope < 0.1%) slope in the Fogera plain close to the Robit Bata watershed by Enku et al. (2017). In ...
Study region: Robit Bata and Dengeshita watersheds, Upper Blue Nile basin, Ethiopia Study focus: Sustainable development of groundwater in the Ethiopian Highlands requires recharge measurements. The Water Table Fluctuation (WTF) method has been used to measure recharge. Lateral flow in sloping hillside aquifers violates the assumptions on which the original WTF method is based. We modified the original WTF method to include lateral flow controlled by gravity. New hydrological insights: Previously it was shown that the sum of recharge over the travel time in a sloping aquifer is equal to the base flow. Since recharge cannot be measured directly, the recharge calculated with the modified WTF method was tested using baseflow measurement in two contrasting highland watersheds, Robit Bata and Dengeshita, where groundwater depth and streamflow data were monitored from 2015 to 2018. Baseflow was determined with the one-parameter digital filter technique. Predicted and observed monthly baseflow agreed well with R 2 > 0.9 and RMSE < 20 mm during calibration and validation. Recharge in Robit Bata, in which the aquifer underlays 55% of the watershed, was 293 mm a − 1 of 1378 mm a − 1 precipitation. In Dengeshita, where the entire watershed has an aquifer, the average annual recharge was 525 mm a − 1 of 1550 mm a − 1. Our findings indicate that the modified WTF method is simple and practical for determining aquifer recharge for highlands and other sloping aquifers.
... Consequently, in contrast to recharge in Akaki catchment, the average recharge rate produced by this study from the soil water balance method may not be an underestimation due to insignificant preferential recharge through fractures (mainly diffuse recharge through the soil) in the Tana basin. However, in the floodplain where water from the upstream catchment stagnates, and irrigation takes place in less than 4% of the Fogera floodplain (comprising flood plains of both Gumera and Ribb catchment) (Enku et al., 2017) groundwater recharge from soil water balance method may underestimate the actual recharge. This is because the ponding water from the flood and return flow from irrigation may cause an additional recharge which cannot be captured by this method. ...
Ribb river catchment, one of the four major catchments in the Lake Tana Basin, is located in North-western highlands of Ethiopia. In terms of hydrogeology, it is the least studied catchment in the Lake Tana Basin. The study aimed at investigation of groundwater recharge using both chemical (chloride mass balance method) and physical (soil moisture balance, water table fluctuation and baseflow separation) methods, groundwater flow, and hydrochemistry.
For the Thornthwaite and Mather (1957) soil moisture balance method, meteorological data (rainfall, temperature, windspeed, relative humidity, and sunshine hours) and Penman-Monteith potential evapotranspiration of 6 stations, the soil map of the HWSD and the raster land cover map of the ESA-CCI are used to map the spatial and temporal variability of groundwater recharge. Groundwater level of 13 monitoring well networks and specific yield of weathered basalt (4%) and clay and sandy clay (4.5%) are used for recharge estimation using the water table fluctuation method, while 11 rainwater and 20 groundwater chloride concentrations are applied in the chloride mass balance method. Baseflow separation has been done using the antecedent recession curve of Ribb river hydrograph from the discharge data of two recording stations. Conceptual groundwater flow and aquifer types in the catchment are investigated using multiple topographic profiles, and geologic map in global mapper 7 GIS environment. Chemistry of groundwater, rainwater, and river water are analysed at the Laboratory of Applied Geology and Hydrogeology (Ghent University).
The mean annual groundwater recharge rate obtained by soil moisture balance, water table fluctuation, chloride mass balance, and baseflow separation methods are 365, 189, 154, 57.4 mm/year respectively. Deeper groundwater discharge to Lake Tana and the floodplain, while shallow groundwater discharge to local valleys from recharge areas in the upstream, and the northern and southern periphery of the catchment. Four aquifer types (Termaber Basalt, mountain top, river valley and floodplain aquifers) are distinguished. Mountain top aquifers are recharge areas, whereas floodplain deposits are discharge areas where recharge rate is relatively low. Ca-HCO3 rainwater types in the study area are the result of the influence of carbonate dust. Major groundwater types in the study area are Ca-HCO3, Ca-Mg-HCO3, Na-HCO3 and Ca-Cl-HCO3 type. Dissolution of silicate minerals characterizes the evolution of the groundwater chemistry.
... The net recharge values considered in our study averaged 0.7 mm/d, or about 255 mm/year. These net recharge rates are the distributed flux values reaching the groundwater system through percolation (Harbaugh, 2005), and these estimates are consistent with the ranges recharge rates reported by other studies in Ethiopia (Enku et al., 2016;Kebede et al., 2005;Mechal et al., 2015;Tilahun et al., 2020;Walraevens et al., 2015Walraevens et al., , 2009. The drought year of 2015 accounted for a substantial drop in recharge values. ...
Groundwater (GW) is the main source of domestic water supply in Ethiopia (85%), however, despite widespread acknowledgement of its potential for resource-based development and climate change adaptation, the sector is still quite under-investigated. This is mainly due to the scarcity of in situ data, which are essential to building robust impact models. To address this, we developed a fine-resolution (500m) GW model using MODFLOW-NWT, focusing on the Gilgel-Abay Catchment located in the Upper Blue Nile basin, fed with daily distributed input forcings of recharge and streamflow simulated by the Coupled Routing and Excess Storage (CREST) hydrological model. The model was calibrated against instantaneous observation records of GW table for 38 historical wells, and validated at selected sites using time series data collected from the Citizen Science Initiative (PIRE CSI), and the Innovation Lab for Small Scale Irrigation (ILSSI) project. An of 14.4m (1.8% of range) was achieved for calibration and same for validation was 18.21m and 15.76m at the PIRE CSI and ILSSI sites, respectively. The findings of this research indicate substantial physical GW resource availability in the Gilgel-Abay region. Moreover, we expect the model to have multiscale future applications. These include obtaining dynamically downscaled boundary conditions for a local-scale GW model, to be developed in the next phase of our research. Further, an upscaled version of this model to encompass the entire Tana Basin would be developed to simulate lake-aquifer interactions. Finally, the approach of this research combining different types of datasets (e.g., reanalysis products, satellite data, citizen science data, etc.) is adaptable to other global data-scarce regions. Moreover, the method overcomes specific challenges associated to in situ data scarcity, limited knowledge on GW resources availability in the area, interaction with complex boundary conditions, and sensitivity under meteorological boundary forcings.
... Regional studies in upper Blue Nile (Alemayehu, 2006;Asrat, 2017;Ayenew et al., 2008;Demlie et al., 2007;Kebede, 2005;Kebede et al., 2017) and neighboring basins, specifically Tana sub basin level are undertaken for water resource potential assessment (Abiy et al., 2016;Mamo, 2014;Nigate et al., 2016) and water quality (Enku et al., 2017;Mamo, 2014) issues. A recent study conducted by (Bawoke et al., 2019) in the area, reported that the overall evolution of both surface and groundwater indicated the incidence of pollution by geogenic and anthropogenic effects. ...
This study aimed to evaluate the overall groundwater hydrogeochemical evolution and suitability for drinking in Andasa watershed (Northwestern Ethiopia). To achieve this, analyses of hydrogeochemical, multivariate (PCA, HCA), correlation matrix, and Water Quality Index (WQI) methodologies were employed. A total of 64 groundwater samples which had been collected during winter season (February–April, 2018) were assessed for ionic and composite variables (major physicochemical and composite parameters). The samples have been gathered from deep wells, shallow wells, hand dug wells and springs which are spatially distributed throughout the watershed. The study results revealed that major ions dominating are Ca>Mg>Na>K and HCO3> Cl>SO4> NO3> PO4 > F for cations and anions, respectively. Ca-HCO3, Mg-Ca-HCO3-Cl, Mg-Ca-Na-HCO3-SO4, Mg-HCO3-SO4, Mixed CaMgHCO3, Mixed CaNaHCO3 and Na-HCO3 were recognized as water types where Ca-HCO3 (59.69%) was identified as dominant. Ionic ratio plots are indications for groundwater enrichment by Na+, Ca2+ and Mg2+ cations (silicate minerals hydrolysis). Chloro-alkaline indices resulted for CAI-I (96.88%) and CAI-II (84.48%), showed Ca2++Mg2+ exchange in groundwater with Na++K+ in an aquifer, confirmed for cation exchanging. Four factor loadings (PCA analysis) explained for the existence of geogenic and anthropogenic pollutions. WQI values showed the majority of the samples comprised “good water” (57.81%), distributed in southern and western parts, followed by “excellent water” (20.31%). Additionally, WQI maps portrayed “Good” to “poor water” types in northern portions (waste disposal landfill, urban centers, irrigated areas), while “very poor” to “unsuitable” types are dispersed to northeastern tips. Generally, the study result is believed to give directions for groundwater management options.
... In the study area, there are no comprehensive hydrogeochemical and isotopic studies except those conducted in nearby basins (e.g. Kebede, 2005;Mamo, 2015;Abiy et al., 2016;Nigate et al., 2016;Enku et al., 2017). Hence, this work tries to assess the groundwater evolution, rock-water interaction and anthropogenic effects, and determines the recharge-discharge zones using an integrated hydrogeochemical and isotopic approaches in the Andasa watershed, northwest Ethiopia ( Fig. 1). ...
... The saturated hydraulic conductivity (Table V) varied from 36.1 cm day À1 (field 3) to 164.7 cm day À1 (field 1) with average values of 80 cm day À1 for all fields. Enku et al. (2016) found a 50 cm day À1 average saturated hydraulic conductivity which ranges from 8 to 130 cm day À1 in the same floodplains. Lateral groundwater flow is 0.54 cm day À1 from field 3 to field 2 and 0.493 cm day À1 from field 2 to field 1. ...
Field experiments were conducted (December 2014 to May 2015) in a small irrigation scheme (60 ha) to study the effect of flood irrigation on anisotropic soils with shallow groundwater in the Lake Tana floodplains of Ethiopia. Irrigation (470 ± 33 mm) was measured using V-notches; rainfall did not occur, and the groundwater table was monitored daily using piezometers to estimate recharge from irrigated onion fields using the groundwater table fluctuation method. Recharge was influenced by applied irrigation amount, groundwater table depth, seasonal temperature variations, irrigation application efficiency and crop growth stages. The decreased deep percolation during the hottest periods and peak growth stages negatively influenced the reduction in groundwater decline caused by irrigation. The soil anisotropy also played a major role in the recharge amount: despite clay dominance in the topsoils, rapid groundwater table rises (0.02–0.56 m) were due to the presence of granular and blocky structures. Recharge was also influenced by irrigation efficiency, indicating higher recharge during periods of lower efficiency. The seasonal recharge was 34–46% of applied irrigation and there is much room for improving irrigation efficiency which is only 46 (±12) to 51 (±17)%.
... Hence, the contribution of this lateral subsurface flow is minimum as compared to vertical water flow, which is governed by gravity. Similar result was reported by Enku et al. (2016) for the floodplains. ...
The deep percolation and actual evapotranspiration from flood irrigation in tropical floodplains were predicted using a numerical model, Hydrus-1D, and a bucket type water balance model. Field experiments were conducted on onion and maize crops grown from December 2015 to May 2016 in small irrigation schemes found in the Lake Tana floodplains of Ethiopia. Experimental fields were selected along a topographic transect to account for soil and groundwater variability. Irrigation volumes were measured using V-notches and irrigation depths (400–550 mm) were calculated, and daily groundwater levels were monitored manually from piezometers installed in the fields. The soil profiles were described at each field and physical properties (texture, FC, PWP, BD, and OM) were measured at each horizon which were used to derive model input parameters. Soil hydraulic properties (residual and saturated moisture content, saturated hydraulic conductivity, parameters related to: pore size distribution n, air entry α and pore connectivity l) were derived using KNN pedotransfer functions for tropical soils and fitted using Retention Curve Program for Unsaturated Soils, RETC. The seasonal actual evapotranspiration estimated by Hydrus and water balance models ranged from 320 to 360 mm for onion and from 400 to 470 mm for maize. The seasonal deep percolation estimated from both models was 12–41% of applied irrigation and with this flood irrigation management; the deep percolation is very high. Implementing precise irrigation and water saving practices that minimize deep percolation and unproductive excessive consumptive use are required to achieve the growing food demand with the available water. When less detailed information is available, the water balance model can be an alternative to predict deep percolation and actual evapotranspiration.
The water budget building of a water system lets us understanding the interaction between several variables which are part of the water system and influence in its behavior, being a powerful tool for the water resource management. This fact is particularly very important in urban environment, due to it is necessary establishing the influence of the anthropic activities on the inputs and outputs, and their impact over the water quality and quantity. In this paper, we present a quantitative and qualitative approximation to water budget in the Aburrá valley unconfined aquifer, in an urban environment with significant anthropic interventions, using conceptual and numerical tools to estimate the budget terms. The analysis period is the 2014-2015 hydrological year, which corresponds to a typical dry year. We found the main water input is the potential recharge by precipitation, others potential inputs are the water coming from aqueduct and sewer leaks and the potential adjacent recharge coming from Dunita de Medellín pseudokarstic aquifer.
In recent years, groundwater pumping has increased for domestic, industrial, and irrigation use in the Modjo River catchment. Understanding changes in groundwater levels is crucial for the sustainable use and management of aquifer. This study investigates the groundwater flow system and aquifer response to increased groundwater pumping and reduced recharge using the calibrated steady-state groundwater level and budget as a baseline. The groundwater flow corresponds to the direction of the Modjo River flow, following the topographic gradient. The simulated groundwater budget indicates that recharge from precipitation and surface water (crater lakes and river) are the main inflow to the aquifer, while the outflow from the aquifer is due to groundwater pumping, natural subsurface flow to downstream area, and base flow. Analysis of the different scenarios reveals that both an increase in well pumping and a decline in recharge resulted in a decrease of the base flow to Bishoftu crater lakes and Mojo River, and to the downstream subsurface flow. In conclusion, increasing human demand for groundwater and variability in recharge will affect groundwater contribution to surface water and ultimately will be a source of concern in the future for both environmental flows and groundwater management.
Agricultural intensification to meet the food needs of the rapidly growing population in developing countries is negatively affecting the water quality. In most of these countries such as Ethiopia, information on surface and especially groundwater quality is lacking. This limits the measure that can be taken to stop pollution. We, therefore, investigated the spatial and temporal variation of groundwater quality in the upland watershed. Tikur-Wuha watershed was selected because it is located in the Lake Tana watershed, which is seeing the first signs of eutrophication. Groundwater samples were collected from July 2014 to June 2015 from 19 shallow wells located throughout the watershed. Collected water samples were analyzed both in situ and in the laboratory to determine pH, electric conductivity (EC) and total dissolved solid (TDS), concentration of chemicals (nitrate, dissolved phosphorus, calcium, magnesium, aluminum and iron) and Escherichia coli (E. coli). We found that shallow groundwater had greater chemical concentrations and E. coli level in the monsoon rain phase than in the dry phase. Wells located down slope exhibited greater concentrations than mid- and upper-slope positions, with the exception of the nitrate concentration that was less down slope, due to denitrification in the shallow groundwater. Only E. coli level was above the WHO drinking water quality standards. Further studies on groundwater quality should be carried out to understand the extent of groundwater contamination.
In climate models, lateral terrestrial water fluxes are usually neglected. We estimated the contribution of vertical and lateral groundwater fluxes to the land surface water budget at a subcontinental scale, by modeling convergence of groundwater and surfacewater fluxes. We present a hydrological model of the entire Danube Basin at 5 km resolution, and use it to show the importance of groundwater for the surface climate.
Results show that the contribution of groundwater to evaporation is significant, and can locally be higher than 30 % in summer. We demonstrate through the same model that this contribution also has important temporal characteristics. A wet episode can influence groundwater contribution to summer evaporation for several years afterwards. This indicates that modeling groundwater flow has the potential to augment the multi-year memory of climate models. We also show that the groundwater contribution to evaporation is local by presenting the groundwater travel times and the magnitude of groundwater convergence. Throughout the Danube Basin the lateral fluxes of groundwater are negligible when modeling at this scale and resolution. This suggests that groundwater can be adequately added in land surface models by including a lower closed groundwater reservoir of sufficient size with two-way interaction with surface water and the overlying soil layers.
Development plans are mainly responsible for population changes and the conversion of forest and rangelands into agricultural lands and human settlements. Qualitative and quantitative analysis of population and land use changes are necessary to assess the impacts of change on hydrological processes. However, such important issues have been less considered worldwide particularly in developing countries. Therefore, we selected the Shazand Watershed (1740 Km2) due to rapid urbanization to track the effects of land use and population changes on streamflow and sediment yield. The data were collected from statistical yearbooks and satellite imageries from 1973 to 2008. All available measurements on discharge and suspended sediment concentration at the Pole doab hydrological station were also collected. The study was conducted for the whole period, as well as the pre and post-1991 as a basis for the economic development growth in the region. We found that land use and population changes have occurred in the Shazand Watershed, especially in the vicinity of industrial zones. The results showed that the cities, industrial zones, roads and bare lands quickly increased from 58.8 to 134.3 km2 during post-1991. The flow durations, sediment rating curves and trend analyses indicated distinct variations in the relationship between streamflow and sediment and also caused changes within different periods. Based on the results, the mean annual flow and sediment yield in post industrialization (1991-2008) were respectively 0.84 and 1.19 times of those for pre industrialization period and the annual sediment yield increased from 25,000 to 29,850 Mg.
Lake Tana is one of nature’s best gifts to Ethiopia with unique and diversified socioeconomic benefits. It is the largest natural reservoir in Ethiopia and the head water of Abbay (Blue Nile) basin. The Lake sub basin is endowed with good potentials of water and land resources and has been identified as one of the most important area for various socioeconomic developments by the federal democratic republic government of Ethiopia. In recognition to this some national and regional water resources projects of various kinds are being developed and the development of some more additional projects are also deemed to be realized very quickly in the near future.
However, there is a lack of appropriate water resources systems planning and management tools that assure the sustainability of current and future development endeavors in the sub basin. The sub basin hydrology and water resources systems are not well studied and documented in scientific journal articles. Thus everyone involved directly or indirectly in the management of the sub basin’s water resources systems is asking about the sustainability and viability of all ongoing and envisaged development plans. There is a great fear that some of these endeavors may bring some undesirable effect which will threaten the sustainability of the sub basin’s resources. In view of this an attempt is made to conceptualize the sub basin’s hydrology and water resources systems problems in a simplified way, in light of the available data, by using an ideal equivalent catchment. Following the conceptualization a stochastic model has been fitted to the net supply of the sub basin and different synthetic net supply series were generated. The stochastic analysis shows that a first order autoregressive model and a normal distribution can adequately represent the net supply. Simulating the Lake mass balance using the synthetic generated net supply series helps to know the performance measuring indices of the Lake in various alternative development scenarios.
The modeling and simulation attempts have revealed that operating the Lake at a minimum operating level of 1784.55m asl make the Lake to offer both hydropower supply and navigations services at equal reliabilities. Besides, it is also identified that more than 10% reduction in net supply capacity of the sub basin as a result of upstream irrigation developments has a serious consequence in the reliabilities of the services offered by the Lake. The conceptualization and subsequent modeling and simulation frame works can be easily applied and used as guiding tool in the planning and management practices of the sub basin water resources systems. However, an attempt for developing a more advanced and robotic water resources systems planning and management tools are still very crucial and mandatory for the sustainable development of the sub basin’s water resources systems.
Lake Tana, biggest lake in Ethiopia, is very important water resource for community living and depending on the lake’s resources. However, the recent development activities at the catchment areas have negatively affected the water of the lake, and the climate of the region. This study delineated how land cover modification, climate change, population increase and development activities within the catchment have influenced the hydrological dynamics of Lake Tana. The land cover modification of southern part of catchment of Lake Tana in the years 1973, 1986, 1995 and 2008 were examined using land sat images. For the last 30 years, temperature, rainfall and lake level data were evaluated to identify change in climate and lake level. The annual means of temperature, rainfall, and lake level with their decadal variation and trends were analyzed with the support of linear regression model to assess trend change. Population change for the last 25 years and development activities were also assessed. The results have revealed that within the last 35 years more than 6.2% of lake’s area was converted to other land covers. Parallel to this period within the catchment there was declining of rainfall and lake level and rise in temperature. There was also increase of population and intensification of use of lake’s water in recent times. Thus, it is implied that change in Lake Tana water was caused by human factors and climate changes.
The geomorphological map of Lake Tana basin (15 077 km², Nile
basin, Ethiopia) was prepared from fieldwork data, maps and satellite
data that were processed in a GIS system. It contains four major
components: (i) hydrography, (ii) morphology and -metry, (iii) materials
and (iv) processes. The scale is 1:500 000. The geomorphological setting
of the basin consists of lavas that erupted from fissures or (shield)
volcanoes during the Tertiary and Quaternary eras, were uplifted and
ultimately sculpted by (mainly water) erosion. Lake Tana emerged by the
combination of a lava barrier blocking the Blue Nile to the south and by
epirogenetic subsidence. Since the time that the lake reached its
maximum extent, extensive floodplains were created, river valleys have
been filled with sediment and higher laying topography has been eroded.
Today, the lake plays a lesser role in landscape formation because of a
decreased lake extent (3041 km² now) as compared to the ancient
maximum (6602 km²). Dominant processes today are merely fluvial and
denudative. Recent (1886-2010) changes in lake coast are small with
exception of the delta of the major feeding river, Gilgel Abay, which
increased disproportionally the last 15 years. This indicates a large
input of sediment which is mainly due to rivers flowing through
Quaternary lavas. The recent sediment input increase is most probably
related to human induced land-use changes.
Abstract:In Ethiopia the population is rapidly expanding. As a consequence the landscape is rapidly changing. Eucalyptus plantations are increasing and irrigation projects are implemented. The hydrological effects of the changing landscape on river (low) flows have not been well documented and therefore the amount of water available in the future might be over optimistic. The objective of this paper is to establish how low flows have been impacted by new developments in irrigation and by landscape change. For this paper, we choose the Gilgel Abay in the headwaters of the upper Blue Nile basin, since it has both good quality discharge data and it is located in the Tana Beles growth corridor. Numerical and statistical means were used to analyze the 25 years of available low flow data. We found a statistically significant decreasing trend (P<0.00001) of low flow in the Gilgel Abay. From 1980’s to 1990’s the low flow decreased by 25% and from 1990’s to 2000’s the
low flow was reduced by 46%. The deterministic analysis with the Parameter Efficient Distributed (PED) model supported the statistical findings and indicated that in the middle of the nineteen nineties, after irrigation projects and eucalyptus plantations increased greatly, the low flows decreased more rapidly.
Key words:Eucalyptus; Blue Nile; Gilgel Abay; low flow; irrigation potential; PED model; watershed
Climate change is likely to have severe effects on water availability in Ethiopia. The aim of the present study was to assess the impact of climate change on the Gilgel Abay River, Upper Blue Nile Basin. The Statistical Downscaling Tool (SDSM) was used to downscale the HadCM3 (Hadley centre Climate Model 3) Global Circulation Model (GCM) scenario data into finer scale resolution. The Soil and Water Assessment Tool (SWAT) was set up, calibrated, and validated. SDSM downscaled climate outputs were used as an input to the SWAT model. The climate projection analysis was done by dividing the period 2010-2100 into three time windows with each 30 years of data. The period 1990-2001 was taken as the baseline period against which comparison was made. Results showed that annual mean precipitation may decrease in the first 30-year period but increase in the following two 30-year periods. The decrease in mean monthly precipitation may be as much as about -30% during 2010-2040 but the increase may be more than +30% in 2070-2100. The impact of climate change may cause a decrease in mean monthly flow volume between -40% to -50% during 2010-2040 but may increase by more than the double during 2070-2100. Climate change appears to have negligible effect on low flow conditions of the river. Seasonal mean flow volume, however, may increase by more than the double and +30% to +40% for the Belg (small rainy season) and Kiremit (main rainy season) periods, respectively. Overall, it appears that climate change will result in an annual increase in flow volume for the Gilgel Abay River. The increase in flow is likely to have considerable importance for local small scale irrigation activities. Moreover, it will help harnessing a significant amount of water for ongoing dam projects in the Gilgel Abay River Basin.
The filtering function of soil is an important ecosystem service for groundwater and surface water protection. The efficiency of soils as a filter depends on the behaviour of pollutants in the soil and the hydrological transport processes. This paper aims to identify knowledge gaps in processes influencing pollutant behaviour in soils and their potential transport to groundwater. Currently most soil-filter function research is approached from two disciplines, one originating from agronomical/environmental sciences; one from more fundamental hydrological process research. Combining insights and approaches from both disciplines through collaboration could lead to better understanding of this complex system and enhance assessments of management strategy changes, both over the long term as well as in different climatic settings.
Lake Tana's flow system is governed by four main components: the inflow from surrounding river catchments into the lake, the outflow at Bahir Dar through the Blue Nile, the direct rainfall on the lake and the direct evaporation from the lake. While recent studies applied simple pragmatic approaches to estimate runoff from ungauged catchments, here emphasis is placed on more advanced approaches based on regionalization and spatial proximity principles. In the regionalization approach, model parameters of the conceptual HBV (Hydrologiska Byråns Vattenbalansavdelning) rainfall-runoff modelling of gauged catchments are transferred to ungauged catchments to allow runoff simulation. Parameter transfer was attempted through regression, proximity procedures and catchment size. This yielded 42, 47 and 46%, respectively, of the total river inflow for the three procedures. Lake areal rainfall is estimated by interpolation of the rain gauges around the lake, open water evaporation is estimated by the Penman-combination equation while observed inflows and outflow data are directly used in the lake water balance. The water balance closure term was established by comparing the measured lake levels with the calculated levels. Results show that runoff from ungauged catchments is around 880 mm per year for the simulation period 1995–2001 with a water balance closure error of 5%. In addition, use is made of river and lake water chemistry to arrive at an estimate of the unknown inflow and outflow components through the mixing cell approach. The results obtained with this method also provide independent information with regard to the errors in the individual water balance components. Copyright © 2009 John Wiley & Sons, Ltd.
Rapid groundwater drawdown in Gaocheng City, the alluvial plain of the Taihang Mountain in the North China Plain, has become the biggest threat to agricultural sustainability. In order to determine the factors resulting in the groundwater decline and to develop a practical plan for long-term groundwater use, water-table fluctuation data were collected over a period of 25 years. The analysis showed that although the drawdown of the water-table was mainly due to water used for winter wheat production and other crops, another reason for groundwater level decline was the tremendous decline of upstream groundwater recharge. It was estimated that, compared to the 1970s, decrease in upstream groundwater recharge in the 1990s resulted in about 1.2 m/a of groundwater level decline. Thus, decline of upstream groundwater recharge rather than agricultural water use was the main reason for the recent groundwater drawdown. On the other hand, gradually improved agricultural practices have saved a great deal of water since the 1970s. The analysis also revealed that, although the groundwater level declined during the wheat-growing season, corn-growing season and over the whole year strongly correlated with the amount of precipitation in that period, aside from one year of extremely high precipitation, precipitation did not recharge groundwater directly but affected groundwater levels through a decrease in irrigation water use. Finally, in order to maintain the groundwater balance, agricultural practices have to save about 180 mm/a of irrigation water from their present level.
One of the most pressing global issues currently facing mankind is the increase in world population and its impact on the availability of freshwater. Recent estimates of water stocks and flows through the world's hydrologic cycle and their spatiotemporal variability illustrate the nature of current and projected water disparities throughout the world. As all such problems manifest themselves at smaller scales, a major challenge in freshwater assessments is how to handle this on different geographical scales. Increasing use of water is creating water shortages in many countries that are projected to have significant population increases in the next 25 years. Humankind is projected to appropriate from 70% to 90% of all accessible freshwater by 2025. Agriculture is the dominant component of human water use, accounting for almost 70% of all water withdrawals. Hence, many of the solutions to water-related food and environmental security come from within agriculture by increasing the efficiency and productivity of water use. Many factors significantly impact the increasing water demand, including population growth, economic growth, technological development, land use and urbanization, rate of environmental degradation, government programs, climate change, and others. Demand management, not increasing supply availability, is the realistic way forward. Although, thanks to market forces, the threatened exhaustion of nonrenewable resources has not happened, renewable resources, such as freshwater, remain problematic because they are vulnerable to human overuse and pollution. Climate change adds further risks and uncertainties to the global picture requiring the adoption of adaptive management in water resources based on monitoring and reevaluation. Although climate change may be perceived as a long-term problem, it needs to be addressed now because decisions today will affect society's ability to adapt to increasing variability in tomorrow's climate. If we are to balance freshwater supply with demand, and also protect the integrity of aquatic ecosystems, a fundamental change in current wasteful patterns of production and consumption is needed. Recognition of the links between rapidly growing populations and shrinking freshwater supplies is the essential first step in making water use sustainable.
In climate models, lateral terrestrial water fluxes are usually neglected. We estimated the contribution of vertical and lateral groundwater fluxes to the land surface water budget at a subcontinental scale, by modeling convergence of groundwater and surfacewater fluxes. We present a hydrological model of the entire Danube Basin at 5 km resolution, and use it to show the importance of groundwater for the surface climate.
Results show that the contribution of groundwater to evaporation is significant, and can locally be higher than 30 % in summer. We demonstrate through the same model that this contribution also has important temporal characteristics. A wet episode can influence groundwater contribution to summer evaporation for several years afterwards. This indicates that modeling groundwater flow has the potential to augment the multi-year memory of climate models. We also show that the groundwater contribution to evaporation is local by presenting the groundwater travel times and the magnitude of groundwater convergence. Throughout the Danube Basin the lateral fluxes of groundwater are negligible when modeling at this scale and resolution. This suggests that groundwater can be adequately added in land surface models by including a lower closed groundwater reservoir of sufficient size with two-way interaction with surface water and the overlying soil layers.
Lake Tana is the largest fresh water body situated in the north-western highlands of Ethiopia. In addition to its ecological services, it serves for local transport, electric power generation, fishing, recreational purposes, and source of dry season irrigation water supply. Evidence shows that the lake has dried at least once at about 15,00017,000 before present owing to a combination of high evaporation and low precipitation events. Past attempts to understand and simulate historical fluctuation of Lake Tana based on simplistic water balance approach of inflow, outflow, and storage have failed to capture well-known events of drawdown and rise of the lake that have happened in the last 44years. This study tested different stochastic methods of lake level and volume simulation for supporting Lake Tana operational planning decision support. Three stochastic methods (perturbations approach, Monte Carlo methods, and wavelet analysis) were employed for lake level and volume simulation, and the results were compared with the stage level measurements. Forty-four years of daily, monthly, and mean annual lake level data have shown a Gaussian variation with goodness of fit at 0.01 significant levels of the KolmogorovSmirnov test. The stochastic simulations predicted the lake stage level of the 1972, 1984, and 2002/2003 historical droughts 99% of the time. The information content (frequency) of fluctuation of Lake Tana for various periods was resolved using Wigner's Time-Frequency Decomposition method. The wavelet analysis agreed with the perturbations and Monte Carlo simulations resolving the time (1970s, 1980s, and 2000s) in which low frequency and high spectral power fluctuation has occurred. The Monte Carlo method has shown its superiority for risk analysis over perturbation and deterministic method whereas wavelet analysis reconstructed historical record of lake stage level at daily and monthly time scales. Copyright (c) 2012 John Wiley & Sons, Ltd.
In response to anthropogenic disturbances, alluvial rivers adjust their geometry. The alluvial river channels in the upper Blue Nile basin have been disturbed by human-induced factors since a longtime. This paper examines channel adjustment along a 38-km stretch of the Gumara River which drains towards Lake Tana and then to the Blue Nile. Over a 50 years period, agriculture developed rapidly in the catchment and flooding of the alluvial plain has become more frequent in recent times. The objectives of this study were to document the changes in channel planform and cross-section of the Gumara River and to investigate whether the changes could have contributed to the frequent flooding or vice versa. Two sets of aerial photographs (1957 and 1980) were scanned, and then orthorectified. Recent channel planform information was extracted from SPOT images of 2006 and Google Earth. Channel planform and bed morphology (vertical changes) were determined for these nearly 50 years period. The vertical changes were determined based on aggradation along a permanent structure, historic information on river cross-sections at a hydrological gauging station, and field observations. The results indicate that the lower reach of Gumara near its mouth has undergone major planform changes. A delta with approx. 1.12 km 2 of emerged land was created between 1957 and 1980 and an additional 1 km 2 of land has been added between 1980 and 2006. The sinuosity of the river changed only slightly: negatively (-1.1% i.e. meandering decreased) for the period from 1957 to 1980 and positively (+3.0%) for the period 1980-2006. Comparison of cross-sections at the hydrological gauging station showed that the deepest point in the river bed aggraded by 2.91 m for the period 1963-2009. The importance of sediment deposition in the stream and on its banks is related to land degradation in the upper catchment, and to artificial rising of Lake Tana level that creates a backwater effect and sediment deposition in Gumara.
Mendae Plain at Abraha Atsbaha (Tigray Region, northern Ethiopia) is an agricultural area, which has been very drought-prone in the past. In the last decade, agricultural development has boosted, due to the intensive use of large diameter wells that tap the phreatic aquifer. Pumped water is used for irrigation during the long dry season (October to May). Since 15 years, water harvesting measures have been implemented, mainly in the form of infiltration ponds and trenches that enhance local infiltration of rainfall runoff from hillslopes. To investigate the sustainability of the groundwater exploitation and the efficiency of the measures, the different recharge and discharge components of the water balance of data-scarce Mendae plain have been identified and quantified, using different methods. Diffuse aquifer recharge is calculated from a soil moisture balance based on meteorological data, and with the chloride mass balance method, based on groundwater analyses. Diffuse recharge is much higher on cultivated land plots than on non-cultivated bare soils. Rainfall infiltration in ponds and trenches is estimated based on the inflow catchment derived from the topography. Groundwater flow to a nearby river is obtained by balancing in- and outflow by the other components over an 11 year period. The balance components are integrated into a lumped parameter model that was run for the period from 2000 to 2010. The results show that infiltration in ponds and trenches contributes between 30 and nearly 50% of total aquifer recharge, with the highest values in dry years. Changes in aquifer storage over time are an indicator for the evolution of groundwater levels in the aquifer and confirm the occurrence of two dry periods in 2002-2005 and 2008-2009. This article is protected by copyright. All rights reserved.
A critical analysis of the present situation on the global water resources assessment is made. Basic data and methodological approaches used by the author for the assessment and prediction of water resources, water use and water availability on the global scale are briefly described. On the basis of data generalization of the world hydrological network new data are given on the dynamics of renewable water resources of the continents, physiographic and economic regions, selected countries as well as on the river water inflow to the world ocean. The results of the assessments for the 20th century and for the future before 2010–2025 on the water supply for municipal, industrial and agricultural needs as well as an additional evaporation from reservoirs are presented. Loads on water resources and water availability depending on socio-economic and phisiographic factors are analyzed; regions of water scarcity and water resources deficit are discovered. Possible ways of water supply improvement and elimination of water resources deficit in different regions and countries are discussed.
Developing countries are vulnerable to negative impacts of climate change due to over reliance on climate-sensitive sectors, mainly agriculture. Limited adaptive capacity makes them vulnerable to climate-induced hazards. However, over the years, indigenous knowledge systems (IKS) have proven effective in promoting sustainable development particularly for those in subsistence agriculture. For example, in Lake Victoria basin, local communities have coped and adapted to climate-induced hazards using traditional systems and IKS. This chapter presents findings of a cross-sectional survey on the use of IKS in subsistence agriculture to enhance climate risk management and mitigation of community vulnerability in a changing climate. Data were collected by household questionnaires, key informants’ interviews, and focus group discussions. Results showed overall, significantly high community awareness levels prevail in study area, implicating climate change as the main challenge facing agricultural sector. Nevertheless, as climate change adaptation and mitigation measures, local communities use myriad of IKS to improve resilience and productivity. They use IKS in soil conservation, weather/climate forecasting, selection of planting seeds, and preservation of seeds/crops. This study, therefore, recommends incorporating IKS into scientific knowledge systems to promote climate change adaptation and mitigation among vulnerable communities dependent on climate-sensitive resources.
In the Sahelian region, the high precipitation intensity and the daily rainfall extreme values are currently the main cause of soil erosion and land degradation. In addition, solid transport often leads to reservoir siltation and reduction of the amount of water available for agriculture. To cope with these issues, Soil and Water Conservation (SWC) measures have been regularly employed in the Sahelian area. However, a proper cost-effectiveness analysis of the impact of SWC interventions on the catchment sediment budget normally requires quantitative surveys on erosion and sedimentation processes. Where data for calibration and validation of models are scarce, an overall methodology to evaluate the economical sustainability of a proposed intervention can be of paramount importance. The study herein proposed aims to assess the monetary sustainability of SWC measures in limiting the reservoir siltation of the Laaba dam (Yatenga District, Northern Burkina Faso). In particular, the catchment sediment budget was estimated by means of morphological and pedologic parameters and dam sedimentation rates; a cost-effectiveness analysis was then performed to assess the economic sustainability of a possible SWC intervention. The proposed methodology showed interesting potentials for land and water management in Burkina Faso, particularly when data and financial resources are limited and where the application of detailed process-based models is not possible. Copyright © 2013 John Wiley & Sons, Ltd.
The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can be used as tracers of infiltration over sometimes considerable distances, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Kenya and Ethiopia are considered, including Lakes Naivasha, Baringo, Awasa and Zwai.In addition to their physical tracing role, stable isotopes can reveal information about processes of fluid-rock interaction. The general lack of O isotope shifting in rift hydrothermal systems suggests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate waters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promoted by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle between Kenya and Ethiopia.
Accurate estimation of evapotranspiration (ET) is essential in water resources management and hydrological practices. Estimation of ET in areas, where adequate meteorological data are not available, is one of the challenges faced by water resource managers. Hence, a simplified approach, which is less data intensive, is crucial. The FAO-56 Penman–Monteith (FAO-56 PM) is a sole global standard method, but it requires numerous weather data for the estimation of reference ET. A new simple temperature method is developed, which uses only maximum temperature data to estimate ET. Ten class I weather stations data were collected from the National Meteorological Agency of Ethiopia. This method was compared with the global standard PM method, the observed Piche evaporimeter data, and the well-known Hargreaves (HAR) temperature method. The coefficient of determination (R2) of the new method was as high as 0.74, 0.75, and 0.91, when compared with that of PM reference evapotranspiration (ETo), Piche evaporimeter data, and HAR methods, respectively. The annual average R2 over the ten stations when compared with PM, Piche, and HAR methods were 0.65, 0.67, and 0.84, respectively. The Nash–Sutcliff efficiency of the new method compared with that of PM was as high as 0.67. The method was able to estimate daily ET with an average root mean square error and an average absolute mean error of 0.59 and 0.47 mm, respectively, from the PM ETo method. The method was also tested in dry and wet seasons and found to perform well in both seasons. The average R2 of the new method with the HAR method was 0.82 and 0.84 in dry and wet seasons, respectively. During validation, the average R2 and Nash–Sutcliff values when compared with Piche evaporation were 0.67 and 0.51, respectively. The method could be used for the estimation of daily ETo where there are insufficient data. Copyright © 2013 John Wiley & Sons, Ltd.
Models accurately representing the underlying hydrological processes and sediment dynamics in the Nile Basin are necessary for optimum use of water resources. Previous research in the Abay (Blue Nile) has indicated that direct runoff is generated either from saturated areas at the lower portions of the hillslopes or from areas of exposed bedrock. Thus, models that are based on infiltration excess processes are not appropriate. Furthermore, many of these same models are developed for temperate climates and might not be suitable for monsoonal climates with distinct dry periods in the Nile Basin. The objective of this study is to develop simple hydrology and erosion models using saturation excess runoff principles and interflow processes appropriate for a monsoonal climate and a mountainous landscape. We developed a hydrology model using a water balance approach by dividing the landscape into variable saturated areas, exposed rock and hillslopes. Water balance models have been shown to simulate river flows well at intervals of 5 days or longer when the main runoff mechanism is saturation excess. The hydrology model was developed and coupled with an erosion model using available precipitation and potential evaporation data and a minimum of calibration parameters. This model was applied to the Blue Nile. The model predicts direct runoff from saturated areas and impermeable areas (such as bedrock outcrops) and subsurface flow from the remainder of the hillslopes. The ratio of direct runoff to total flow is used to predict the sediment concentration by assuming that only the direct runoff is responsible for the sediment load in the stream. There is reasonable agreement between the model predictions and the 10-day observed discharge and sediment concentration at the gauging station on Blue Nile upstream of Rosaries Dam at the Ethiopia–Sudan border. Copyright © 2009 John Wiley & Sons, Ltd.
The level of Lake Tana, Ethiopia, fluctuates annually and seasonally following the patterns of changes in precipitation. In this study, a mass balance approach is used to estimate the hydrological balance of the lake. Water influx from four major rivers, subsurface inflow from the floodplains, precipitation, outflow from the lake constituting river discharge and evapotranspiration from the lake are analysed on monthly and annual bases. Spatial interpolation of precipitation using rain gauge data was conducted using kriging. Outflow from the lake was identified as the evaporation from the lake's surface as well as discharge at the outlet where the Blue Nile commences. Groundwater inflow is estimated using MODular three-dimensional finite-difference ground-water FLOW model software that showed an aligned flow pattern to the river channels. The groundwater outflow is considered negligible based on the secondary sources that confirmed the absence of lake water geochemical mixing outside of the basin. Evaporation is estimated using Penman's, Meyer's and Thornwaite's methods to compare the mass balance and energy balance approaches. Meteorological data, satellite images and temperature perturbation simulations from Global Historical Climate Network of National Oceanographic and Atmospheric Administration are employed for estimation of evaporation input parameters. The difference of the inflow and outflow was taken as storage in depth and compared with the measured water level fluctuations. The study has shown that the monthly and annually calculated lake level replicates the observed values with root mean square error value of 0·17 and 0·15 m, respectively. Copyright © 2009 John Wiley & Sons, Ltd.
The groundwater flow contribution into Lake Tana was simulated using Gumera River sub-basin as a case study. Draining into Lake Tana, the Gumera sub-basin boundary was delineated from a 90-m Shuttle Radar Terrain Mapping (SRTM) digital elevation model (DEM) using ArcHydro tools. The sub-basin boundary served as the divide line of the groundwater flow while stream networks were used as internal drainage lines. Input parameters such as transmissivity and hydraulic conductivity were obtained from past studies and experts' knowledge. Based on the geological information of the sub-basin, unconfined subsurface flow condition was considered and simulated using MODular 3D finite difference ground-water FLOW model (MODFLOW). Analytical solutions were also used for comparison purposes. The result indicates that head contours are aligned to the streams showing their relationship as a subdued form of the surface water flow, which are dictated by the morphology of the basin. This suggested the need to account groundwater contribution of the sub-basins to Lake Tana as baseflow, and to estimate groundwater inflow from the floodplain separately. The contribution from the floodplain was estimated using a finite differences method which showed an inflow of 0·15 billion cubic meter (BCM) from rivers into the floodplain out of which 0·09 BCM is flowing out to the Lake Tana and 0·06 BCM stored in the aquifers of the floodplain. The stored quantity could meet irrigation demands as well as contribute to the preservation of the ecological structure and function of the area. The study suggests further research on validation of parameters through inverse modeling approaches and optimization of water allocations. Copyright © 2009 John Wiley & Sons, Ltd.
Rapidly depleting unconfined aquifers are the primary source of water for irrigation on the North China Plain. Yet, despite its critical importance, groundwater recharge to the Plain remains an enigma. We introduce a one-dimensional soil-water-balance model to estimate precipitation- and irrigation-generated areal recharge from commonly available crop and soil characteristics and climate data. To limit input data needs and to simplify calculations, the model assumes that water flows vertically downward under a unit gradient; infiltration and evapotranspiration are separate, sequential processes; evapotranspiration is allocated to evaporation and transpiration as a function of leaf-area index and is limited by soil-moisture content; and evaporation and transpiration are distributed through the soil profile as exponential functions of soil and root depth, respectively. For calibration, model-calculated water contents of 11 soil-depth intervals from 0 to 200 cm were compared with measured water contents of loam soil at four sites in Luancheng County, Hebei Province, over 3 years (1998–2001). Each 50-m2 site was identically cropped with winter wheat and summer maize, but received a different irrigation treatment. Average root mean-squared error between measured and model-calculated water content of the top 180 cm was 4·2 cm, or 9·3% of average total water content. In addition, model-calculated evapotranspiration compared well with that measured by a large-scale lysimeter. To test the model, 12 additional sites were simulated successfully. Model results demonstrate that drainage from the soil profile is not a constant fraction of precipitation and irrigation inputs, but rather the fraction increases as the inputs increase. Because this drainage recharges the underlying aquifer, improving irrigation efficiency by reducing seepage will not reverse water-table declines. Copyright © 2003 John Wiley & Sons, Ltd.
The Horn of Africa extends from the Sahara desert eastwards to the Red Sea, the Gulf of Aden and the Indian Ocean. Due to its complex volcano-tectonic evolution over the past 15 million years (Mohr 1971), the region is characterised by considerable changes in elevation within short distances. The Ethiopian highlands, which rise to altitudes exceeding 4000m above sea level, form an extensive uplifted plateau, delimited by pronounced escarpments on both east and west. In Southern Ethiopia the 1000 km-wide uplifted volcanic province is divided asymmetrically into northwestern and southeastern plateaux by the Main Ethiopian Rift which runs SSW-NNE and represents the northern end of the continental East African Rift. Eastward, the continental Afar Rift (Ethiopia and Republic of Djibouti) sits astride the Gulf of Aden—Red Sea sea-floor spreading axis, with several areas below sea level (e.g.,–155mat Lake Asal). These marked altitudinal gradients result in a wide variety of climates and environments, making the region particularly suitable for investigating past environmental change. The region is subject to the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) and is very sensitive to monsoon variability. Several studies have revealed climate variability from millennial to inter-annual timescales during the Late Quaternary (Gasse 2000).
Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO2 influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO3 type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO3 type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ18O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ18O groundwaters. Altitudinal depletion of δ18O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude.
The annual water budget of Lake Tana is determined from estimates of runoff, rainfall on the lake, measured outflow and empirically determined evaporation. Simulation of lake level variation (1960–1992) has been conducted through modeling at a monthly time step. Despite the ±20% rainfall variations in the Blue Nile basin in the last 50 years, the lake level remained regular. A preliminary analysis of the sensitivity of level and outflow of the lake suggests that they are controlled more by variation in rainfall than by basin-scale forcing induced by human activities. The analysis shows that a drastic (40–45%) and sustained (7–8 years) rainfall reduction is required to change the lake from out flowing to terminal (cessation of outflow). However, the outflow from the lake shows significant variation responding to the rainfall variations. Unlike the terminal lakes in the Ethiopian rift valley or the other large lakes of Tropical Africa, at its present hydrologic condition, the Lake Tana level is less sensitive to rainfall variation and changes in catchment characteristics.
The isotope balance approach, which used (18)O content of waters, has been used as an independent tool to estimate inflow to Lake Tana of surface water flows from ungauged catchment of Lake Tana (50% of the total area) and evaporative water loss in the vast plains adjoining the lake. Sensitivity analysis has been conducted to investigate the effects of changes in the input parameters on the estimated flux. Surface water inflow from ungauged catchment is determined to be in the order of 1.698×10(9) m(3)a(-1). Unaccounted water loss from the lake has been estimated at 454×10(6) m(3)a(-1) (equivalent to 5% of the total via surface water). Since the lake is water tight to groundwater outflow, the major error introduced into the water balance computation is related to evaporative water loss in water from the flood plains. If drained, the water which is lost to evaporation can be used as an additional water resource for socio-economic development in the region (tourism, agriculture, hydropower, and navigation). Hydrological processes taking place in the vast flood plains of Lake Tana (origin of salinity, groundwater surface water interaction, origin of flood plain waters) have been investigated using isotopes of water and geochemistry as tracers. The salinity of shallow groundwaters in the flood plains is related to dissolution of salts accumulated in sediments covering former evaporation pools and migration of trace salt during recharge. The waters in the flood plains originate from local rainfall and river overflows and the effect of backwater flow from the lake is excluded. Minimum linkage exists between the surface waters in the flood plains and shallow groundwaters in alluvio lacustrine sediments suggesting the disappearance of flood waters following the rainy season, which is related to complete evaporation or drainage than seepage to the subsurface. There is no groundwater outflow from the lake. Inflow of groundwater cannot be ruled out. Discharge of groundwater to the lake is presumed to take place along rocky bottom in southern sector from Quaternary volcanics covering the southern sector of the catchment.
In many landscapes, vegetation extracts water from both the unsaturated and the saturated zones. The partitioning of evapotranspiration (ET) into vadose zone evapotranspiration and ground water evapotranspiration (GWET) is complex because it depends on land cover and subsurface characteristics. Traditionally, the GWET fraction is assumed to decay with increasing depth to the water table (DTWT), attaining a value of 0 at what is termed the extinction depth. A simple assumption of linear decay with depth is often used but has never been rigorously examined using unsaturated-saturated flow simulations. Furthermore, it is not well understood how to relate extinction depths to characteristics of land cover and soil texture. In this work, variable saturation flow theory is used to simulate GWET for three land covers and a range of soil properties under drying soil conditions. For a water table within half a meter of the land surface, nearly all ET is extracted from ground water due to the close hydraulic connection between the unsaturated and the saturated zones. For deep-rooted vegetation, the decoupling of ground water and vadose zone was found to begin at water table depths between 30 and 100 cm, depending on the soil texture. The decline of ET with DTWT is better simulated by an exponential decay function than the commonly used linear decay. A comparison with field data is consistent with the findings of this study. Tables are provided to vary the extinction depth for heterogeneous landscapes with different vegetation cover and soil properties.
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