This paper presents a case study conducted in the Upper Argos River, in southeast Spain, to verify the applicability of the SWAT model for prediction of the water discharge and sediment load in a Mediterranean semiarid karst basin. For this purpose, the monthly and yearly discharge and sediment load records at the Argos reservoir gauge during the period 1976–2000 were used to calibrate the model, while data from 2001 to 2017 were used for validation. For both stages of the modeling the performance of the NSE, RSR, and PBIAS indices was good in the case of the monthly flow rate (NSE = 0.62 and 0.70 for calibration and validation, respectively; RSR = 0.61 and 0.54; PBIAS = −20.60% and − 16.09%) and acceptable for the estimation of the monthly sediment load (NSE = 0.52 and 0.58; RSR = 0.70 and 0.64; PBIAS = 10.65% and 15.20%). These indices showed a substantial improvement in the annual simulations, particularly in the calibration, for which the respective values of NSE and RSR were 0.89 and 0.32 for the flow rate and 0.81 and 0.42 for the sediment transport.
Differences in deposit geometry and texture with depth along ephemeral gravel-bed streams strongly reflect fluctuations in bedload which are due to environmental changes at the basin scale and to morphological channel adjustments. This study combines electrical resistivity tomography (ERT) with datasets from borehole logs to analyse the internal geometry of channel cross-sections in a gravel-bed ephemeral stream (southeast Spain). The survey was performed through longitudinal and transverse profiles in the upper channel stretch, of 14 to 30 m in length and 3 to 6 m in depth, approximately. ERT values were correlated with data on sediment texture as grain size distribution, effective grain sizes, sorting, and particle shape (Zingg’s classification). The alluvial channel-fills showed the superposition of four layers with uneven thickness and arrangement: (1) the softer rocky substrate (<1000 Ω.m); (2) a thicker intermediate layer (1000 to 2000 Ω.m); and (3) an upper set composed of coarse gravel and supported matrix, ranging above 2000 Ω.m, and a narrow subsurface layer, which is the most resistive (>5000 Ω.m), corresponding to the most recent armoured deposits (gravel and pebbles). The ERT results coupled with borehole data allowed for determining the horizontal and vertical behaviour of the materials in a 3D model, facilitating the layer identification.
This study has evaluated the runoff and erosion rates in torrents of Southern Italy, two forested headwaters with very similar climatic, hydrological and geomorphological characteristics; in one headwater, 15 check dams were installed in the mid-1950s, while the other is not regulated with engineering works. To this aim, the hydrological variables have been modeled over 15 years after check dam installation using the HEC-HMS (Hydrologic Engineering Center-Hydrologic Modeling System) model coupled to the MUSLE (Modified Universal Soil Loss Equation) equation. The model simulations have shown that check dams have not played a significant role in reducing the surface runoff compared to the unregulated torrent; in both catchments, the well-developed forest cover determined very low runoff coefficients (lower than 0.3%) with a scarce runoff generation capacity. Additionally, the reduction in peak flow due to the check dams was not significant, on average −7.4% compared to the unregulated headwater. Check dams have retained sediments for about 8-10 years after their installation, reducing erosion by about 35%, although soil loss was much lower than the tolerance limit in both catchments. After the sediment retention capacity of the dam sediment wedge was depleted, the sediment yield in the regulated torrent was even higher (by about 20%) compared to the unregulated catchment. Overall, the study has shown that the use of check dams as a catchment management strategy of forested headwaters under semi-arid Mediterranean conditions should be considered with caution, since the structures could be ineffective to reduce water and sediment flows during floods or, in some cases, check dams may increase erosion rates.
This research studies the effect of climate change on the hydrological behavior of two semi-arid basins. For this purpose, the Soil and Water Assessment Tool (SWAT) model was used with the simulation of two future climate change scenarios, one Representative Concentration Pathway moderate (RCP 4.5) and the other extreme (RCP 8.5). Three future periods were considered: close (2019–2040), medium (2041–2070), and distant (2071–2100). In addition, several climatic projections of the EURO-CORDEX model were selected, to which different bias correction methods were applied before incorporation into the SWAT model. The statistical indices for the monthly flow simulations showed a very good fit in the calibration and validation phases in the Upper Mula stream (NS = 0.79–0.87; PBIAS = −4.00–0.70%; RSR = 0.44–0.46) and the ephemeral Algeciras stream (NS = 0.78–0.82; PBIAS = −8.10–−8.20%; RSR = 0.4–0.42). Subsequently, the impact of climate change in both basins was evaluated by comparing future flows with those of the historical period. In the RCP 4.5 and RCP 8.5 scenarios, by the end of the 2071–2100 period, the flows of the Upper Mula stream and the ephemeral Algeciras stream will have decreased by between 46.3% and 52.4% and between 46.6% and 55.8%, respectively.
Los cursos efímeros (ramblas, rieras, cauces secos salvo en avenidas repentinas y eventuales) tienen una gran presencia en el territorio mediterráneo, donde constituyen la mayor parte de la red fluvial. Son sistemas naturales fundamentales en el ciclo hidrológico para el transporte de agua, sedimentos y nutrientes, y, por tanto, excelentes indicadores del cambio climático y global. Su puesta en valor, el reconocimiento de su papel, de sus valores hidromorfológicos y de sus servicios ecosistémicos son totalmente necesarios para entender su grado de resiliencia y contribución a la adaptación al cambio climático. Y es muy urgente trabajar en su gestión, restauración y conservación, porque en general se encuentran sometidos a fuertes presiones y están enormemente dañados. La presente guía alerta sobre los múltiples impactos que sufren, divulga este gran patrimonio mediterráneo, tan subestimado y poco conocido, y propone 33 buenas prácticas para su gestión y restauración. Es una guía que puede aportar ideas a las personas responsables de la gestión, pero está destinada al conjunto de la sociedad, porque el reto es muy complejo: tenemos que recuperar cursos efímeros a partir del conocimiento y la sensibilización. Y actuar con rapidez, porque ya es tarde y hasta ahora prácticamente no se ha hecho nada por respetar, proteger y recuperar estos sistemas fluviales imprescindibles de nuestro territorio. Este es nuestro reto.
Ephemeral streams are highly dependent on rainfall and terrain characteristics and, therefore, very sensitive to minor changes in these environments. Western Mediterranean area exhibits a highly irregular precipitation regime with a great variety of rainfall events driving the flow generation on intermittent watercourses, and future climate change scenarios depict a lower magnitude and higher intensity of precipitation in this area, potentially leading to severe changes in flows. We explored the rainfall-runoff relationships in two semiarid watersheds in southern Spain (Algeciras and Upper Mula) to model the different types of rainfall events required to generate new flow in both intermittent streams. We used a nonlinear approach through Generalized Additive Models at event scale in terms of magnitude, duration, and intensity, contextualizing resulting thresholds in a long-term perspective through the calculation of return periods. Results showed that the average ~ 1.2-day and
Stream power represents the rate of energy expenditure along a stream reach and can be calculated using topographic data acquired via structure-from-motion (SfM) photogrammetry and terrestrial laser scanning (TLS). This study sought to quantitatively relate morphological adjustments in the Azohía Rambla, a gravel-bed ephemeral stream in southeastern Spain, to stream power (ω), critical power (ωc), and energy gradients (∂ω/∂s), along different reference channel reaches of 200 to 300 m in length. High-resolution digital terrain models (HRDTMs), combined with ortophotographs and point clouds from 2018, 2019, and 2020, and ground-based surveys, were used to estimate the spatial variability of morphological sediment budgets and to assess channel bed mobility during the study period at different spatial scales: reference channel reaches (RCRs), pilot bed survey areas (PBSAs), and representative geomorphic units (RGUs). The optimized complementary role of the SfM technique and terrestrial laser scanning allowed the generation of accurate and reliable HRDTMs, upon which a 1-D hydrodynamic model was calibrated and sediment budgets calculated. The resulting high-resolution maps allowed a spatially explicit analysis of stream power and transport efficiency in relation to volumes of erosion and deposition in the RCR and PBSA. In addition, net incision or downcutting and vertical sedimentary accretion were monitored for each flood event in relation to bedforms and hydraulic variables. Sediment sources and sinks and bed armoring processes showed different trends according to the critical energy and stream power gradient, which were verified from field observations. During flows exceeding bankfull discharges (between 18 and 24 m3 s−1 according to channel reach), significant variations in ∂ω/∂s values and ω/ωc ratios (e.g., −15 < ∂ω/∂s < 15 Wm−3; ω/ωc > 2 for a peak discharge of 31 m3 s−1) were associated with a large amount of bedload mobilized upstream and vertical accretion along the middle reach (average rise height of 0.20 to 0.35 m for the same event). By contrast, more moderate peak flows (≤10 m3 s−1) only produced minor changes resulting in surface washing, selective transport, and local bed scouring. View Full-Text Keywords: SfM photogrammetry; terrestrial laser scanning; stream power; morphological sediment budget; bedforms; gravel-bed ephemeral channel; southeastern Spain
In Spanish semi-arid environments, rainfall has an irregular distribution and duration due to the influence of the relief, and the alternation of Mediterranean and subtropical (persistent) anticyclones and cold air pockets at high atmospheric levels. The study area is part of the Betic mountains and it conditions a föehn effect that attenuates the action of the west winds. In the study area, the rainfall intensity has not been recorded in several control and measured points until twenty years ago, so the temporal analyzes are relatively recent (2000-2020). The analysis suggests a change in the rainfall ratio (hourly / daily), due to changes in duration, mean rainfall and maximum hourly rainfall. This research was funded by ERDF / Spanish Ministry of Science, Innovation and Universities - State Research Agency (AEI) / Project CGL2017-84625-C2-1-R. State Program for Research, Development and Innovation focused on the Challenges of Society. We also would like to extend our thanks to AEMET, IMIDA, SHC and Government of Spain, for their collaboration.
Ephemeral rivers hydromorphological processes are intermittent and many times of fast response. Therefore they remain still quite unknown. The geomorphological mapping of river forms and geomorphological units is a useful tool to recognize the evolution, changes and the response of river adjustments of hydrological events. A diachronic geomorphological mapping has been done in some ephemeral rivers located in Ebro basin, Segura basin and Calabrian ephemeral rivers. We are presenting the specific results of six reaches distributed by the Ebro basin (Tudela, Reajo, Alpartir, Cariñena, Valcodo, Sosa and Seco). The first historical aerial image is that of the American Flight B of 1956-57, another of the mid 80’s, the last official ortophotography available (around 2017), and a specific flight with an unmanned aerial vehicle (UAV) done during the winter of 2019. An altimetry correction has been performed on the first two images. Different categories have been identified within the channel (active channel, principal channel and secondary channel), the sediment bars (vegetated, scant vegetated and non-active paleo-bars), the deposits coming from bank failures or tributaries, rocky areas, exhumed old sediment areas, consolidated or unconsolidated granular bed. The categories were mapped at different scales depending on the image quality (for example, from ≤ 1/300 scale of the UAV to ≤ 1/1,000 scale of the American flight). This evolutionary cartography allows comparing the geomorphology of each river reach among different dates, considering the different resolution of the images and its limitations (i.e. previously, the results were unified to compare among them), and relating to the fluvial processes and changes on the river and basin. This research was funded by ERDF/Spanish Ministry of Science, Innovation and Universities—State Research Agency (AEI) /Project CGL2017-84625-C2-1-R; State Program for Research, Development and Innovation Focused on the Challenges of Society.
Dimensionless morphological ratios (DMR) generally are used in systemic proposals for stream classification and river restoration projects. Often, such morphometric parameters, including field data from channel cross sections, develop into a template for a given geomorphic reference site. In this study, high-resolution Digital Terrain Models (HRDTMs), combined with orthophotographs from 2018 and ground-based surveys, were used to analyze the spatial variability of DMR in a semi-arid ephemeral stream subject to changes in stream power under bankfull conditions. In particular, a channel reach of 2.7 km in length along the Upper Mula stream in southeastern Spain was chosen to test the relationships between the two types of variables. Rosgen's DMR (width to depth ratio, entrenchment ratio, bank height ratio) and hydraulic data at bankfull stage (flow velocity, Froude number, shear stress, mean stream power and energy gradient, among others) were calculated by 1D hydrodynamic modeling and HRDTMs prior entry of field information. The resulting maps allowed comparison of stream power with DMR in relation to the channel stretch class and bed stability. The results showed similar spatial patterns for the width to depth ratio and the bank height ratio. The average values estimated in bend stretches were lower than along straight and slightly sinuous stretches and very similar to those of sub-reaches from the inflection point to the meander bend apex. However, the entrenchment ratio followed a different pattern, according to which the straight and slightly sinuous stretches were the most entrenched and the bend stretches presented a more moderate average entrenchment ratio. In addition, the energy balance and power gradient also experienced spatial variations in relation to the bed stability and DMR. Only in highly incised sub-reaches were such relationships not significant. The lack of a significant correlation between excess energy and bank height ratio or width to depth ratio over short lag distances was also verified, regardless of the affected bedforms. An ANOVA showed important differences between the straight and slightly sinuous stretches and bend stretches, which were strongly influenced by the incision and entrenchment ratios and the maximum bankfull depth.
The impact of climate change on future soil loss is commonly assessed with soil erosion models, which are suggested to be an important source of uncertainty. Here we present a novel soil erosion model ensemble to assess model uncertainty in climate change impact assessments. The model en semble consisted of five continuous process‐based soil erosion models that run at a daily time step (i.e. DHSVM, HSPF, INCA, MMF, SHETRAN). The models were implemented in the SPHY hydrological model and simulate detachment by raindrop impact, detachment by runoff and immediate deposition. The soil erosion model ensemble was applied in a semi‐arid catchment in the southeast of Spain. We applied three future climate scenarios based on global mean temperature rise (+1.5, +2 and +3°C). Data from two contrasting regional climate models were used to assess how an increase and a decrease in projected extreme precipitation affect model uncertainty. Soil loss is projected to increase (up to 95%) and decrease (up to ‐30%) under climate change, mostly reecting the change in extreme precipitation. Model uncertainty is found to increase with increasing slope, extreme precipitation and runoff, which reveals some inherent differences in model assumptions among the five models. Moreover, the model uncertainty increases in all climate change scenarios, independent of the projected change in annual precipitation and extreme precipitation. This stresses the importance to consider model uncertainty through model ensembles of climate, hydrology, and soil erosion in climate change impact assessments. This article is protected by copyright. All rights reserved.
Los cursos de agua efímeros del ámbito mediterráneo plantean muchas dificultades para que puedan ser objeto de programas de restauración. Los problemas derivan de la gravedad de los impactos geomorfológicos que sufren, de su funcionamiento repentino y esporádico-de ocurrencia a veces impredecible-, de su escasísima consideración en el marco de la gestión y planificación hidrológica, de las dificultades de enfoque en la correcta restauración geomorfológica, de su percepción social negativa y de su alejamiento del ideario de las políticas conservacionistas convencionales. Frente a ello, es necesario y urgente avanzar en soluciones de rehabilitación, tanto generalistas como específicas para este tipo de cursos fluviales. La restauración geomorfológica se impone como la más importante en este contexto, ya que los impactos son fundamentalmente sobre los procesos geomorfológicos y las propias for ma s de los cauces, considerando además el hecho de que no se puede contar con caudales salvo precisamente con los geomórficos, siendo fundamental el papel de los sedimentos en los procesos y formas. Las líneas de actuación restauradora deben centrarse en mejorar el conocimiento y la sensibilización sobre estos cursos, en lograr el adec ua do espacio fluvial, en la eliminación de barreras y obstáculos, con especial atención a l os vados, y en la prohibición de dragados y extracciones y la necesaria reposición sedimentaria que los compense. Palabras clave: evaluación hidromorfológica, irregularidad hidrológica, sucesos extremos, carga sedimentaria, actuaciones e impactos Abstract Restoration projects find numerous difficulties in Mediterranean ephemeral streams. Problems derive from (i) the severity of the geomorphological impacts ; (ii) their sudden and sporadic activity-frequently unpredictable-; (iii) their very low consideration for the hydrological management and planning framework; (iv) the difficulties of focusing on correct geomorphological restoration; (v) their negative social perception; and (vi) their distance from the ideology of conventional conservationist policies. It is necessary and urgent to progress in rehabilitation solutions for thi s type of streams, both general and specific. Geomorphological restoration is the most important action in this context. I mpa c ts on ephemeral rivers affect primarily geomorphological processes and channel morphologies, because geomorphic flows are the only relevant flow and sediments play a fundamental the role in processes and forms. The lines of restorative action should focus on improving knowledge and awareness on these streams, on achieving adequate river space, on the elimination of barriers and obstacles, with special attention to fords, and on the prohibition of dredging and gr a vel mining and the necessary sediment replacement to compensate these extractions. Palabras clave: hydromorphological assessment, hydrological irregularity, extreme events, sediment load, actions and impacts 1. Estado de la cuestión
A set of dimensionless morphological ratios are commonly used as a basis for studies of stream stability and natural channel restoration. Often, such ratios have been obtained from morphometric measurements made in the field and developed into a template for a given geomorphic reference site. The main objective of the present study is to analyze the spatial patterns of dimensionless ratios along an ephemeral channel. A Mediterranean ephemeral channel in semi-arid basin located in the Southeastern Iberian Peninsula (Alto Mula Basin) was examined.
A set of dimensionless morphological ratios are commonly used as a basis for studies of stream stability and natural channel restoration. Often, such ratios have been obtained from morphometric measurements made in the field and developed into a template for a given geomorphic reference site. In this study, high resolution aerial photographs from 2018 and ground-based surveys were used to map the spatial variability of dimensionless ratios (DR) in an ephemeral channel. In particular, three reaches of 0.5-1 km long were chosen along upper Mula stream, in Southeast Spain. Different ratios (width/depth, entrenchment, bank height ratio, root depth/bank height, among other) were estimated from measured channel dimensions, such as bankfull width, maximum and mean bankfull depth, flood prone width, bankfull depth in pools, riffles and runs. Field indicators (the back of point bars, significant breaks in slope, changes in vegetation, the highest scour line or the top of the bank) and a DEM developed from UAV and GNSS data were used to obtain the bankfull stage along each reach. All the data then was transferred to HEC-RAS to obtain the channel profile and cross-sections (separated from each other every 1-5 m). We used a GIS to integrate the high resolution DEM, field measurements and hydraulic geometry data, to calculate the dimensionless ratios and to map the channel reaches. Finally, a statistical analysis of spatial data was performed with R in order to model the morphological relationships for the DR as a function of bed slope and planform parameters.
Lithological and aridity conditions in the central area of the Ebro Basin favour the abundance of intermittent and ephemeral rivers. These rivers suffer important environmental problems but lack specific scientific studies, and in most cases they are not even considered water bodies in their management. A methodological proposal is launched combining photointerpretation and evolutionary cartography with different field monitoring techniques (tomography, river bed coring, granulometry, bed load transport). The recognition of aerial photographies and the verification of hydrological records have allowed the identification of existing ephemeral rivers with at least a 20 m wide alluvial channel. Geomorphological classification has been made based on planform basic characters. Subsequently, reference sites of each of the considered river types have been selected. The detailed geomorphological characterization of the selected sites/reaches includes the analysis of their evolution, highlighting among all of them, for its very active channel dynamics, the Tudela stream (Navarre), in which several meander cutoffs have been recorded in the last decades. This research constitutes a starting point of a comparison and joint work with ephemeral streams of other basins, which follows the enhancement of these rivers and the requirement of protection and restoration actions.
Many models have been developed to predict the sediment transport in watercourses. This paper attempts to test the effectiveness of log-linear models (LLM) to estimate the suspended (S-LMM), dissolved (D-LLM), and total suspended (T-LLM) load into a Mediterranean semiarid karst stream (the Argos River basin, in southeast Spain). An assessment of the supposed validity of each model and a leave-one-out cross-validation were carried out to determine their degree of statistical robustness. The T-LLM model showed higher prediction accuracy (R2 = 0.98, RMSE = 0.15, and PE = ±5.4–6.6%) than the D-LLM model (R2 = 0.97, RMSE = 0.16, and PE = ±5.5–6.8%) or the D-LLM model (R2 = 0.77, RMSE = 0.71, and PE = ±101–493%). In addition, different model variants, according to two flow patterns (FP1 = base flow and FP2 = rising water level), were developed. The FP2-SLLM model provided a very good fit (R2 = 0.94, RMSE = 0.34, and PE = ±25.3–61.5%), substantially improving the results of the S-LLM model.