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A User-Friendly Interactive Tool for Estimating Reference ET Using ASCE Standardized Penman-Monteith Equation
Abstract
The Accurate daily reference evapotranspiration (ET) values are needed to estimate crop water demand for irrigation management and hydrologic modeling purposes. The Bushland Reference ET Calculator (BET) was developed to implement a user-friendly interface for calculating hourly and daily grass and alfalfa reference ET using the Java Programming Language. The calculator uses the American Society of Civil Engineers (ASCE) Standardized Reference ET equation for calculating both grass and alfalfa reference ET at hourly and daily time steps from a single set or time series of weather data. Daily reference ET can be calculated either by calculating the hourly reference ET values and summing them up or by calculating a daily value using daily statistics of the climatic data (means, maxima, and minima). Graphing capabilities include line graph and scatter plot for quality assurance and quality control purposes. Descriptive statistics can be calculated for selected or all of the variables. Although the "Bushland Reference ET Calculator" was designed and developed for use mainly by producers and crop consultants to manage irrigation scheduling, it can also be used in educational training, research, and other practical applications. This article demonstrates the use of the Bushland Reference ET Calculator that is available from the USDA-ARS Conservation and Production Research Laboratory web site to interested users at no cost. © 2016 American Society of Agricultural and Biological Engineers.
... to farmers to assist with irrigation scheduling [26][27][28]. The mathematical processes required to estimate crop water demand are beyond the capabilities of smallholder farmers. ...
... However, effective communication and adoption of these complex methods by farmers, particularly resource poor farmers, are rarely reported. Some have attempted to make the functionality of predictive techniques available through the use of spreadsheet and app-based calculators [26][27][28]. However, the novelty of this work is reflected in what appears to be an absence of literature outlining approaches to make the processes and outputs of crop water demand estimation methods, such as the Penman-Monteith method, available in a form that does not require complicated calculations and is therefore suited to smallholder farmers. ...
Hand-held hoses and watering cans are widely used by smallholder farmers to irrigate vegetables in Cambodia and Laos. Overwatering is common. Technology change (e.g., low-pressure drip irrigation) has been used to improve irrigation efficiency but can be unaffordable for many smallholder farmers. The purpose of this study was to identify an appropriate method of predicting crop water demand, develop and field-test improved irrigation schedules for smallholder leafy vegetable farming based on that method, and then develop extension tools to communicate the schedules to smallholder farmers. Improved irrigation schedules for leafy vegetables were developed based on a crop water use prediction technique that is well established (the Penman–Monteith method) but beyond the capacity of smallholder farmers to implement without access to simple aids. Compared to conventional practice, the method approximately halved water and labour use and improved irrigation water productivity 2–3 fold in field research and demonstration trials. Simplified extension tools to assist smallholder farmers with practice change were developed. This work showed that significant efficiencies could be gained through improved irrigation scheduling without changing application technology.
... We have performed additional model evaluation by comparing simulated soil water with measured volumetric soil water from the top 1.4 m soil profile reported in Bordovsky et al. (2011) from similar cotton-grain sorghum rotation experiments and similar irrigation treatments in adjacent plots under the same center pivot irrigation system. We have also compared simulated reference ET during the cotton growing season with the standardized reference ET estimated by Bushland ref-ET software (Gowda et al., 2016). The ref-ET software uses the American Society of Civil Engineers (ASCE) Standardized Reference ET equation proposed by Walter et al. (2000), and provides standard ET results that can be compared with other ET methods, as was done in previous studies (Adhikari et al., 2016;DeJonge and Thorp, 2017). ...
... The simulated and measured volumetric soil water in the top 1.4 soil profile matched closely with mean absolute error (MAE) less than 0.01 m 3 m − 3 , and RMSE ranging from 4.4 % to 11.7 % (Fig. S2). The simulated reference crop ET during the seven cotton growing years, calculated from the planting to harvest matched closely with the reference ET obtained from the standardized ET method (Gowda et al., 2016), with MAE of 0.27-0.5 mm d − 1 and RMSE of 6.0-8.1 % (Fig. S3). Overall, the comparison of measured and simulated phenology, seed cotton yield, and soil water under different irrigation levels indicated a fair model performance (as per RMSE < 30 % criteria suggested by Bannayan and Hoogenboom (2009)). ...
The Texas High Plains (THP) is a major cotton-producing region in the United States. Sustaining cotton production under declining groundwater availability in the underlying Ogallala Aquifer and changing climate remains a key challenge for stakeholders in this region. The objectives of this study were to assess climate change impacts on cotton yield and irrigation water use, and evaluate six ideotypes for adaptation. In this study, we used the DSSAT-CSM-CROPGRO-Cotton model for simulating cotton production under 18 projected future climate scenarios and with six potential adaptation ideotypes at Bushland, Halfway and Lamesa in the northern, central, and southern parts of the THP region, respectively. Seed cotton yield and irrigation water use between baseline (1976–2005) and future periods (mid-century:2036–2065 and late-century: 2066–2095) were compared. The irrigated seed cotton yield is expected to increase by 12–21 % at cooler northern sites, and decrease by 2% at the warmer southern site, in the mid-century compared to the baseline. For the same period, seasonal irrigation water use is expected to increase by 6–11 % and dryland seed cotton yield is expected to change by +6 % to −11 % across the locations. The increases in irrigated seed cotton yield were attributed to increased vegetative growth under elevated CO2, while the decline in dryland seed cotton yield was due to poor boll retention at high growing season temperatures. Six potential climate change adaptive ideotypes with greater drought and heat tolerances, higher yield potential, and longer maturity were designed and compared to the reference cultivar. For irrigated conditions, increasing area of full leaf and enhancing partitioning of assimilates to reproductive growth (high yield potential) were preferred, because these characteristics increased seed cotton yield substantially (by 3–9 %) with a marginal change in irrigation water use (by −1 to 3 %). For dryland production, a long maturity ideotype with longer boll filling duration was the most effective ideotype with a substantial increase in seed cotton yield by 11–45 %. The results from this study will be useful to THP cotton producers and water managers in making appropriate decisions for adapting cotton production to projected changes in future climate and groundwater availability.
Crop Water Need (ET crop) is referred to as the amount of water needed by a crop to grow. ET crop has high significance to identify the adequate amount of irrigation need. In this paper, a decision support system is proposed to identify Crop Water Need. The proposed decision support system is implemented through sensors and android based smartphone. Internet of Things (IoT) based temperature sensor (DHT11) is used to acquire the real time environmental factors that affect the ET crop. The sensor will communicate with android based smartphone application using Bluetooth Technology (BT-HC05). This proposed system has been compared with available evapotranspiration and existing manual method of evapotranspiration and it was found that proposed system is more correlated than existing manual method of evapotranspiration. The correlation coefficient obtained between proposed system and available evapotranspiration is 0.9783. The proposed decision support system is beneficial for farmers, agriculture researchers and professionals.
Water productivity of crops must be increased to meet global demand for farm products while conserving limited water resources. Throughout the western United States, highly productive agricultural regions face more frequent and severe droughts and must allocate limited water resources among competing uses. Irrigated agriculture accounts for the majority of consumptive water use and has a proportionately important role in managing water shortage. Water productivity accounting helps inform objective decision making regarding these allocations at the regional, local, and farm scales. Publicly available data from several government agencies were re-analyzed and combined to quantify interannual changes in consumptive water use. Water productivity was determined using county-level crop survey data, satellite-based maps of evapotranspiration, and weather records for 21 counties located in the Eastern Snake Plain in the US state of Idaho for nine years during the period 2009–2019. Changes in water productivity over this period suggest trends that correspond to crop-specific irrigation practices, interannual variability in water supply, and regional attempts to curtail water consumption and improve water use efficiency. Results indicate that, at regional scales, water productivity for alfalfa can increase despite or as a result of water limited conditions, increasing from 0.5 to 1.75 kg ha⁻¹ m⁻³ in some individual counties over the 10 year period, which may indicate increasingly efficient water use. Water productivity of barley and wheat crops also varied year to year, but did not demonstrate clear independence from total water consumption. Instead, irrigated areas and crops with adequate water supply follow a general trend of increased yield with increased water consumption, and the resulting water productivity approximately follows a linear function of actual evapotranspiration.
Evapotranspiration (ET) processes at the leaf to landscape scales in multiple land uses have important controls and feedbacks for local, regional, and global climate and water resource systems. Innovative methods, tools, and technologies for improved understanding and quantification of ET and crop water use are critical for adapting more effective management strategies to cope with increasing demand for freshwater resources under global climate change. This article introduces an ASABE Special Collection of 12 articles on ET monitoring and modeling research for multiple land uses and scales. The collection focuses on recent advances in four critical topical areas: (1) reference ET (REF-ET) method development and applications, including crop management and irrigation scheduling, limitations due to sensor inaccuracies and variability, and sensitivity to climatic drivers (three articles); (2) ET process and pathway characterization, including canopy interception, transpiration, and soil evaporation measured using various state-of-the-art techniques on crop lands and plantation forests, and effects of soil moisture on grassland water balance (three articles); (3) ET simulation within hydrological models (SWAT, MIKE SHE, RZWQM, RZ-SHAW, RegCM-BATS, DRAINMOD-FOREST, and Thornthwaite water balance) as well as related processes, such as crop growth and ET/PET ratios, for grass, crop, and forest lands (four articles); and (4) geospatial technology applications, such as using remote sensing to estimate ET and its components (soil evaporation and transpiration) for various land uses (two articles). Study sites represent a range of spatial scales and ecohydrological settings, including grasslands in Inner Mongolia dry lands in northern China, semiarid high plains in Texas, corn production regions from Iowa to Colorado, forest plantations on the humid Atlantic Coastal Plain, developed coastal areas on the island of Taiwan, and the continental U.S. Results from these studies will help guide current development and assessment of REF-ET, ET, and monitoring and modeling of their components in multiple scales and ecosystems. The studies also establish a platform for addressing potential inaccuracies in data from weather sensors and algorithms used in remote sensing products for estimating ET and its parameters, including uncertainties in REF-ET estimates, for tall forest vegetation in particular. Furthermore, the studies offer insights into the interactions between climatic variability and change and vegetation through the ET process. © 2016 American Society of Agricultural and Biological Engineers.
A sensitivity analysis was conducted to determine the relative effects of measurement errors in climate data input parameters on the accuracy of calculated reference crop evapotranspiration (ET) using the ASCE-EWRI Standardized Reference ET Equation. Data for the period of 1995 to 2008 from an automated weather station located at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas were used for the analysis. Results indicated that grass (ETos) and alfalfa (ETrs) reference crop ET were most sensitive to measurement errors in wind speed and air temperature followed by incoming shortwave (solar) radiation, and that data sensitivity was greater during the mid-summer growing season in this semi-arid region. Given the highly advective conditions of the Texas High Plains and the relative sensitivity of ET calculations to errors in wind speed, special care is recommended in siting, sensor placement, and sensor maintenance for agriculturally-based ET weather stations.
A sensitivity analysis was conducted to determine the relative effects of measurement errors in climate data input parameters on the accuracy of calculated reference crop evapotranspiration (ET) using the ASCE-EWRI Standardized Reference ET Equation. Data for the period of 1995 to 2008 from an automated weather station located at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas were used for the analysis. Results indicated that grass (ET os) and alfalfa (ET rs) reference crop ET were most sensitive to measurement errors in wind speed and air temperature followed by incoming shortwave (solar) radiation, and that data sensitivity was greater during the midsummer growing season in this semi-arid region. Given the highly advective conditions of the Texas High Plains and the relative sensitivity of ET calculations to errors in wind speed, special care is recommended in siting, sensor placement, and sensor maintenance for agriculturally-based ET weather stations.
Comparison among commonly used reference evapotranspiration (ET) equations in the United States and the recently recommended ASCE standardized reference ET equation was made as part of the ASCE standardization effort. Analyses used hourly and daily weather data from 49 geographically diverse sites in the United States. Calculations were performed for both grass and alfalfa reference crops in a consistent manner, using weather data that passed integrity and quality assessment checks. Comparisons were made between reference ET computed by the various methods and the ASCE Penman-Monteith (PM) equation used for a daily calculation time step. In addition, calculations using hourly time steps and summed daily were compared with daily calculations for the same method as well as against the ASCE-PM method. Results showed that the ASCE standardized equation agreed best with the full form of ASCE-PM. The results provide a basis for objectively assessing the relative performance of reference ET equations in a variety of climates and support adoption of a standardized equation as recommended by the ASCE Task Committee.
Potential evapotranspiration (PET) is an important index of hydrologic budgets at different spatial scales and is a critical variable for understanding regional biological processes. It is often an important variable in estimating actual evapotranspiration (AET) in rainfall-runoff and ecosystem modeling. However, PET is defined in different ways in the literature and quantitative estimation of PET with existing mathematical formulas produces inconsistent results. The objectives of this study are to contrast six commonly used PET methods and quantify the long term annual PET across a physiographic gradient of 36 forested watersheds in the southeastern United States. Three temperature based (Thornthwaite, Hamon, and Hargreaves-Samani) and three radiation based (Turc, Makkink, and Priestley-Taylor) PET methods are compared. Long term water balances (precipitation, streamflow, and AET) for 36 forest dominated watersheds from 0.25 to 8213 km2 in size were estimated using associated hydrometeorological and land use databases. The study found that PET values calculated from the six methods were highly correlated (Pearson Correlation Coefficient 0.85 to 1.00). Multivariate statistical tests, however, showed that PET values from different methods were significantly different from each other. Greater differences were found among the temperature based PET methods than radiation based PET methods. In general, the Priestley-Taylor, Turc, and Hamon methods performed better than the other PET methods. Based on the criteria of availability of input data and correlations with AET values, the Priestley-Taylor, Turc, and Hamon methods are recommended for regional applications in the southeastern United States.
The ASCE Evapotranspiration in Irrigation and Hydrology Committee (ASCE-ET) is recommending, for the intended purpose of establishing uniform evapotranspiration (ET) estimates and transferable crop coefficients, two Standardized Reference Evapotranspiration Surfaces: (1) a short crop (similar to grass) and (2) a tall crop (similar to alfalfa), and one Standardized Reference Evapotranspiration Equation. The standardized equation is derived from the ASCE-Penman Monteith equation, by simplifying several terms within that equation. The standardized equation, with appropriate constants provided in an accompanying table, is used to calculate evapotranspiration for the standardized short reference (ETos) and/or evapotranspiration for the standardized tall reference (ETrs). One constant is in the numerator and one is in the denominator. The constant in the right-hand side of the numerator (Cn) is a function of the time step and aerodynamic resistance (i.e., reference type). The constant in the denominator (Cd) is a function of the time step, bulk surface resistance, and aerodynamic resistance (the latter two terms vary with reference to type, time step, and daytime/nighttime).
Maximizing the net benefits of irrigated plant production through appropriately dsigned agricultural water management programs is of growing importance in Nebraska, and other western and Midwestern states, because many areas are involved in management and policy changes to conserve irrigation water. In Nebraska, farmers are being challenged to practice conservation methods and use water resources more efficiently while meeting plant water requirements and maintaining high yields. Another challenge Nebraska experiences in it's approximately 3.5-million-ha irrigated lands is limited adoption of newer technologies/tools to help farmers better manage irrigation, conserve water and energy, and increase plant water use efficiency. In 2005, the Nebraska Agricultural Water Management Demonstration Network (NAWMDN or Network) was formed from an interdisciplinary team of partners including the Natural Resources Districts (NRD); USDA-NRCS; farmers from south central, northeast, west central and western Nebraska; crop consultants; and University of Nebraska-Lincoln faculty. The main goal of the Network is to enable the transfer of high quality research-based information to Nebraskans through a series of demonstration projects established in farmers' fields and implement newer tools and technologies to address and enhance plant water use efficiency, water conservation and reduce energy consumption for irrigation. The demonstration projects are supported by the scientifically-based field research and evaluation projects conducted at the University of Nebraska-Lincoln South Central Agricultural Laboratory located near Clay Center, Nebraska. The Network was formed with only 15 farmers as collaborators in only one of the 23 NRDs in 2005. As of late 2009, the number of active collaborators has increased to over 300 in 12 NRDs and 35 of 93 counties. The Network is impacting both water and energy conservation due to farmers adopting information and newer technologies for irrigation management. The NAWMDN is helping participants to improve irrigation management and efficiency by monitoring plant growth stages and development, soil moisture, and crop evapotranspiration. As a result, they are reducing irrigation water application amounts and associated energy savings is leading to greater profitability to participating farmers. This article describes the goals and objectives of the Network, technical and educational components, operational functions, and procedures used in the NAWMDN. The quantitative impacts in terms of water and energy conservation are reported. © 2010 American Society of Agricultural and Biological Engineers.
Crop coefficients derived from properly designed, operated and maintained lysimeters provide the most accurate values throughout the growing season and are critical in the computation of hourly and daily, regionally based, crop evapotranspiration (ET) values. Multi-stage crop coefficients can be derived from continuously recording lysimeters, increasing the accuracy of both daily and seasonal irrigation crop demand estimates. These crop coefficients can be used with calculated, network based, reference crop ET to develop and disseminate locally representative crop water use estimates. Subsequently, using these accurate values in estimating crop water demand results in improved validity of regional water demand models, better assessments of proposed water policy measures, and enhanced integrity with crop consultants, water districts, and agricultural producers, ultimately resulting in better (more efficient) irrigation management and water conservation.
Two theoretical approaches to evaporation from saturated surfaces are outlined, the first being on an aerodynamic basis in which evaporation is regarded as due to turbulent transport of vapour by a process of eddy diffusion, and the second being on an energy basis in which evaporation is regarded as one of the ways of degrading incoming radiation. Neither approach is new, but a combination is suggested that eliminates the parameter measured with most difficulty-surface temperature-and provides for the first time an opportunity to make theoretical estimates of evaporation rates from standard meteorological data, estimates that can be retrospective. Experimental work to test these theories shows that the aerodynamic approach is not adequate and an empirical expression, previously obtained in America, is a better description of evaporation from open water. The energy balance is found to be quite successful. Evaporation rates from wet bare soil and from turf with an adequate supply of water are obtained as fractions of that from open water, the fraction for turf showing a seasonal change attributed to the annual cycle of length of daylight. Finally, the experimental results are applied to data published elsewhere and it is shown that a satisfactory account can be given of open water evaporation at four widely spaced sites in America and Europe, the results for bare soil receive a reasonable check in India, and application of the results for turf shows good agreement with estimates of evaporation from catchment areas in the British Isles.
In Nebraska, historically, there have been differences among the water regulatory agencies in terms of the methods used to compute reference evapotranspiration (ETref) to determine actual crop water requirements and hydrologic balances of watersheds. Because simplified and/or empirical temperature or radiation-based methods lack some of the major weather parameters that can significantly affect grass and alfalfa-reference ET (ETo and ETr) the performance of these methods needs to be investigated to help decision makers to determine the potential differences associated with using various ETref equations relative to the standardized ASCE Penman-Monteith (ASCE-PM) equations. The performance of 12 ETo and five ETr equations were analyzed on a daily basis for south central Nebraska from 1983 to 2004. The standardized ASCE-PM ETo and ETr values were used as the basis for comparisons. The maximum ASCE-PM ETo value was estimated as 12.6 mm d(-1), and the highest ETr value was estimated as 19 mm d(-1) on June 21, 1988. On this day, the atmospheric demand for evaporation was extremely high and the vapor pressure deficit (VPD) reached a remarkably high value of 4.05 kPa. The combination-based equations exhibited significant differences in performance. The 1963 Penman method resulted in the lowest RMSD of 0.30 mm d(-1) (r(2)=0.98) and its estimates were within 2% of the ASCE-PM ETo estimates. The 1948 Penman estimates were similar to the 1963 Penman (r(2)=0.98, RMSD=0.39 mm d(-1)). Kimberly forms of alfalfa-reference Penman equations performed well with RMSD of 0.48 mm d(-1) for the 1972 Kimberly-Penman and 0.67 mm d(-1) for the 1982 Kimberly-Penman. The locally-calibrated High Plains Regional Climate Center (HPRCC) Penman method, ranked 6th, performed well and underestimated the ASCE-PM ET by 5% (RMSD=0.56 mm d(-1)). Most of the underestimations occurred at the high ET range (>11 mm) and this was attributed to the upper limits applied by the HPRCC on VPD, (2.3 kPa) and wind speed (5.1 m s(-1)). The lack of ability of the radiation methods in accounting for the wind speed and relative humidity hindered the performance of these methods in the windy and rapidly changing VPD conditions of south central Nebraska. The 1977 FAO24 Blaney-Criddle method was the highest ranked (seventh) noncombination method (RMSD=0.64 mm d(-1), r(2)=0.94). The FAO24 Penman estimates were within 4% of the ASCE-PM ETo. Overall, there were large differences between the ASCE-PM ETo and ETr versus other ETref equations that need to be considered when other forms of the combination or radiation and temperature-based equations are used to compute ETref. We recommend that the ASCE-PM ETo or ETr equations be used for estimating ETref when necessary weather variables are available and have good quality. The results of this study can be used as a reference tool to provide practical information, for Nebraska and similar climates, on the potential differences between the ASCE-PM ETo and ETr and other ETref equations. Results can aid in selection of the alternative method(s) for reasonable ETref estimations when all the necessary weather inputs are not available to solve the ASCE-PM equation.
The sensitivity of the standardized ASCE grass-reference Penman-Monteith evapotranspiration ASCE-PM ET o equation to climate variables in different regions has not yet been studied. Sensitivity analyses for the standardized daily form of the ASCE-PM equation were conducted on wind speed at 2 m height U 2 , maximum and minimum air temperatures T max and T min , vapor pressure deficit VPD, and solar radiation R s in the following regions of the United States: semiarid Scottsbluff, Nebraska, and Bushland, Texas, a Mediterranean-type climate Santa Barbara, California, coastal humid Fort Pierce, Florida, inland humid and semihumid Rockport, Missouri, and Clay Center, Nebraska, and an island Twitchell Island, California. The sensitivity coefficients were derived for each variable on a daily basis. In general, ET o was most sensitive to VPD at all locations, while sensitivity of ET o to the same variable showed significant variation from one location to another and at the same location within the year. After VPD, ET o was most sensitive to U 2 in semiarid regions Scottsbluff, Clay Center, and Bushland during the summer months. The R s was the dominant driving force of ET o at humid locations Fort Pierce and Rockport during the summer months. At Santa Barbara, the sensitivity of ET o to U 2 was minimal during the summer months.. A 1 m · s −1 increase in U 2 resulted in 0.42, 0.18, 0.37, 0.28, 0.31, 0.20, and 0.26 mm increases in ET o at the same locations. A unit increase in T max resulted in the largest increase in ET o at Bushland, and a unit increase in R s caused the largest increases in ET o at Fort Pierce. A 1 MJ m −2 ·d −1 increase in R s resulted in 0.05, 0.08, 0.06, 0.11, 0.05, 0.06, and 0.06 mm increases in ET o at the same locations. A 0.4 kPa increase in VPD resulted in 1.13, 0.54, 1.29, 0.57, 1.04, 1.10, and 1.22 mm increases in ET o at the same locations. The U 2 had the most effect on ET o at Scottsbluff and Bushland, the two locations where dry and strong winds are common during the growing season. The sensitivity coefficient for R s was higher during the summer months and lower during the winter months, and the opposite was observed for VPD except for Twitchell Island. The decrease of the sensitivity coefficients for R s corresponding to an increase in the sensitivity coefficient for VPD is due to a decrease in the energy term in favor of the increase in significance of the aerodynamic term of the standardized ASCE-PM equation in summer versus winter months. Because the ASCE-PM and the Food and Agriculture Organization paper number 56 Penman-Monteith FAO56-PM equations are identical when applied on a daily time step, the results of the sensitivity analyses and sensitivity coefficients of this study should be directly applicable to the FAO56-PM equation.
Earlier studies (Singh and Xu, 1997; Xu and Singh, 2000, 2001) have evaluated and compared various popular empirical evapotranspiration equations that belonged to three categories:(1) mass-transfer based methods, (2) radiation based methods, and(3) temperature-based methods; and the best and worst equations of each category were determined for the study regions. In this study a cross comparison of the best or representative equation forms selected from each category was made. Five representativeempirical potential evapotranspiration equations selected from the three categories, namely: Hargreaves and Blaney-Criddle (temperature-based), Makkink and Priestley-Taylor (radiation-based) and Rohwer (mass-transfer-based) were evaluatedand compared with the Penman-Monteith equation using daily meteorological data from the Changins station in Switzerland.The calculations of the Penman-Monteith equation followed theprocedure recommended by FAO (Allen et al., 1998). Thecomparison was first made using the original constant valuesinvolved in each empirical equation and then made using therecalibrated constant values. The study showed that: (1) theoriginal constant values involved in each empirical equationworked quite well for the study region, except that the valueof = 1.26 in Priestley-Taylor was found to be too high and therecalibration gave a value of = 0.90 for the region.(2) Improvement was achieved for the Blaney-Criddle method by addinga transition period in determining the parameter k. (3) The differences of performance between the best equation forms selected from each category are smaller than the differences between different equations within each category as reportedin earlier studies (Xu and Singh, 2000, 2001). Further examinationof the performance resulted in the following rank of accuracy ascompared with the Penman-Monteith estimates: Priestley-Taylor andMakkink (Radiation-based), Hargreaves and Blaney-Criddle (temperature-based) and Rohwer (Mass-transfer).
The objective of this study is to evaluate the potential utility of the USGS Global Data Assimilation System (GDAS) 1-degree,
daily reference Evapotranspiration (ET0) products by comparing them with observed Oklahoma mesonet daily ET0 over a 2year period (2005–2006). The comparison showed a close match between the two independent ET0 products, with bias within a range of 10% for most of the sites and the overall bias of − 2.80%. The temporal patterns are
strongly correlated, with a correlation coefficient above 0.9 for all groups. In summary, we conclude that (1) the consistent
low bias shows the original GDAS ET0 products have high potentials to be used in land surface modeling; (2) the high temporal correlations demonstrate the capability
of GDAS ET0 to represent the major atmospheric processes that control the daily variation of surface hydrology; (3) The temporal and
spatial correspondences in trend between independent datasets (GDAS and MESONET) were good. The finding in Oklahoma, a different
hydro-climate region from a similar regional study conducted in California by Senay et al. (J Am Water Res Assoc 44(4):969–979,
2008), reconfirms the reliability and potential of using GDAS reference ET for regional energy balance and water resources management
in many parts of the world.
KeywordsEvapotranspiration (ET)–Reference ET–GDAS–Oklahoma MESONET
In this study, the performance of three empirical methods for estimating reference evapotranspiration (ET0): Makkink (Mak) and Priestley–Taylor (PT) (radiation-based) and Hargreaves–Samani (HARG) (temperature-based) were assessed in semi-arid regions. The values of ET0 derived using these three methods were compared to those estimated using the reference FAO Penman–Monteith (FAO-PM) method under semi-arid conditions of the Tensift basin (central of Morocco) and the Yaqui Valley (Northwest Mexico). The results showed that the HARG method is the best one to estimate ET0 over both semi-arid test sites. Conversely, the performance of the other two empirical methods was poor except under humid conditions. However when the parameters α and Cm figurate in the PT and Mak equations are locally calibrated, the performance of these two methods greatly improved. Additionally, this study showed that, when measurements of meteorological parameters needed for estimating ET0 (which are not always available especially in developing countries) are lacking, the climatic data generated with numerical weather prediction models provide an alternative and effective solution to estimate ET0. In this regard, data generated using a weather forecast model (ALADIN) over the Tensift basin showed that the HARG model is the most accurate one for estimating the spatio-temporal variability of ET0.
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