Potential Evapotranspiration Methods Research Articles

Linacre Derived Potential Evapotranspiration Method and Effect on Supplementary Irrigation Water Needs of Tomato/Cabbage/Carrot

The study examined the dynamic nature of water balance parameters over Kano town, a semi-arid environment and impact of Linacre derived potential evapotranspiration method on the supplementary irrigation water needs of selected crops. Monthly Rainfall and Temperature data were collected from the Nigerian Meteorological Agency, Lagos for the period 1953-2012. The study observed that there is a steady decline in annual precipitation over Kano from the first decade (1953-1962) to the fifth decade (1993-2002), after which there was a sign of weak recovery in the last decade (2003-2012). For water loss through potential evapotranspiration, there was a steady rise from the first decade (1953-1962) to the fifth decade (1993-2002), and then followed by a sudden decline in the last decade (2003-2012). The total average of water storage on the other hand, first experienced a rise between the first two decades (1953-1962) and (1963-1972), followed by a steady decline, up until the fifth decade (1993-2002) and finally a rise in the last decade (20032012). The total average of soil water deficit experienced a steady rise between the first and the fifth decades (1953-1962) to (1993-2002), this was followed by a decline in the last decade (20032012). Finally, the total average of water surplus experienced a steady decline between the first and the fifth decades. The observed decline in precipitation, storage, and water surplus, and the rise in water loss from potential evapotranspiration and soil water deficit, suggests that there have been changes in the climatic pattern over Kano and this could be seen in the supplementary irrigation water needs of Tomato/Cabbage/Carrot.

Uncertainty of Hydrological Model Components in Climate Change Studies over Two Nordic Quebec Catchments

Abstract Projected climate change effects on hydrology are investigated for the 2041–60 horizon under the A2 emission scenarios using a multimodel approach over two snowmelt-dominated catchments in Canada. An ensemble of 105 members was obtained by combining seven snow models (SMs), five potential evapotranspiration (PET) methods, and three hydrological model (HM) structures. The study was performed using high-resolution simulations from the Canadian Regional Climate Model (CRCM–15 km) driven by two members of the third-generation Canadian Coupled Global Climate Model (CGCM3). This study aims to compare various combinations of SM–PET–HM in terms of their ability to simulate streamflows under the current climate and to evaluate how they affect the assessment of the climate change–induced hydrological impacts at the catchment scale. The variability of streamflow response caused by the use of different SMs (degree-day versus degree-day/energy balance), PET methods (temperature-based versus radiation-based methods), and HM structures is evaluated, as well as the uncertainty due to the natural climate variability (CRCM intermember variability). The hydroclimatic simulations cover 1961–90 in the present period and 2041–60 in the future period. The ensemble spread of the climate change signal on streamflow is large and varies with catchments. Using the variance decomposition on three hydrologic indicators, the HM structure was found to make the most substantial contribution to uncertainty, followed by the choice of the PET methods or natural climate variability, depending on the hydrologic indicator and the catchment. Snow models played a minor, almost negligible role in the assessment of the climate change impacts on streamflow for the study catchments.

Implications of potential evapotranspiration methods for streamflow estimations under changing climatic conditions

ABSTRACTSeveral potential evapotranspiration (PET) estimation methods are commonly used to quantify water and energy budgets. As each PET method can differ, their effects on projected streamflows under changing climatic conditions are critical to quantify the sensitivities of these methods. We used the Coupled Model Intercomparison Project (CMIP5) climate dataset and Hydrological Simulation Program – FORTRAN (HSPF) model to compare the following five PET methods [Hargreaves (HG), Hamon (HM), Thornthwaite (TW), Priestley–Taylor (PT) and Penman–Monteith (PM)] for the Susquehanna River Basin in the northeastern United States. As PET is used as an input, various configurations of HSPF driven by these five PET estimates were used to calibrate HSPF with observed streamflow data from 41 gaging stations. We also used nine global climate model inputs to derive five PET estimates, which were subsequently used as inputs to compute streamflow projections for 2020–2099. An increase in precipitation from 6.2 to 7.2% and an increase in temperature from 1.8 to 2.7 °C were projected, while changes in PET and actual evapotranspiration (AET) were found to substantially differ among the PET methods. The HM method shows an increase in AET of between 14 and 24%, while the other methods show an increase of between 7 and 12%. It is concluded that streamflow projections are sensitive to the selection of the PET methods in the HSPF model; a decrease of up to 5.5% and increase of up to 3.6% are projected for PET levels estimated by using the HM and HG methods, respectively; and both HM and TW are found to be suitable for simple seasonal water balance analyses conducted at regional scales.

Estimation of the Virtual Water Content of Main Crops on the Korean Peninsula Using Multiple Regional Climate Models and Evapotranspiration Methods

Sustainable agriculture in the era of climate change needs to find solutions for the retention and proper utilization of water. This study proposes an ensemble approach for identifying the virtual water content (VWC) of main crops on the Korean Peninsula in past and future climates. Ensemble results with low uncertainty were obtained using three regional climate models, five potential evapotranspiration methods, and the Environmental Policy Integrated Climate (EPIC) crop model. The productivity results of major crops (rice and maize) under climate change are likely to increase more than in the past based on the ensemble results. The ensemble VWC is calculated using three types of crop yields and fifteen consumptive amounts of water use in the past and the future. While the ensemble VWC of rice and maize was 1.18 m3 kg−1 and 0.58 m3 kg−1, respectively, in the past, the future amounts were estimated at 0.76 m3 kg−1 and 0.48 m3 kg−1, respectively. The yields of both crops showed a decline in future projections, indicating that this change could have a positive impact on future water demand. The positive changes in crop productivity and water consumption due to climate change suggest that adaptation to climate change can be an opportunity for enhancing sustainability as well as for minimizing agricultural damage.

Characterizing present and future drought changes over eastern China

ABSTRACTThis study aimed to characterize present and future drought changes over eastern China using observations and Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations. We used the standardized precipitation evapotranspiration index (SPEI) to characterize droughts at the timescales of 3‐ and 12‐month. We distinguished the spatial patterns of drought regimes (DRs) using the rotated empirical orthogonal function (EOF) on the SPEI. Our results identified three DRs centered in northern China (DR1), northeastern China (DR2), and southern China (DR3). Droughts in these regimes have increased in recent decades. Most CMIP5 models reproduce at least two of the DRs. The rotated EOF results indicate that the three DRs can jointly explain 46–68% of the SPEI variance (compared with approximately 60% in the observations). Among the 33 CMIP5 models analysed in this study, 13 simulated all three DRs well and showed a strong correlation pattern (>0.5). Further analysis indicated that both the external natural and greenhouse gas forcing experiments in CMIP5 reproduced the DRs, implying that natural variability and anthropogenic activity play important roles in the formation of these DRs. With continued climate warming, the three DRs over eastern China will persist. Despite the uncertainties in drought changes that mainly depend on the potential evapotranspiration methods, climate model results suggest that droughts would be aggravated under warming scenarios. The probability of severe droughts increase by the end of the century: 33% in DR1, 25% in DR2, 34% in DR3 in RCP4.5 and almost double in RCP8.5 scenario in Thornthwaite method based SPEI estimates.

Potential evapotranspiration method influence on climate change impacts on river flow: a mid-latitude case study

Projected changes in 21st century climate are likely to impact water resources substantially, although much uncertainty remains as to the nature of such impacts. A relatively under-explored source of uncertainty is the method by which current and scenario evapotranspiration (ET) are estimated. Using the Waikaia River (New Zealand) as a case study, the influence of a potential ET (PET) method is investigated for a scenario of a 2°C increase in global mean temperature (the presumed threshold of ‘dangerous’ climate change). Six PET methods are investigated, with five general circulation models (GCMs) used to provide an indication of GCM uncertainty. The HBV-Light hydrological model is used to simulate river runoff. Uncertainty in scenario PET between methods is generally greater than between GCMs, but the reverse is found for runoff. The cause of the reduction in uncertainty from PET to runoff is unclear: the catchment is not water-limited during the summer half-year, indicating that it is not because of actual ET failing to reach the potential rate. Irrespective of the cause, these results stand in contrast to previous estimations of relatively high sensitivity of runoff projections to PET methods, indicating that further work is required to understand the controls on this source of uncertainty.

Evaluation of groundwater potential recharge models considering estimated bare soil evaporation, in a semi-arid foothill region

Actual evapotranspiration/evaporation is essential in proper determination of potential recharge of semi-arid regions. Eight potential evapotranspiration methods with dual crop coefficient methodology were used to estimate actual evaporation (Ea) values from bare soil. Method comparison with measured Ea values using lysimeter data for the hydrological years 2011/12 and 2012/13 identified better performance for the FAO-56-Penman-Monteith (NRMSE < 9%), Hargreaves-Samani (NRMSE < 13%), Priestley-Taylor (NRMSE < 15%) and Makkink (NRMSE < 14%) methods. Data of 2011/12, best Ea estimates and a water balance approach were used to calibrate six potential recharge models with linear, quadratic, exponential, cubic and power functions. Calibrated models were validated in simultaneous estimation of potential recharge and soil moisture using independent data for 2012/13. While models with linear/quadratic functions produced the weakest potential recharge estimates (ΔQ > 10%), the remaining models were acceptable. Soil moisture was acceptably simulated utilizing six models and four best evaporation methods (NRMSE < 20%). However, combination of the FAO-56-Penman-Monteith model with an exponential function produced the best Ea values, soil moisture and potential recharge.

Incorporation of the Penman–Monteith potential evapotranspiration method into a Palmer Drought Severity Index Tool

Calculating the Palmer Drought Severity Index (PDSI) using the Thornthwaite potential evapotranspiration (PET) method has come under recent scrutiny for overestimating drought conditions when air temperatures exceed the historical baseline. With increasing air temperatures around the world, calculating the PDSI using the Thornthwaite PET method may no longer be the appropriate option. Therefore, various researchers have called for the use of the physically-based Penman–Monteith PET method to estimate PDSI. We present an addition to the PDSI tool developed by Jacobi et al. (2013) that includes an option to use the Penman–Monteith method to calculate PET as an input to the PDSI calculations. Comparisons with a global PDSI dataset also estimated using the Penman–Monteith method indicate good spatial correlation. This addition to the Jacobi et al. (2013) tool allows users to easily calculate a suite of Palmer drought indices using Penman–Monteith potential evapotranspiration for current and future drought projections at any spatial scale.

Evaluation and modification of potential evapotranspiration methods in Beijing, China

Abstract: In this study, seven widely used potential evapotranspiration (ETo) methods were evaluated by comparing with the FAO-56 Penman-Monteith method (PM method) to provide useful information for selecting appropriate ETo equations under data-limited condition in Beijing, China. Statistical methods and parameters, namely linear regression, root mean squared error (RMSE) and mean bias error (MBE), were used to evaluate the seven ETo methods. Results showed that ETo estimated using Kimberly–Penman method have fairly close agreement with the PM method (referring to standard ETo), considering the coefficient of determination (R2) of 0.96, RMSE of 0.42 mm/day, and a coefficient of efficiency (E) of 0.96. Locally calibrated Penman and Doorenbos–Pruitt methods also have better agreement with the PM method, correspondingly with R2 of 0.99 and 0.95, RMSEs of 0.24 mm/day and 0.21 mm/day, and coefficients of efficiency of 1.02 and 0.99, respectively. The ETo is the most sensitive to vapor pressure deficit (VPD) and net radiation in the Beijing area. Hence, the VPD-based and VPD-radiation combined ETo methods were developed and calibrated. Results showed that the two developed methods performed well in ETo estimation. By fully considering the data-limit situation, the calibrated Turc method, VPD-based method and VPD-radiation-combined method may be attractive alternatives to the more complex Penman−Monteith method in Beijing. Keywords: potential evapotranspiration, Penman-Monteith method, determination coefficient, coefficient of efficiency, Beijing DOI: 10.3965/j.ijabe.20130604.002 Citation: Liu H J, Li Y, Zhang R H, Huang G H. Evaluation and modification of potential evapotranspiration methods in Beijing, China. Int J Agric & Biol Eng, 2013; 6(4): 9－18.

Hydrological Variability and Uncertainty of Lower Missouri River Basin Under Changing Climate

AbstractThe lower Missouri River Basin has experienced increasing streamflow and flooding events, with higher risk of extreme hydrologic impacts under changing climate. The newly available North American Regional Climate Change Assessment Program (NARCCAP) climate projections were used as atmospheric forcing for Soil and Water Assessment Tool (SWAT) model which runs with varying potential evapotranspiration (PET) methods to assess the hydrological change and uncertainty of 2040‐2069 over 1968‐1997. The NARCCAP temperature and precipitation predictions were refined using a bias correction method. The results show that, following the seasonal variability of precipitation, various water fluxes would increase in most seasons except the summer. Expected precipitation tends to increase in intensity with little change in frequency, triggering faster surface water concentration to form floods. The greatest streamflow increase would occur from November to February, increasing by around 10% on average. An increase of 3% occurs in the other months except for July and August in which river discharge decreases by around 2%. The climate predictions contribute more uncertainty annually, but PET algorithms gain more influence in winter or when other weather factors such as wind play a relatively more important role on evapotranspiration flux. This study predicts an even wetter environment compared to the historically very wet period, with the possibility of more flooding.

Geostatistical improvements of evapotranspiration spatial information using satellite land surface and weather stations data

The objective of the present study was to use the simple cokriging methodology to characterize the spatial variability of Penman–Monteith reference evapotranspiration and Thornthwaite potential evapotranspiration methods based on Moderate Resolution Imaging Spetroradiometer (MODIS) global evapotranspiration products and high-resolution surfaces of WordClim temperature and precipitation data. The climatic element data referred to 39 National Institute of Meteorology climatic stations located in Minas Gerais state, Brazil and surrounding states. The use of geostatistics and simple cokriging technique enabled the characterization of the spatial variability of the evapotranspiration providing uncertainty information on the spatial prediction pattern. Evapotranspiration and precipitation surfaces were implemented for the climatic classification in Minas Gerais. Multivariate geostatistical determined improvements of evapotranspiration spatial information. The regions in the south of Minas Gerais derived from the moisture index estimated with the MODIS evapotranspiration (2000–2010), presented divergence of humid conditions when compared to the moisture index derived from the simple kriged and cokriged evapotranspiration (1961–1990), indicating climate change in this region. There was stronger pattern of crossed covariance between evapotranspiration and precipitation rather than temperature, indicating that trends in precipitation could be one of the main external drivers of the evapotranspiration in Minas Gerais state, Brazil.

Estimation method of grass net radiation on the determination of potential evapotranspiration

Evapotranspiration depends primarily on the local energy balance, which is the most important variable of the physical environment when it comes to irrigation scheduling, especially throughout the production formation stages that are limited by water factors. The current manuscript will verify the applicability of estimates of grass net radiation to the performance of estimation methods of potential evapotranspiration under the climatic conditions in Piracicaba, Sao Paulo, Brazil. In order to proceed to simple linear regression studies, meteorological data were monitored by an automatic weather station throughout a period of 87 days for the calculation of potential evapotranspiration, taking into account both the estimated and calculated grass net radiation values. The use of grass net radiation estimated by a model based on the local energy balance showed an excellent performance for estimating potential evapotranspiration by means of the Penman–Monteith, adjusted Priestley–Taylor, and simplified Penman approaches, along with a good agreement (R2 > 0.91) related to the potential demand values obtained with grass net radiation measured by net radiometers. Copyright © 2012 Royal Meteorological Society

Demand and Supply of Water for Agriculture: Influence of Topography and Climate in Pre-Alpine, Mesoscale Catchments

With climate change, water may become limited for intensive agriculture even in regions presently considered “water-rich”. Information about the potential water requirement and its temporal and spatial variability can help to develop future water management plans. A case study was carried out for Switzerland with its highly complex pre-alpine topography and steep gradients in climate. The hydrological model WaSiM-ETH was used to simulate net irrigation requirement (NIR) for cropland, grassland and orchards using criteria to define irrigation periods based either on the water stress level (expressed by the ratio of actual (aET) to potential evapotranspiration ((pET) (Method 1) or on thresholds for soil water potential (Method 2). Simulations for selected catchments were carried out with a daily time step for the period 1981-2010 using a 500 × 500 m spatial resolution. Catchment-scale NIR ranged between 0 and 4.3 million m3 and 0 and 7.3 million m3 for the two methods, respectively, with no trend over the observation period in any catchment. During the heat wave in 2003, NIR increased by a factor of 1.5 to 2.3 relative to the mean, and in catchments where discharge is directly dependent on precipitation, NIR in the summer of 2003 reached the limits of river water availability. In contrast, in a region with water supply from glacier melt water, highest NIR in 2003 still remained far below total river discharge. The results show that NIR varies strongly between years and across the landscape, and even in a presently cool-temperate climate, irrigation may put pressure on regional water resources under extreme climatic conditions that may become more frequent by the end of the 21st century.

Estimation of regional renewable water resources under the impact of climate change

The purpose of this paper is to develop an estimation model for regional renewable water resources under climate change conditions. Working with Japan’s high-resolution GCM-type JMA/MRI TL959L60 future rainfall modeling data in SRES-A1B condition, first we revise the modeling data through the Quadrant Transformation Method (QTM), and then use the Thornthwaite method for potential evapotranspiration to establish regional evapotranspiration data and estimate future renewable water resources. This study focuses on Taiwan’s four water resource regions to estimate renewable water resources for each region between 2080 and 2099. The results indicate that average annual renewable water resource volume for Taiwan as a whole will drop by 12.3% from 67.5 to 59.2 km3, as compared with the average from 1949 through 2000.

Evaluation of some empirical methods of estimating potential evapo-transpiration for determination of length of growing season in a tropical wet and dry climate

A comparison of 4 empirical methods of calculating potential evapo-transpiration (PE) using climatic data for the estimation of length of growing season from rainfall – potential evapo-transpiration model was carried out using 15 years of estimates recorded in Shaki, Southwestern Nigeria. The length of the growing season estimated from the 4 empirical methods of PE were related to the moisture requirements of some selected tropical crops. Data were analyzed by one – way ANOVA taking each yearly mean as the average of the 15 years (1991 - 2005) for the station. Results show that the methods varied greatly in their ability to define the magnitude and variability of the length of growing season. The original Penman and the modified Penman of McCulloch models showed no significant difference in their use for the determination of length of growing season. However, the Thornthwaite, the Priestly and Taylor showed differences in the estimates of length of growing season. Priestly and Taylor gave the least estimate of length of growing season. The relative reliability of each method in terms of accuracy of both measured and extrapolated meteorological data utilized was also discussed. Key words: Potential evapo-transpiration, length of growing season, moisture requirement, wet and dry climate.

Basin-wide actual evapotranspiration estimation using NOAA/AVHRR satellite data

The present study elaborates on the estimation of actual evapotranspiration at watershed scale using mathematical models and remote sensing techniques. A water balance model was updated and used to simulate runoff from a watershed, based on input data of precipitation and potential evapotranspiration. The water balance simulation considers the loss and routing functions to estimate actual evapotranspiration. Two methods were used to estimate the areal precipitation, the precipitation gradient method and the Thiessen polygons method adjusted for the mean elevation of the watershed. Areal potential evapotranspiration was calculated using three empirical and semi-empirical methods based on temperature, precipitation, and solar radiation. The potential evapotranspiration methods used were the Thornthwaite, Blaney–Criddle, and the modified Penman–Monteith. The water balance model was calibrated with the observed monthly runoff. The actual basin-wide evapotranspiration was estimated using the water balance model. Monthly composites of the normalized difference vegetation index (NDVI), derived from the National Oceanic and Atmospheric Administration’s (NOAA)/Advanced Very High Resolution Radiometer (AVHRR) data were correlated and linear relationships were developed with the water balance computed monthly actual evapotranspiration rates for four watersheds of Central Thessaly, Greece. The performance of the developed Actual evapotranspiration–NDVI relationships was examined using various statistical tests. The NDVI-derived actual evapotranspiration agrees well, in general, with the actual evapotranspiration calculated from the water balance method for both wet and water-limiting conditions.

EPIC estimates of soil water, nitrogen and carbon under semiarid temperate conditions

The Environmental Policy Integrated Climate (EPIC) model is an important support tool for environmental management. Previous tests of the model have determined that it is suitable for long-term yield estimation, but it is less precise in assessing annual yield variability. To determine the reasons for the discrepancies between estimated and measured yields, we tested the ability of EPIC version 5300 to predict soil water and soil nitrogen dynamics, using data from a long-term spring wheat (Triticum aestivum L.) rotation experiment in the semiarid prairie region of Canada. Potential evapotranspiration (PET) estimates varied among methods tested: Priestley-Taylor and Penman-Monteith methods resulted in PET means that were about twice those obtained with the Hargreaves and Baier-Robertson methods. The higher PET means were associated with an excessive estimation of net radiation. We used the Baier-Robertson method to generate the other estimates reported herein. EPIC generally overestimated total soil water, but it still allowed clear differentiation among rotation phases and times of the year, and provided adequate estimates of water during the critical shot-blade stage. Water estimates by soil layer were also generally overpredicted, especially at depths from 0.15 to 0.60 m, but we were able to differentiate among rotation phases and times of the year. Precision of these latter estimates was generally low, accounting at most for 27% of the variability, and varied by soil layer, rotation phase and time of the year. Nitrate-N estimates tended to be lower than measured values, especially at depths below 0.3 m and during vegetative growth phases. However, the estimates also allowed us to distinguish among the rotation phases and times of the year. Total N and organic C were satisfactorily estimated by EPIC. In general, EPIC provided adequate long-term estimates of the environmental quality indicators tested. Key words: Environmental quality, environmental modelling, sustainability, spring wheat, fallow, potential evapotranspiration methods

Crop yield, soil temperature and sensitivity of EPIC under central-eastern Canadian conditions

The Environmental Policy Integrated Climate (EPIC) model can be an important tool for agricultural and environmental management. Application of EPIC in central-eastern Canada requires testing to evaluate its performance and to indicate eventual changes that ultimately can lead to a more accurate model. We compared crop yields and soil temperatures estimated using EPIC (version 5300) with measured data from two sites — Barrhaven, Ontario and St. Antoine, Quebec — where corn (Zea mays L.) and soybean (Glycine max L.) were grown during 4 yr. The sensitivity of several EPIC outputs to variations in selected input parameters was also determined. EPIC was run with either the Penman-Monteith (PM) or the Baier-Robertson (BR) potential evapotranspiration (PET) method. Mean estimated and measured soybean yields were not different, independent of PET method and location. Mean corn yields were underestimated at Barrhaven with the PM method, whereas they were overestimated by the BR method at St. Antoine. Using the PM method resulted in no difference between estimated and measured yields in 2 out of 5 corn years, while the BR method ensued in no difference in 3 out of 5 years. For soybean, only the BR method resulted in 1 yr of yield different than measured. Performance of the model in relation to experimental error and efficiency of the model varied with crop and location, and indicated that EPIC caused less error and was more efficient in estimating soybean than corn yields. Comparing limited measured soil temperature data with EPIC estimates reveled that EPIC tended to underestimate soil temperature; however no effect on estimated yield was observed. The relative sensitivity of EPIC outputs was least for yield and greatest for leached nitrate-N. The latter also displayed the highest variability in sensitivity. EPIC studies focusing on leached nitrate-N should require the most accurate crop, soil, hydrological and weather data in order to minimize errors. Key words: Environmental modelling, soybean yield, corn yield, sensitivity analysis

Estimating spring wheat yield variability with EPIC

The Environmental Policy Integrated Climate (EPIC) model has been used on the semiarid temperate Canadian Prairies to estimate crop yield, soil erosion loss, and water and nitrate dynamics. While its estimates of long-term average yields are accurate for most purposes, additional model development is desirable to fully reflect year-to-year variability. We tested the precision of EPIC (version 5300) in estimating mean yields and in replicating yearly yield variability as influenced by the potential evapotranspiration (PET) method, using field data from a 27-yr crop rotation experiment at Swift Current, Saskatchewan. Rotations tested ranged from continuous wheat (Triticum aestivum L.) to fallow-wheat-wheat. Mean estimated yields were compared with measured yields (MY) and detrended yields (DY). Estimated yields and MYs were further compared by regression, ratio of variances due to lack-of-fit and to experimental errors (R), and model efficiency (EF). Mean yields estimated using the Penman-Monteith and the Priestly-Taylor PET methods resulted in significant underestimations, associated with high annual PET values, and were not analysed further. The Hargreaves (H) and Baier-Robertson (BR) PET methods resulted in mean yields not different than MY or DY for most cases, especially the BR method. EPIC with the H method accounted for 18 to 66% of the variability in annual yield estimation, whereas the BR method accounted for 29 to 60%. These were slightly, but not significantly, lower than results obtained with regionally derived statistical crop models. Overall EPIC with the BR PET method provided yield estimates accurate and precise enough for long term studies. The relatively high R and low EF values obtained, though, suggest further improvements in EPIC are necessary to better replicate yearly yield variability. Analysis of yield residuals indicated that EPIC may not be simulating accurately enough the water balance and its effects throughout the off-season and in the early part of the growing season. Key words: Semiarid temperate climate, crop model, potential evapotranspiration

Potential Evapotranspiration Methods for the Assessment of River Runoff under Climate Change

Based on an overview of the main potential evapotranspiration methods, the sensitivity of four potential evapotranspiration methods to temperature in three representative river basins of different climatic and hydrological natures was analyzed. With the application of the potential evapotranspiration methods in regional hydrological models, the impact of potential evapotranspiration methods on the assessment of river runoff under climate change was studied.