Potential Evapotranspiration Estimation Methods Research Articles

Influences of Potential Evapotranspiration Estimation Methods on SWAT's Hydrologic Simulation in a Northwestern Minnesota Watershed

The Soil and Water Assessment Tool (SWAT), a widely used watershed hydrology and water quality model, provides three different methods (Hargreaves, Priestley-Taylor, and Penman-Monteith) for estimating potential evapotranspiration (PET) and the corresponding actual evapotranspiration (AET). Although these methods have been extensively tested, the effects of using them within SWAT's framework are largely unknown. The objective of this study was to test the three PET methods within SWAT's framework using data collected in the Wild Rice River watershed, located in northwestern Minnesota. The performance of the SWAT models was measured using three statistics: the Nash-Sutcliffe coefficient (Ej2), coefficient of determination (R2), and performance virtue (PVk). The three models were independently calibrated and validated using the observed daily stream flows at two USGS gauging stations. The simulated stream discharges were compared with the corresponding observed values and the estimated evapotranspiration examined in accordance with the wet-environment areal evapotranspiration (ETW) derived from the evaporation data for Williams Lake, located about 100 km southeast of the study watershed. The use of the three PET methods resulted in different values for two calibration parameters, namely the soil evaporation compensation factor and SCS curve number. At the lower station, which is near the watershed outlet, the observed annual mean discharge (8.33 m3/s) during the model validation period was predicted to be 10.25, 10.87, and 9.69 m3/s by SWAT-Penman, SWAT-Priestley, and SWAT-Hargreaves, respectively. The annual mean discharge (10.83 m3/s) was more accurately predicted during the model calibration period, with an absolute error of less than 0.5 m3/s. The prediction errors for the upper station were comparable with those for the lower station. In addition, all three models exhibited good performance when simulating the monthly, seasonal, and annual mean discharges (Ej2 >0.75 and PVk >0.80) and satisfactory performance when predicting the daily stream flows (Ej2 >0.36 and PVk >0.70). In estimating evapotranspiration for the study watershed, SWAT-Hargreaves seemed to be slightly superior to the other two models, while SWAT-Priestley might be more appropriate for an ETW value greater than 8.0 mm/d. Nevertheless, the AET values estimated by the three models shared a concurrent spatial pattern and temporal trend, and were insignificantly different from each other at a 5% significance level (p-values > 0.05). The results indicated that after calibration, using the three ET methods within SWAT produced very similar hydrologic (AET and discharge) predictions for the study watershed.

Sensitivity of some potential evapotranspiration estimation methods to climate change

Climatic change and its impact on environment, and thus the consequent effects on human, animal and plant life, is a hot topic for discussion at national and international forums both at scientific and political levels. However, the basis for such discussions are scientific reports. Unless these are based on sound foundation, the consequent effects will be costly to exchequer. The paper of McKenney and Rosenberg (McKenney, M.S. and Rosenberg, N.J., 1993. Sensitivity of some potential evapotranspiration estimation methods to climate change. Agric. For. Meteorol., 64: 81–110.) is looked into in this context.

Sensitivity of some potential evapotranspiration estimation methods to climate change

A simulation approach was used to generate estimates of the sensitivity of potential evapotranspiration ( ET p) to climate change using eight alternative ET p estimation methods. The methods, which differ in structure and data requirements, were: Thornthwaite, Blaney-Criddle, Hargreaves, Samani-Hargreaves, Jensen-Haise, Priestley-Taylor, Penman, and Penman-Monteith. The simulations were performed using climate data from five sites in the North American Great Plains. The results indicate that the methods differ, in some cases significantly, in their sensitivities to temperature and other climate inputs. The degree of agreement among the methods is affected, to some extent, by location and by time of year. When two GCM-derived scenarios of climatic change were applied, the predicted response of ET p varied in magnitude and in some cases in sign, according to the estimation method used. The differences among methods can be attributed both to differences in their sensitivities to climate, and to differences in the climatic factors they consider. The implications of these findings for studies of climatic change are discussed.