EVAPOTRANSPIRATION PREDICTIONS: A COMPARISON AMONG GLEAMS, OPUS, PRZM-2 AND RZWQM MODELS IN A HUMID AND THERMIC CLIMATE.

Authors: MA, Q - UNIVERSITY OF GEORGIA; HOOK, J - UNIVERSITY OF GEORGIA; Wauchope, Robert - Don

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: This project was undertaken to evaluate several computer simulation models as to how well they simulate field pesticide water pollution data. These models try to predict how a pesticide will behave in the environment by mathematically simulating processes like rainfall and plant growth and evaporation of pesticides. One important process is evapotranspiration - the uptake of water by plant roots, transport of the water upward to the plant foliage, and evaporation of the water from the plant leaves. This process can control the water flow in soil. Four important models, including two that are used worldwide and two that are ARS experimental models, were compared. Generally the models were within a factor of two in predicting how much water would evaporate over a given time. The accuracy of the models' predictions depended on how well they could predict the water content in the soil. This is the first paper ever published in which hfour models have been compared in this manner. The results should be useful to those using the well-established models and provides a comparison of these "benchmark" models with the newer ARS products.

Technical Abstract: Simulation model errors in predicting processes like evapotranspiration (ET), which is rarely measured during model validation studies, can significantly affect predictions of chemical fate and transport. This study compared approaches and predictions for ET by GLEAMS, Opus, PRZM-2, and RZWQM and determined effects of predicted ET on simulations of other hydrology components. The ET was investigated for two years of various fallow-corn growing seasons under sprinkler irrigation. The comparison included annual cumulative daily potential ET (ETp), actual ET, and partitioning of total ET between soil evaporation (Es) and crop transpiration (Et). When measured pan evaporation was used for calculating ETp (the pan evaporation method), Opus, PRZM-2, and RZWQM predicted, respectively, 74%, 65%, and 59% of the 10-year average ET reported for a nearby site. When the energy-balance equations were used for calculating ETP (combination methods), GLEAMS, Opus, PRZM-2, and RZWQM predicted 84%, 105%, 60%, and 72% of the reported ET, respectively. The pan evaporation method predicted similar amounts of ET to the combination methods for bare soil, but predicted less ET when both Es and Et occurred. RZWQM reasonably predicted partitioning of ET to Es, while GLEAMS and Opus over-predicted partitioning. A close correlation between soil water storage in the root zone and ET suggests that accurate soil water content predictions were fundamental to ET predictions.