Evapotranspiration and Irrigation Algorithms in Hydrologic Models: Present Status and Opportunities.

Hydrologic models are used extensively for predicting water availability and water quality responses to alternative irrigation, tillage, crop, and fertilizer management practices and global climate change. Modeling results have been frequently used by regulatory agencies for developing remedial measures for impaired water bodies and for water planning purposes. However, comprehensive calibration and validation of these models for simulating daily ET and timing and rate of irrigation, the two most important parts of the water budget, was done only at the time of model development during 1990s. The crop database and production functions embedded in these models need to be revised for accurately simulating the water budget including ET and irrigation demand and consequent water quality and crop yield. This is largely due to previous unavailability of long term ET and irrigation databases for different cropping systems, which may be further complicated by advancements in crop breeding that resulted in high yield and drought tolerant crop varieties. Consequently, most modelers seem to neglect ET and irrigation simulations from hydrologic models and concentrate on matching model predictions to measured runoff for model calibration by adjusting sensitive model parameters related to ET. This approach can provide a false sense that the model is performing properly as modeled values match the observed runoff well during the calibration period. Unfortunately, the model may be giving the right answer for wrong reasons that become a major problem when these improperly calibrated models are used for evaluating the alternative management practices. Propagated modeling errors, therefore, go unnoticed in the absence of measured data. In this presentation, we discuss current limitations in hydrologic models and ongoing research activities at the USDA-ARS Conservation and Production Research Laboratory in Bushland, Texas, which is known globally for lysimeter research on quantifying ET in agricultural systems managed under both irrigated and dryland management.