Seasonal and interannual variability in evapotranspiration of native tallgrass prairie and cultivated wheat ecosystems

Year-round measurements of evapotranspiration (ET) were made, using the eddy covariance technique, in tallgrass prairie and winter wheat ecosystems in north-central Oklahoma during 1996-2000. Seasonal and interannual variability in water vapor flux was examined in terms of relevant controlling variables. During the growing season, the daily ET was 3.5-5.0 mm day−1 for the prairie and 2.5-7.0 mm day−1 for wheat. Annual ET ranged 640-810 mm for the prairie, and 710-750 mm for wheat. “Non-growing” season ET was about 25% of the annual ET in the prairie (during November-April) and about 50% of the annual ET in wheat (during July-February). Differences in ET between the two ecosystems and the corresponding interannual variability were related mostly to the effects of soil moisture stress and variations in green foliage area, while the weather parameters had a smaller impact. The ET model of Priestley and Taylor (1972) [Priestley, C.H.B., Taylor, R.J., 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weather Rev. 100, pp. 81-92.] was modified to incorporate the effects of soil moisture and foliage area, and tested against field measurement. The modification improved the prediction of ET significantly as compared to the original model. On average, the overestimation of the actual ET by the original model reduced from 47 to 9% at the prairie site, and from 20 to 4% at the wheat site. Improved performance of the modified Priestley-Taylor model over a wide range of environmental conditions makes it potentially a practical tool for predicting ET in grasslands and agricultural systems.