Modis-Aided Water-Balance Investigations in the Republican River Basin, USA

Spatially distributed monthly evapotranspiration (ET) rates over the 2000–2009 period were specified for the three states [Colorado (CO), Kansas (KS), Nebraska (NE)] sharing the Republican River basin with the help of a calibration-free ETestimation method using Moderate Resolution (of about one km) Imaging Spectroradiometer (MODIS) data. Mean annual ET rate is 444 mm [i.e., 103% of the mean annual precipitation (P) of 430 mm] in CO, 576 mm in NE (102% of P = 563 mm), 540 mm (101% of P = 536 mm) in KS upstream of Hardy, NE; and 568 mm (95% of P = 596 mm) for the entire KS basin area. It is estimated that irrigation (predominantly center pivot) may lead to an additional annual ET of about 200 mm in CO and NE, and to 140 mm in part of KS that is upstream of Hardy, NE over that of native grass. The MODIS-based ET method yields estimates within 10% of water-balance derived mean annual subcatchment ET rates with correlation coefficient values of 0.990.96, depending on which estimate (low, average or high) of the specific yield value of the unconfined aquifer is applied in the water balance equation. The present ET-estimation method may help decision makers on how to share the Republican River flow among the neighboring states.