Overview of Development and Use of Crop ET Estimates with Hydrological Models for Interstate Compacts

Crop Evapotranspiration (ET) is an important element in hydrological modeling in river basins with significant irrigation. The State of Kansas currently participates in ongoing modeling in two river basins for the purpose of monitoring compliance with interstate compacts: the Arkansas River in Colorado and Kansas and the Republican River in Kansas, Nebraska and Colorado. The decree in Kansas v. Colorado (No. 105, Original, U.S. Supreme Court) provides for annual updates of the Arkansas River Hydrological-Institutional Model (H-I Model) to compute the effects of well pumping and offsets on Usable Stateline flows. The model was originally developed for the litigation, and has been modified for use in monitoring compact compliance. The Republican River Groundwater Model was developed through negotiation as part of a settlement in the case of Kansas v. Nebraska & Colorado (No. 126 Original, U. S. Supreme Court). The Republican River model is used each year to determine the Calculated Beneficial Consumptive Use attributed to groundwater pumping in the Republican River Basin in the States of Nebraska, Colorado and Kansas. The settlement stipulation requires that each state calculate the crop irrigation requirement by county and compare those estimates to the pumping. This paper discusses the development and function of crop ET in each setting.