Interpolation of METRIC Evapotranspiration Maps for Future Eastern Snake Plain Aquifer Model Versions Using Normalized Difference Vegetative Indices
Report
Evapotranspiration associated with irrigated cropland is an essential component of the water budget in modeling the Eastern Snake Plain Aquifer. Rounding acreage to 2,000,000 irrigated acres and assuming that net evapotransipration (ET) depth is 2.5 feet, the annual volume of ET is approximately 5,000,000 acre feet. This is equivalent to an annual average of 6,900 cubic feet per second, which exceeds the combined discharge of springs in the Thousand Springs area. In Eastern Snake Plain Aquifer Model versions 1.1 and 2.0 (ESPAM1.1 and ESPAM2.0 respectively), crop evapotranspiration has been represented using county-wide crop mix from National Agricultural Statistics Service and other data, crop coefficients from ETIdaho data, and reference evapotranspiration from ETIdaho data. Adjustments for chronic departures from standard conditions were made using ET adjustment factors. These were calculated using ET data for a limited number of years processed using METRIC methodology. In ESPAM1.1 these were global adjustments, both spatially and temporally; that is, a single pair of ET adjustment factors (sprinkler and gravity) served for all irrigated parcels and all stress periods. In ESPAM2.0 these varied spatially but not in time. In ESPAM1.1, ad-hoc manual adjustments were made for acute year-to-year departures from non-standard conditions due to acute water supply constraints. In ESPAM2.0 this adjustment was formalized in the On-Farm algorithm of the MKMOD recharge-calculation software.
