Evapotranspiration Toolbox for the Upper Rio Grande Water Operations Model

The Bureau of Reclamation (Reclamation) and others have determined a need for rapid improvement in measuring and predicting daily riparian and crop water use in the Rio Grande Basin. Reclamation is developing an Evapotranspiration Toolbox (ET Toolbox) for estimating these daily water use requirements at a resolution useful for implementation in the Upper Rio Grande Water Operations Model (URGWOM). The URGWOM is a multi-agency effort to develop a numerical computer surface water model that will cover the Rio Grande from its headwaters in Colorado to Fort Quitman, Texas. The primary purpose of this model will be a daily water operations accounting tool that can be used for basin-wide water management and planning.

The goal of the ET Toolbox project is to develop a methodology for automatically inputting daily riparian and crop water use estimates, open water evaporation estimates, and rainfall estimates to the URGWOM. The initial development work has focused on the Middle Rio Grande area from Cochiti Dam to San Marcial, which is just south of the Bosque del Apache National Wildlife Refuge in New Mexico. ET including riparian vegetation, irrigated crops, and open water evaporation accounts for about 60 percent of the water depletions over this reach of the Rio Grande.

The ET Toolbox is an extension of Reclamation's Agricultural WAter Resources Decision Support (AWARDS) system that provides Internet access to high resolution rainfall and daily crop water use estimates for improving the efficiency of water management and irrigation scheduling. The ET Toolbox uses the National Weather Service's Stage III multi-sensor (radar and gage) hourly precipitation estimates at a continuous spatial coverage of the Hydrologic Rainfall Analysis Project's (HRAP) 4 km x 4 km grid resolution. These Stage III rainfall estimates offset a portion of ET surface water depletion estimates.

The ET Toolbox integrates the Stage III rainfall estimates with weather data from automated stations, Geographic Information System (GIS) mapping, remote sensing vegetation imagery, communication technology, and ET based on the Modified-Penman methodology, to estimate daily water requirements for each HRAP grid cell in the vicinity of the Rio Grande. Eight different GIS vegetation data sets that cover portions of the Middle Rio Grande area have been evaluated. The most recent being aircraft photos taken during a June 1999 ET field study. All of the vegetation data sets are transposed to the HRAP grid cell resolution and compared to determine changes in the vegetation and water depletion over time.

The primary purpose of the ET Toolbox is to estimate daily rainfall and water depletions along the Rio Grande for each HRAP grid cell and the specified river reaches. These daily ET estimates and summary year-to-date cumulative ET estimates are already available to users and water managers via the Internet. The daily cumulative river reach ET estimates will be input to the URGWOM by RiverWare, which was selected as the modeling software. RiverWare contains water accounting and ownership tools and peripheral water budget and flood routing tools that are being configured for the URGWOM.