Reclamation is charged with measuring consumptive water use (water that is physically removed, either through export or evaporation) within the Upper Colorado River Basin. Irrigated agriculture accounts for the majority of consumptive water use within the Basin, and Reclamation measures this use by multiplying estimates of crop area (m2) derived from remote sensing by daily crop-specific water use estimates (m d-1), then summing these daily results to generate seasonal and annual values. Locally measured weather data (solar radiation, air temperature, wind speed, humidity, and precipitation) define the daily water requirement of a clipped grass reference crop (ETo), while locally calibrated crop coefficients relate the daily water use of specific crops to that of the reference crop.
Existing datasets are leveraged during the irrigated crop mapping procedure. Digital geographic databases of potentially irrigated land from the Upper Colorado Basin states define the areas where Reclamation maps irrigation status, and crop types for irrigated areas are provided by the National Agricultural Statistics Service (NASS) Cropland Data Layer (CDL). Reclamation maps irrigation status on a pixel-by-pixel basis using a multidate Normalized Difference Vegetation Index (NDVI) thresholding procedure. NDVI images are generated from atmospherically corrected Landsat imagery provided by the U.S. Geological Survey. For areas where cloud cover limits the number of successful image acquisitions, Landsat 7 data gaps can produce areas with an inadequate number of image “looks” to reliably ascertain irrigation status. For these areas, gaps in the Landsat 7 image data are filled with mean values of valid NDVI data occurring within polygons defining individual agricultural fields.
A color-coded irrigated crop map for the Gunnison River Basin, Colorado, displayed over a natural color mosaic of Thematic Mapper (TM) images acquired during the summer of 2011. Grass hay (light green) and alfalfa (pink) dominate the higher elevation portions of the basin, while crop diversity increases in the Uncompahgre Valley on the west side of the image.