ABSTRACT
MIrAD-US ........................................................................................ 185 6.3.5 GIAM and MIrAD-US Spatial Structure ......................................... 188 6.3.6 Assessment of CIAFs in GIAM and MIrAD-US ............................. 188
6.3.6.1 Area Comparison to USDA-NASS Irrigation Statistics ............................................................. 188
6.3.6.2 Areas of Agreement and Disagreement ............................. 190 6.3.6.3 Quantifying the Agreement Between MIrAD-US
and GIAM .......................................................................... 192
6.3.7 Relating Average Annual Precipitation with Irrigation in the Conterminous United States ............................................................. 194
6.3.8 Limitations and Caveats ................................................................... 195 6.4 Discussion of the Geospatial Assessment of MIrAD-US and
Comparison with GIAM ............................................................................... 195 6.5 Future Evaluation Efforts and Conclusions .................................................. 196 Acknowledgments .................................................................................................. 197 References .............................................................................................................. 197
Although there are over 22 million hectares (Mha) of irrigated land in the United States [1], consistent, current, and detailed geospatial information on the distribution and location of all irrigated fields is not readily available. The United States Department of Agriculture (USDA) National Agricultural Statistics Service (NASS) publishes the amount of irrigated area by county (administrative unit) and by crop every year (in acres) in its annual crop statistics and every five years in the US Census of Agriculture [1]. The statistics published by the USDA are collected via questionnaires and summarized by county. The current wall-to-wall land cover database of the conterminous United States [2], mapped with 2001-era Landsat satellite imagery at 30-m2 resolution, does not include an irrigated lands class. All agricultural land cover is categorized as either cultivated crops or pasture/hay. Also, within the last few years, complementary research has resulted in geospatially continuous estimates of land surface evapotranspiration based on remote-sensing inputs [3,4]. These efforts model evapotranspiration across rain-fed and irrigated landscapes and have been successful in modeling surface energy fluxes. However, up to now, these and other efforts have not separated land use types, are subnational in scope (Afghanistan is the study area in the case of Senay et al. [4]), and are not yet operational. To our knowledge, few efforts have resulted in a cohesive and contemporary (post-2000) map of irrigation status across the entire United States with a spatial resolution that provides subcounty mapping details.
