Capturing the effect of human influence in downscaling irrigating water availability estimates: an evaluation in the western US.

Irrigation is critical for agricultural production in the United States. This is especially true for the western United States where reservoirs and irrigation infrastructure have allowed the transport of water across watersheds, creating highly productive agriculture in arid ecosystems. In exploring future climate change adaptation and mitigation opportunities for agriculture, it is critical to consider irrigation water availability as influenced by this human element as well as the fact that there are other non agricultural competing needs for water. Given data constraints, this aspect is hard to quantify, often overlooked in modeling efforts, and there isn't a clear answer to the question " In the United States, how much irrigation water is available currently and in the future at a sufficiently precise spatial scale to be relevant for climate adaptation and mitigation studies ?"

There is a body of existing work that quantifies this information at large spatial scales. For example, the International Food Policy Research Institute's IMPACT model application divides the United States into 16 large food production units (FPUs) to address this question. We explore simple ways to downscale these FPU scale results to a smaller crop reporting district scale while accounting for the human influence and infrastructure effects. We utilize a combination of information from USDA Census of Agriculture and the USGS Water Use Surveys to determine scaling factors to redistribute irrigation water availability from FPU to USDA Crop Reporting District scales. We then utilize regional expertise in the western US to evaluate the ability of this rescaling approach to account for the human and infrastructure influence in redistributing irrigation water availability. Our results indicate that this simple downscaling approach generally captures the human influence. However, there is spatial variability in the ability to capture this influence, and the approach is better suited in areas where water is either fully or over allocated. This effort helps set the stage for defining how this critical issue can be addressed in the future.