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Climate Response to Irrigation in the American West

Significant changes in population and cultivation of farmland in Western U.S. over the past 200 years has required the use of extensive irrigation to sustain crop growth in the region. This irrigation has altered the land surface processes in the region and subsequently the local climates by increasing soil moisture levels higher than they naturally would be if no irrigation had taken place. Analysis of feedback processes show both positive and negative feedbacks are present and impact climate parameters such as surface temperature, precipitation, and albedo. Literature suggests that in the western plains the dominance of positive or negative feedback is determined by the intensity, spatial scale, and duration of irrigation, as well as synoptic scale patterns. The intermediate model QTCM was used to simulate irrigation of Western U.S. growing season (May-Nov) over the period of 1990-1999. Monthly climatological sensitivity experiments were conducted over a range of irrigation forcings from realistic values to near saturation of the soil. The results show a slight climatic response to realistic forcing and a significant climatic response to saturated forcing. All experiments indicated a positive feedback and agree with previous studies of large scale increases in precipitation, decreases in surface temperature, and increases in albedo over irrigated regions. Additionally simulations suggest that the QTCM model has a threshold of sensitivity to the irrigation forcing of 1.0 mm/day. Any forcing greater than this does not show significantly further climatic impact in the model. Future work for refining the accuracy of irrigation forcing would include using non-uniform distribution of irrigation in a higher resolution model, considerations of altitude, refining of seasonal dependence of crop growth, and a trend analysis. Globally research of regions such as China and India have not been well explored and experience even more irrigation than Western U.S. Irrigation raster maps could be incorporated into a GCM to simulate global irrigation and study climatic impacts of realistic forcings.