These estimated precipitation data were compiled using the WestMap web site (http://www.cefa.dri.edu/Westmap/). We selected pixels on the map shown on their web site that were in the core of our study areas: one near Palm Springs, California and the other at Sugarloaf Mountain in the Tonto National Forest of Arizona. WestMap uses PRISM data to make point measurements of climate data and a digital elevation model of terrain to create estimates of monthly climate elements. Estimates are derived for a 4km grid, for ease in mapping and GIS applications. PRISM is an integrated set of rules, decision making, and calculations designed to imitate the process an expert climatologist would go through when mapping climate data. We were interested in precipitation data for two hydroperiods: winter precipitation (October-March) and summer precipitation (June-September). These two periods are important for desert tortoise ecology since they trigger germination of food plants in the spring and in the summer.