Mean annual precipitation map for Idaho. Research technical completion report, 14-08-0001-G1419-32

Sound estimates of the amounts and distributions of mean annual precipitation in Idaho have become increasingly important in recent years because of stringent design requirements and increasingly tighter supplies of water. Agriculture, hydropower, wildlife, recreation and others are putting forth claims to a scarcest resource. The current mean annual precipitation (MAP) map of Idaho was prepared in 1965. Since then, more data have been collected, particularly in the higher elevation zones where the previous map had to rely on models to provide estimates. The Soil Conservation Service's SNOTEL system provides data for these very important high elevation zones where the majority of Idaho's water originates. Lower elevation data are obtained primarily from the National Weather Service Cooperative network. Monthly data were compiled for the base period of 1961 to 1985. Missing data were estimated by using an average system surrounding stations. All of these data were then plotted using a contouring package. For this report, no manual smoothing was done as the primary objective was to prove out the mapping technique. For the next phase of the project, various interpolation techniques will be used as well as regression models in areas of sparse or uncertain data. The coefficient of variation of the annual precipitation was also plotted. This resulted in a very smooth contour suggesting that lines of equal coeffient of variation could be overlayed upon the MAP , giving users an idea of the variation in precipitation as well as the mean value. Submitted to the U.S. Geological Survey