Estimating monthly distribution of average annual precipitation in mountainout areas of Montana

Many computerized models and Geographic Information Systems (GIS) are being used to estimate average annual precipitation in mountains of the western United States. In most areas, there is a good correlation between average annual precipitation and elevation and most models use elevation to distribute annual precipitation to unmeasured areas. There is an increasing need to distribute these annual values to monthly averages. Some models use elevation to distribute monthly precipitation. Some models use the monthly distribution for long-term valley stations to distribute annual precipitation in higher elevations. Studies in Montana show there is a significantly different distribution of monthly precipitation between valley and mountain areas, particularly in the winter and spring months. Comparisons for some watersheds in Montana are shown and proposed procedure for distributing annual precipitation into realistic monthly increments is presented.