A STATISTICAL TOPOGRAPHIC MODEL FOR MAPPING CLIMATOLOGICAL PRECIPITATION OVER MOUNTAINOUS TERRAIN

The demand for climatological precipitation fields on a regular grid i s growing dramatically as ecological and hydrological models become in creasingly linked to geographic information systems that spatially rep resent and manipulate model output. This paper presents an analytical model that distributes point measurements of monthly and annual precip itation to regularly spaced grid cells in midlatitude regions. PRISM ( Precipitation-elevation Regressions on Independent Slopes Model) bring s a combination of climatological and statistical concepts to the anal ysis of orographic precipitation. Specifically, PRISM 1) uses a digita l elevation model (DEM) to estimate the ''orographic'' elevations of p recipitation stations; 2) uses the DEM and a windowing technique to gr oup stations onto individual topographic facets; 3) estimates precipit ation at a DEM grid cell through a regression of precipitation versus DEM elevation developed from stations on the cell's topographic facet; and 4) when possible, calculates a prediction interval for the estima te, which is an approximation of the uncertainty involved. PRISM exhib ited the lowest cross-validation bias and absolute error when compared to kriging, detrended kriging, and cokriging in the Willamette River basin, Oregon. PRISM was also applied to northern Oregon and to the en tire western United States; detrended kriging and cokriging could not be used, because there was no overall relationship between elevation a nd precipitation. Cross-validation errors in these applications were c onfined to relatively low levels because PRISM continually adjusts its frame of reference by using localized precipitation-DEM elevation rel ationships.