These include models of vegetation, water supply, water quality, drought severity, fire risk, crop production, soil erosion and others. The demand for precipitation fields on a regular grid and in digital form is growing dramatically as models become increasingly linked to geographic information systems that spatially represent and manipulate model output. There are many methods of interpolating precipitation from monitoring stations to grid points. Some provide estimates of acceptable accuracy in flat terrain, but none have been able to adequately explain the extreme, complex variations in precipitation that occur in mountainous regions. Even then, there is no provision for easily updating the precipitation maps with new data or developing maps for other years or months.
Orographic effects operate at a hierarchy of spatial scales.
For example, a large mountain barrier can enhance precipitation over a broad area windward of the crest through forced uplift of moist air flow. Imbedded within this major effect are minor, smaller-scale precipitation perturbations caused by, say, vertical air motion associated with river valleys carved into the barrier. In addition, the density of most routine precipitation networks is sufficient to resolve only the larger-scale orographic effects at best. The relationship between precipitation and elevation varies from one slope face to another, depending on location and orientation. Each topographic facet is a contiguous area over which the slope orientation is reasonably constant.