TOPOGRAPHIC AND ATMOSPHERIC INFLUENCES ON PRECIPITATION VARIABILITY OVER A MOUNTAINOUS WATERSHED

Using rotated principal component analysis (PCA), unique, orthogonal spatial patterns of daily and monthly precipitation on a well-instrumented, mountainous watershed in Idaho are examined for their relationship to topography, geographic location, and atmospheric variability. Precipitation pattern and homogeneous precipitation region differences between daily and monthly timescales and between winter and summer seasons were identified using the rotated PCA procedure. In general, monthly data produced regional boundaries more closely aligned with topography, reflecting the integration of many storm events on monthly timescales. Spatial fields, derived from mapping rotated component loadings at 46 precipitation stations on a 234-km(2) watershed, were found to be highly correlated with topography and geographic location. The eight-year time series of the components for specific watershed regions were found to be moderately related to linear combinations of meteorological variables derived from a single radiosonde station approximately 50 km from the waterhed. This would indicate the potential usefulness of data from a single location, such as a general circulation model grid point, to provide clues about spatial pattern changes and regional precipitation fluctuations even on a small watershed, if sufficient information about local climate (i.e., topographic influences) is first established.