Spatial scaling of hydrological and biogeochemical aspects of high-altitude catchments in the Sierra Nevada, California, USA

Using data from alpine and subalpine watersheds in the southern and central Sierra Nevada, California, we develop the concept of representative elementary area (REA) for a hydrological variable (maximum snow depth at the beginning of the melt season), and for outflow concentrations and the annual export of a biologically important ion (nitrate) and the products of weathering (base cations and silica). Up to 1000 snow-depth measurements were taken each year since 1990 at maximum accumulation in the upper Marble Fork watershed (a 1900-ha headwater watershed of the Kaweah River, 36 degrees 36'22N, 118 degrees 40'59W). The random nature of snow distribution is illustrated in semi variograms, which usually have an autocorrelation range of < 30 m, i.e., adjacent pixels with similar terrain and radiation inputs can have dissimilar snow depths. There is a consistent year-to-year relationship between the mean Marble Fork basin snow-depth at maximum accumulation and mean depth!, in the sub-basins; the REA appears to be on the order of 50 ha. Our results suggest that the REA for annual volume-weighted mean (VWM) nitrate concentration, nitrate yield, and possibly the temporal variation in outflow concentrations lies in the range of 35 to 70 ha. A similar-sized REA seems to apply to the annual export and VWM concentrations of base cations and silica.