Ecohydrological and Topographical Controls on Soil Moisture and Soil Temperature for a Snow-dominated Watershed in Pacific Northwest of North America

Abstract:

In mountainous, snow-dominated watersheds, the relative influence of and interactions among factors controlling the spatio-temporal distributions of soil moisture and soil temperature, including slope, elevation, precipitation magnitude and type, incoming solar radiation, and vegetation, are poorly understood at the watershed scale. We investigated these processes by using the Weather Research and Forecasting model to dynamically downscale the Global Forecast System model to a 4 km resolution for western Montana for years 2000–2006. We used the resulting regional climate data to force a physics-based ecohydrologic model, ECH2O, over the Bitterroot River basin (6,500 km²). The model was run at daily time steps in a 250-m resolution grid and was calibrated against measured streamflow and snow water equivalent, as well as satellite-derived snow covered distribution, gross primary production, evapotranspiration, leaf area index, and land-surface temperature. Soil moisture, soil temperature, runoff, and other ecohydrologic variables were simulated. We focus on analyzing the sensitivity of soil moisture and soil temperature to elevation by studying soil moisture-temperature curves for six elevation intervals spanning the elevation range of ~ 2 km. Results show that as elevation increases, the ranges of variation for depth-averaged soil moisture and soil temperature throughout the year evolve differently. A negative correlation between soil temperature and soil moisture is apparent at all elevations, being stronger in the valley bottom and at low altitudes. The spatial variability of soil moisture and soil temperature increases from the valley bottom toward low and moderate elevations due to more transient and complex patterns of snow cover. A stronger hysteresis between these two variables was also detected as elevation increases with two loops, one corresponding to late summer and fall precipitation and the other to the onset of spring snowmelt. The soil moisture-temperature curves show different characteristics within elevation bands, and are representative of the climate conditions and the snow accumulation and melting regime. The altitudinal analysis of soil moisture and temperature presented here provides insight into which elevation zones may be most vulnerable to climate change.