Abstract

Soil moisture in mountain catchments is highly spatial heterogeneous due to steep topographic gradients, complex soil and vegetation patterns and seasonally varying energy and precipitation inputs. In an idealized setting, a randomized soil moisture sampling design with high spatial frequency can be used to resolve the spatial heterogeneity of soil moisture at catchment scales. However, this bottom-up approach is constrained by the feasibility of high frequency measurements particularly in mountain environments with limited accessibility. Thus, in these mountain environments, an alternative, top-down approach is often needed. In this study, we propose the top-down approach sampling design of soil moisture and sapflux measurement based on an ecohydrologic model and clustering analysis. The sampling strategy is explicitly designed to capture the effect of inter-annual climate variability on ecohydrolgy response of mountain catchments located in King River Experiment Watersheds, Sierra National Forest. The ecohydrolgic model (RHESSys model) is calibrated with existing collected data sets including snow depth, soil moisture, sapflux, evapotranspiration from a flux tower and streamflow. The model is used to generate spatial-temporal patterns of snow accumulation and melt, soil moisture and transpiration and compute inter-annual mean and coefficient of variation of five hydrologic similarity indices. Similarity indices are chosen to reflect seasonal trajectories of snowmelt, root-zone soil moisture storage and evapotranspiration. Clustering analysis, using Partitioning Around Medoid (PAM), is used to partition the watershed based on these similarity indices. For the Kings River Experimental Watersheds, clustering distinguished six clusters and a representative plot per cluster. These results were used to identify additional strategic sampling points within the watershed. For each of these points, we installed soil moisture sensors (5TE) at the two depths (30m and 90m) and at the five soil pits within a 30m plot. A sapflux sensor at the average-size white fir tree per plot was also installed. Initial results from monitoring in summer 2010 are compared with model predictions and used to refine model calibration and uncertainty analysis. Cross-cluster differences in soil moisture and sap-flow trajectories derived from sampling data will be compared with results from initial model to assess the validity of the suggested sampling design.

Citation

Son, K., Tague, C. (2010): A Top-down soil moisture and sap flux sampling design to capture the effect of inter-annual climate variability on ecohydrology in mountain catchments. Fall meeting, American Geophysical Union, December 2010. Abstract GC51D-0775..