Sensitivity of Residual Soil Moisture Content in VIC Model Soil Property Parameterizations for Sub-arctic Discontinuous Permafrost Watersheds

Abstract:

Most soil property representations in large scale hydrological and atmospheric models are derived from empirical relationships of soil texture, wherein the average hydraulic, thermal and thermodynamic processes are described for each layer of the soil column. Of all the soil hydraulic properties, hydraulic conductivity is one of the most difficult to estimate, particularly in permafrost environments. This is because, for large-scale models, it is estimated from soil properties that are originally estimated from other empirical relationships of soil texture, such as residual soil moisture content. Residual soil moisture content determines the amount of available moisture for evapotranspiration and drainage in unsaturated flow. In cold regions, it is also important to estimate the depth of the freezing front by estimating the antecedent soil moisture status before the soils freeze for the winter. This will have direct and indirect effects on the freeze-thaw depth and runoff generation the following spring. Therefore, inaccurate data on residual soil moisture will impact on hydrological modeling of the discontinuous permafrost watersheds in Interior Alaska, where soil water content is highly variable even within a given soil texture. The main objective of this study is to test the sensitivity of models to variation in residual soil moisture for runoff, evaporation, evapotranspiration and soil moisture simulations in discontinuous permafrost watersheds of Interior Alaska. We use the Variable Infiltration Capacity model, a meso-scale hydrological model, and HYDRUS 1D, a software package for simulating water, heat and solute movement, to estimate the soil hydraulics properties at the two contrasting sub-basins of the Caribou Poker Creek Research Watershed. . Preliminary modeling results show that small variations in the residual soil moisture content results in significant differences in the timing and amount of runoff, evapotranspiration and soil moisture storage. Our analysis reveals that, although residual soil moisture is sensitive for both sub-basins, the low permafrost sub-basin is more sensitive than the high permafrost dominated sub-basin. This may be due to the dry mineral soil layer and higher transpiration by the deciduous plants in the low-permafrost ecosystem.