Numerical Modeling of Coupled Water Flow and Heat Transport in Soil and Snow

Abstract

A one-dimensional vertical numerical model for coupled water flow and heat transport in soil and snow was modified to include all three phases of water: vapor, liquid, and ice. The top boundary condition in the model is driven by incoming precipitation and the surface energy balance. The model was applied to three different terrestrial systems: a warm desert bare lysimeter soil in Boulder City, NV; a cool mixed-grass rangeland soil near Laramie, WY; and a snow-dominated mountainous forest soil about 50 km west of Laramie, WY. Comparison of measured and calculated soil water contents with depth yielded modeling efficiency (ME) values (maximum range: −¥ < ME £ 1) of 0.32 £ ME £ 0.75 for the bare soil, 0.05 £ ME £ 0.30 for the rangeland soil, and 0.06 £ ME £ 0.37 for the forest soil. Results for soil temperature with depth were 0.87 £ ME £ 0.91 for the bare soil, 0.92 £ ME £ 0.94 for the rangeland soil, and 0.85 £ ME £ 0.88 for the forest soil. The model described the mass change in the bare soil lysimeter due to outgoing evaporation with moderate accuracy (ME = 0.41, based on 4 yr of data and using weekly evaporation rates). Snow height for the rangeland soil and the forest soil was captured reasonably well (ME = 0.57 for both sites based on 5 yr of data for each site). The model is physics based, with few empirical parameters, making it applicable to a wide range of terrestrial ecosystems. Abbreviations: LAI, leaf area index; ME, modeling efficiency.