Description and Application of H20Trans a Stand Level Hydrologic Model for Western Coniferous Forests

A hydrologic model has been developed to study stand level water flow and tree water stress in western coniferous forests. The model, H20TRANS, has an hourly time resolution and is driven by air and soil temperature, humidity, incoming shortwave radiation and precipitation. Required stand parameters include leaf area index, sapwood basal area and soil water storage capacity. The model incorporates rates of snowmelt, canopy interception, litter and soil evaporation. The model predicts leaf conductance as a function of leaf water potential, absolute humidity deficit, incoming shortwave radiation and air temperature, including a special sub-freezing control. Leaf water potential is calculated from soil water content, root water uptake and soil temperature. Primary model outputs are soil moisture depletion, subsurface outflow, transpiration and tree water stress development as measured by leaf water potential and leaf conductance. A validation exercise was done comparing model prediction to field measurements of seasonal soil moisture depletion and tree water stress development. H20TRANS was then used to calculate hydrologic budgets for two lodgepole pine stands in Wyoming differing in leaf area index, density, basal area, and soil storage capacity. Simulation results show large differences between these stands in both magnitude and seasonal timing of transpiration, subsurface outflow and soil moisture depletion.