Understanding the effect of slope and aspect on land surface processes and water balances using numerical simulation of mountain headwaters

Topographic slope has important impacts on energy budgets as well as the water balance in complex terrain. The effect of slope and aspect are most readily observable in alpine, snow-dominated systems where solar incidence angle and topographic shading play large roles in snowmelt processes and evapotranspiration rates. These systems serve as important headwaters for many major river systems within and outside of the US. Here, we make use of a fully integrated hydrologic model coupled to a land surface model, ParFlow-CLM, with improved functionality to study the sensitivity of land surface temperature, evapotranspiration, snowmelt, and groundwater storage to slope and aspect. We perform this analysis in an idealized single-column numerical simulation. we use ParFlow-CLM to run mathematical experiments studying the effect of slope and aspect on water storage, water age, and watershed signal filtering. We perform this analysis in idealized and real domains using historical atmospheric forcing from the Crested Butte area of Colorado, a headwater system of the Upper Colorado River Basin. Improving model physics allows for truer conceptualization of often highly non-linear hydrologic systems. These improvements to ParFlow-CLM model physics will allow us to untangle the effects of snowmelt delay due to shading and solar incidence angle from the effects of other watershed properties including surface roughness and hydrologic connectivity. It may also allow us to better understand evapotranspiration drivers in energy limited systems where terrain complexity plays a role in the energy budget.