Soil Moisture Dynamics and Evaporation in Arid Alpine Environments

Mountain flows have been studied for several decades now and it is safe to say that their main features are well understood under steady conditions and over idealized terrain. The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program was designed to better understand atmospheric fluid dynamics across all scales over realistic mountainous terrain as well as under transient and steady conditions. As part of MATERHORN, a large field campaign was conducted in May 2013, which is the most synoptically active month in the western Utah desert (USA) where the study took place. There were three main study areas that covered an area of several hundred square kilometers: 1) a playa site, mostly devoid of vegetation, characterized by a flat surface, shallow water table and a heterogeneous soil moisture spatial distribution even in dry conditions; 2) a low-elevation valley floor characterized by Greasewood vegetation; 3) a slope site covered with higher elevation desert vegetation (sagebrush and Mormon tea). Recent studies have shown that soil moisture plays a critical role in the dynamics of mountain flows. We know for instance that high soil moisture levels following a rain event will completely change the energy exchanges at the surface and within the soil, but a detailed understanding of these changes has not been sufficiently quantified in the past. The objectives of this study are thus: 1) to quantify the spatial heterogeneity of soil moisture on the playa site with a series of intensive soil samples measurements; 2) to identify the key controlling mechanisms (i.e. water table depth, net radiation, turbulence, vapor pressure deficit etc.) on evaporation after a rain event in this water-limited environment; 3) to find how the interactions between soil moisture and atmospheric boundary layer turbulent variables vary with each of the three surface types. To do this, we applied the gravimetric method at the playa site to measure volumetric water content in the 0 to 2 cm and 4 cm to 6 cm in the surface soil layer twice per twenty-four hour Intensive Observing Period at 17 sites on a 180 m by 240 m grid. Meanwhile, a continuous time series water table depth was measured in two wells next to the soil sampling sites. Near surface atmospheric variables (i.e. temperature, pressure, turbulent fluxes etc.) were also measured by a flux tower located close to the soil sampling sites. Preliminary data analysis reveals that high spatial variability in surface soil moisture is found under dry conditions, and less so under wet conditions. Overall this study allows us to better understand the mechanisms underlying soil moisture dynamics in desert plays as well as evaporation following occasional rain events.