Playa Soil Moisture and Evaporation Dynamics in an Arid Intermountain Basin

English Abstract:

The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) program was designed to better understand atmospheric fluid dynamic processes across a large range of scales over mountainous terrain. As part of MATERHORN, a large field campaign was conducted in May 2013, which is typically one of the most synoptically active months in the western Utah desert (USA). This study focuses on one of the main field sites, a desert playa, characterized by a shallow water table and significant soil moisture spatial heterogeneity. Recent studies have shown that soil moisture plays a critical role in the dynamics of boundary-layer flows over desert playas. Thus, the objective of this study is to better understand the temporal and spatial evolution of soil moisture and evaporation dynamics under contrasting conditions. During a total of 10 intensive observation periods, soil moisture was monitored with the gravimetric method at 20 sites evenly spread on a 240 x 180 m grid. Near-surface standard meteorological variables, atmospheric fluxes as well as ground heat flux and soil heat transfer properties were also measured. The data analysis reveals that decreasing surface albedo, decreasing Bowen ratio and increasing net radiation with increasing soil moisture all promoted large evaporation rates immediately following rain events. Additionally, it was found that while nocturnal evaporation was negligible during dry periods, it was quite significant (up to 30% of the daily cumulative flux) during nights following rain events. Our results further show that the highest spatial variability in surface soil moisture is found under dry conditions. Finally, we report strong spatial heterogeneities in evaporation rates following a rain event. The cumulative evaporation for the different sampling sites over a five-day period varied from ~0.1 mm to ~6.6 mm. Overall, this study allows us to better understand the mechanisms underlying soil moisture dynamics of desert playas as well as evaporation following occasional rain events.