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Tight coupling between soil moisture and the surface radiation budget in semiarid environments: Implications for land‐atmosphere interactions

Observations are used to examine how soil moisture influences the surface radiation budget, ground heat flux, and available energy in semiarid environments. Defining this relationship is critical to understand interactions between the land surface and the atmosphere, in particular assessing if a feedback exists between soil moisture and rainfall anomalies. We use two summers of data collected from semiarid grassland and shrubland ecosystems in central New Mexico. The response of surface radiation budget components and other variables to soil moisture variations are quantified via linear regression. Then, the variations are scaled over the observed range of soil moisture (15% volumetric water content). The soil temperature is lower by >10°C when the surface soil is wet, compared to when the soil is dry. This temperature decrease results in a measured decrease of 85–100 W m−2 in longwave radiation emitted at the surface. The increase in net longwave radiation is equal in magnitude because downward longwave radiation does not vary with soil moisture. The observed changes in net shortwave radiation are relatively minor (<10 W m−2), as the surface albedo decreases by only 1.5% when soil is wet. Net radiation increases by an amount roughly equal to the decrease in emitted longwave radiation (∼85–100 W m−2). Changes in ground heat flux are not detectable, given the noise in the data. Therefore the available energy, Qa, is higher by 80 W m−2 when the soil is wet. This change is 22% of average Qa at the shrubland site and 19% at the grassland site. The observed soil moisture‐induced Qa variations are large compared to other sources of Qa variability, so they should influence boundary layer moist static energy. However, the intervals during which soil moisture is high and therefore Rn and Qa are enhanced are short, on the order of several days. Therefore feedbacks to rainfall may be limited. Compared to other environments, the influence of soil moisture on Rn and Qa is likely greater in semiarid environments because soil moisture‐induced fluctuations in evaporative fraction and surface temperature are relatively large.