Soil Moisture Dynamics and Thresholds for Surface Energy Balance Regime Transitions: An Observational Analysis at a U.S. Grassland Site

Abstract

Understanding how soil moisture declines following rainfall—when the soil progressively dries due to evaporation and plant uptake—is critical for assessing plant water stress, surface energy partitioning, and land–atmosphere interactions. These periods of moisture loss, commonly referred to as soil moisture drydowns, provide a valuable window into the transition from wet to dry surface conditions. This study focuses on the critical soil moisture threshold (θ*), which marks the transition from energy- water-limited surface evaporation regimes. This transition reflects a key shift in surface energy balance and controls the extent to which evaporation is constrained by moisture availability. While previous research has typically treated θ* as a static value based on soil texture, emerging evidence suggests that it may vary depending on environmental conditions, particularly seasonal climate. This study investigates whether θ* is a fixed property or a dynamic threshold influenced by seasonal variation and available energy. Using in situ data from the Soil Temperature and Moisture Profile (STAMP) system and Infrared Thermometer (IRT) measurements at a semi-arid grassland site in Oklahoma, USA, I identify and analyze soil moisture drydown events. I estimate θ* by applying piecewise linear regression to the relationship between soil moisture and diurnal surface temperature range, isolating the breakpoint that indicates the transition from energy-limited to water-limited evaporation. Results reveal that θ* exhibits systematic temporal variations, particularly across seasons and temperature regimes, suggesting that surface temperature dynamics during drydowns are most likely a response to changes in soil moisture content. These findings challenge the assumption that θ* is solely texture-dependent and highlight the need to account for dynamic environmental controls in modeling surface energy exchange. This research provides new insights into soil moisture-temperature coupling and offers implications for land surface model development, drought forecasting, and vegetation response assessments under a changing climate.