Estimation of Landscape Soil Water Losses from Satellite Observations of Soil Moisture

Abstract

This study presents an observation-driven technique to delineate the dominant boundaries and temporal shifts between different hydrologic regimes over the contiguous United States (CONUS). The energy- and water-limited evapotranspiration regimes as well as percolation to the subsurface are hydrologic processes that dominate the loss of stored water in the soil following precipitation events. Surface soil moisture estimates from the NASA Soil Moisture Active Passive (SMAP) mission, over three consecutive summer seasons, are used to estimate the soil water loss function. Based on analysis of the rates of soil moisture dry-downs, the loss function is the conditional expectation of negative increments in the soil moisture series conditioned on soil moisture itself. An unsupervised classification scheme (with cross validation) is then implemented to categorize regions according to their dominant hydrological regimes based on their estimated loss functions. An east–west divide in hydrologic regimes over CONUS is observed with large parts of the western United States exhibiting a strong water-limited evapotranspiration regime during most of the times. The U.S. Midwest and Great Plains show transitional behavior with both water- and energy-limited regimes present. Year-to-year shifts in hydrologic regimes are also observed along with regional anomalies due to moderate drought conditions or above-average precipitation. The approach is based on remotely sensed surface soil moisture (approximately top 5 cm) at a resolution of tens of kilometers in the presence of soil texture and land cover heterogeneity. The classification therefore only applies to landscape-scale effective conditions and does not directly account for deeper soil water storage.

Mapped Hydroclimatology of Evapotranspiration and Drainage Runoff Using SMAP Brightness Temperature Observations and Precipitation Information

“…In parts of the Midwest and Great Plains, is visibly, albeit slightly, smaller—consequently is slightly larger (approximately 0.4 vs 0.6). The spatial extent and boundary of this regional feature is indeed consistent with hydro‐regime classification in Akbar et al () where a larger percentage of SMAP soil moisture estimates was classified to be in stage I (energy‐limited) evaporation regime or D dominated.…”

“…The temporal dynamics and evolution of soil moisture within a simple hydrologically active control volume, with depth Δz [L], can be described by the water balance formulation: where θ [−] is volumetric soil moisture and P ( t ) is measured precipitation [LT −1 ]. L ( θ ) is the soil water loss function [LT −1 ] that captures moisture divergence from the control volume in the form of ET and D. Δ z [L] is the unknown depth, or characteristic length scale, of the control volume that is required to ensure water balance closure (Akbar et al, ). The L‐band sensing depth is limited to about 50 mm although it varies depending on the soil moisture and temperature profiles.…”

Critical Soil Moisture Derived From Satellite Observations Over Europe

“…Therefore, only analyses using the same soil moisture product can make use of our absolute derived CSM values, while all other studies should rather use it in a relative sense: 21 Vol‐% is drier than 85% of the European grid cell seasonal mean soil moistures. Another study based on satellite‐derived surface soil moisture from National Aeronautics and Space Administration's Soil Moisture Active Passive mission reports a median of CSMs of 18 Vol‐% over the contiguous United States (Akbar et al, 2018). This result, as well as our estimate, is more to the dry side than currently assumed in models, which often become water limited just below or at the field capacity (Teuling, Uijlenhoet, et al, 2009).…”

“…Haghighi et al (2018) determine the CSM in a similar manner but using field observations of SM and EF over semiarid regions outside of the growing season, effectively excluding the effects of plant transpiration from their estimates. Finally, Akbar et al (2018) determine the CSM over the contiguous United States by assessing the characteristics of dry‐downs from satellite surface soil moisture from the National Aeronautics and Space Administration Soil Moisture Active Passive mission during three consecutive summers only.…”

Use of Satellite Soil Moisture to Diagnose Climate Model Representations of European Soil Moisture‐Air Temperature Coupling Strength

“…However, it is acknowledged that a more objective threshold value could be obtained from a closer analysis of the relationship between ET and soil moisture. Recently developed technique for empirically determining soil water loss functions may be suitable for this purpose (Akbar et al, ). Nonetheless, although changing this threshold affects the absolute value of SMTC strength, it does not qualitatively alter our finding that GCMs tend to underestimate European SMTC strength.…”