GC44E-03 Vegetation Rehydration-vs-Growth and Energy-Limited to Water-Limited Evaporation Regime Transitions Inferred from Shifts in the Dynamics of Surface Soil Moisture (Invited)

Plants are often responsive to pulses of resource availability, including water. This water uptake can either be re-hydration or growth. Only one of these responses has implications for the land carbon budget.

The times following pulses of water availability (interstorms) are also characterized by transitions between hydrologic regimes. Following precipitation, the soil drains to the field capacity if sufficiently wetted. During this drainage regime, soil moisture loss under gravity-driven downward flow dominates. At and below field capacity the interstorm soil moisture loss transitions to be dominated by evaporation. Evaporation itself can transition from energy-limited to water-limited regimes during interstorms.

The longterm land carbon, water and energy balances are determined by the timing and duration of transitions in the plant response regimes and in the hydrologic regimes. Whether Earth System models capture these regimes and their transitions are unknown because reliable observational benchmarks are not defined across diverse climates and land use conditions. Moreover shifts in the intermittency of precipitation (but not total accumulation) in a changing climate can have impacts on annual carbon budget and water availability because the timing and frequency of regime transitions are affected.

Here we show that the recent availability of information on surface soil moisture fields based on global microwave radiometry can be used to approach the observational benchmark problem. The regime transitions are evident in the shifts in the dynamics of the observed soil moisture state.