Climatic water deficit (CWD) quantifies evaporative demand exceeding available soil moisture, where S = soil moisture, AET = actual evapotranspiration, D =climatic water deficit.
The term climatic water deficit defined by Stephenson (1998) is quantified as the amount of water by which potential evapotranspiration (PET) exceeds actual evapotranspiration (AET). This term effectively integrates the combined effects of solar radiation, evapotranspiration, and air temperature on watershed conditions given available soil moisture derived from precipitation. Climatic water deficit can be thought of as the amount of additional water that would have evaporated or transpired had it been present in the soils given the temperature forcing. This calculation is an estimate of drought stress on soils and plants and recent studies suggest it may serve as an effective control on vegetation cover types in the Bay Area (Cornwell 2010). In a Mediterranean climate, climatic water deficit can also be thought of as a surrogate for water demand based on irrigation needs, and changes in climatic water deficit effectively quantify the supplemental amount of water needed to maintain current vegetation cover, whether natural vegetation or agricultural crops.