- Metric Name:  Actual Evapotranspiration to Precipitation Fraction During Drought

- Data Vintage:  2021

- Unit Of Measure: Dimensionless fraction (AET in mm/P in mm).

Plants respond to conditions in their immediate vicinity. Thus, to understand the vegetative moisture stress during drought, it is important to measure the local moisture balance. The actual evapotranspiration fraction (AETF) provides such a measure. Specifically, it indicates whether a location is expected to experience local drying during a drought, or whether the location receives sufficient precipitation that it will remain moist even during an extended drought. An extended drought is defined by a 48-month period where the Standardized Precipitation Index (SPI, NCAR 2022) is two standard deviations below the long-term mean (SPI-48 = negative 2). Such a drought is expected approximately once every 50 years in the Sierra Nevada. The southern Sierra 2012-2015 drought was a SPI-48 drought = negative 2.0, which resulted in severe vegetation die-off and a marked reduction in water deliveries.

The AETF ranges from 0 to > 1; a low value indicates a wetter location during drought and a high value indicates a drier location. Locations <1 would be expected to generate runoff, even during a significant drought (SPI-48 drought = negative 2.0), and would be expected to continue generating runoff. Locations > 1 would be expected to desiccate the soil during drought, with negligible runoff, and increasing vegetation drought stress. AET/P does not account for lateral water inflow, either as runoff or irrigation.

Creation Method: Calculated as the ratio of actual evapotranspiration (AET) during 2021 Water Year (WY) and precipitation that would be expected for each pixel under a significant drought ( SPI-48 drought = negative 2.0). AET is calculated based on Landsat observations and eddy covariance, along with information on local monthly irradiance that accounts for Top of Atmosphere and topographic effects. The AET calculated for 2021 is then divided by the precipitation that would be expected for each pixel under a significant drought (SPI-48 drought = negative 2.0). This quantity of precipitation is calculated for each pixel based on local, down-scaled PRISM data for 1991-2020. This fraction provides a measure of the local water balance during drought, with the higher values indicating a drier location. See https://doi.org/10.1029/2012JG002027 and https://doi.org/10.1073/pnas.1319316111 for further information.

- Credits:  CECS; https://california-ecosystem-climate.solutions/

UC Irvine, CECS; https://california-ecosystem-climate.solutions/: 
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