Actual Evapotranspiration and Precipitation Ratio During Drought

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). 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 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.

Data and Resources

Additional Info

Field Value
Version Version 5.0
Last Updated March 28, 2025, 08:36 (UTC)
Created March 28, 2025, 08:36 (UTC)
category /Water Security/Quantity
collection_name California Landscape Metrics
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](https://doi.org/10.1029/2012JG002027) and[ https://doi.org/10.1073/pnas.1319316111](https://doi.org/10.1073/pnas.1319316111) for further information.
data_units Dimensionless fraction (AET in mm/P in mm).
data_vintage 09/2021
date_updated August 2024
element Quantity
encoding utf8
file_name WaterFlux_AETFrac_202109_202312_T1_v5
format GeoTiff
harvest_object_id 8f9d9b8f-8e9f-46b7-86b9-35dae4f7c7ee
harvest_source_id a2637971-af12-457f-ae4a-831d2202a539
harvest_source_title WIFIRE Commons
maximum_value 20.0
metric_definition_and_relevance 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). 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.
minimum_value 0.0
pillar Water Security
spatial {"type": "Polygon", "coordinates": [[[-124.5090833859125, 32.4242698701859], [-113.49380570277376, 32.4242698701859], [-113.49380570277376, 42.1119747450123], [-124.5090833859125, 42.1119747450123], [-124.5090833859125, 32.4242698701859]]]}
tier 1