- Metric Name:  Precipitation Minus Actual Evapotranspiration during normal conditions

- Data Vintage:  2021

- Unit Of Measure: mm/y

Runoff is a measure of the water available for storage. It is determined by both the water supply and the demand of the existing vegetation. Annual mean runoff measures the “average” vegetative demand and thus provides a comparative index on the potential available runoff. Specifically, Annual Mean Runoff is the expected surplus water that would discharge to surface or ground water flows during a series of years with average precipitation. Larger values indicate more runoff under mean conditions.

Creation Method: The Center for Ecosystem Climate Solutions at UC Irvine (CECS) is working with the State and Federal governments in developing scientifically rigorous, stakeholder-informed methods that have produced tailored, integrated data for land management decision makers. The CECS DataEngine model tracks monthly water balance from 1986 to 2021. The Annual Mean Runoff layer is calculated using this CECS DataEngine model logic forced with a series of 4 years that each received precipitation according to the timing and magnitude of the 30-year climate Normal Precipitation (SPI = 0 by definition).

The model water inputs are determined from downscaled PRISM gridded datasets (https://prism.oregonstate.edu/). In the case of the Annual Mean Runoff, this reflects the monthly 30 year Normal for each pixel calculated for 1991-2020. Actual evapotranspiration (AET) is calculated from Landsat observations and eddy covariance during 2021, along with information on local monthly irradiance that accounts for Top of Atmosphere (TOA) and topographic effects, as well as monthly temperature and drought stress. Precipitation Minus Actual Evapotranspiration is calculated as the difference; it provides an excellent measure of the long-term runoff from upland pixels. Areas with a higher P-ET produce greater runoff, and areas with a low P-ET tend to produce little or no runoff during average or dry years. 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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