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* Metric Name: Actual Evapotranspiration to Precipitation Fraction During Drought *  

* Tier: 1  

* Data Vintage: 09/2021  

* Unit Of Measure: Continuous, dimensionless fraction (AET in mm/P in mm).  

* Represent element and pillar: Yes  

* Type and distribution of data: Right-skewed distribution representing continuous values. Raw values range from 0 to 20.  

* Translation: Where is water availability (1-AET/P) during drought the lowest?  

* Translation method and outcome: Raw data values translated to range from +1 to -1 representing more favorable to less favorable conditions, respectively. **** The proposition of more favorable is based on the objective of lower water availability during drought (negative linear slope). More favorable (translated to +1) set at 2.862 and less favorable (translated to -1) set at > 0.

Figure 198. Histogram and scoring criterion of current condition of actual
evapotranspiration/precipitation fraction across the Southern California

Figure 199. Histogram of translated current condition of actual
evapotranspiration/precipitation fraction across the Southern California

Figure 200. Maps displaying raw metric and translated metric of current
condition of actual evapotranspiration/precipitation fraction across the
Southern California.

  

* 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). 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.

  

* Credits: CECS;