Look for complete geospatial metadata in this layer's associated xml document available from the download link

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

* Tier: 1  

* Data Vintage: 09/2021  

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

* Represent element and pillar: Yes  

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

* Translation: Where is water availability (1-AET/P) 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 (negative linear slope). More favorable (translated to +1) set at 0, and less favorable (translated to -1) set at > 2.5. 

Figure 187. Histogram and scoring criterion of current condition of actual
evapotranspiration/precipitation fraction across the Central Coast

Figure 188. Histogram of translated current condition of actual
evapotranspiration/precipitation fraction across the Central Coast

Figure 189. Maps displaying raw metric and translated metric of current
condition of actual evapotranspiration/precipitation fraction across the
Central Coast.

  

* 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 = -2). Such a drought is expected approximately once every 50 years in the Central Coast. The southern Sierra 2012-2015 drought was a SPI-48 drought = -2.0, which resulted in severe vegetation die-off and a marked reduction in water deliveries. 

The AETF ranges from 0 to > 100%; a low value indicates a wetter location
during drought and a high value indicates a drier location. Locations <100%
would be expected to generate runoff, even during a SPI-48 drought = -2.0, and
would be expected to continue generating runoff. Locations >100% would be
expected to desiccate the soil during drought, with negligible runoff, and
increasing vegetation drought stress.

  

* Credits: CECS;