DOI: 10.6071/m3010d
title: Evapotranspiration data from eddy-covariance flux-tower measurements and Landsat imagery in California’s Sierra Nevada from 1985 to 2019

description: The gridded annual evapotranspiration (ET) from 1985 to 2019 were
 calculated based on the correlations between eddy-covariance flux-tower
 measurements of annual evapotranspiration and satellite imagery derived
 Normalized Difference Vegetation Index (NDVI). Annual ET observations from
 12 flux-towers across the Sierra Nevada and Southern California were
 collected from 2001 to 2016, resulting 97 site-years of ET observations in
 five main vegetation types (Evergreen Needleleaf Forest, Grasslands, Mixed
 Forest, Open Shrublands, and Woody Savannas). The NDVI was calculated at
 30-m resolution from USGS Landsat Collection Tier 1 surface reflectance
 over the entire California and aggregated to annual mean values for each
 water year (Oct. to Sept.) from 1985 to 2019. The gridded annual ET was
 calculated at 30 m spatial resolution using the exponential relationship
 between NDVI and ET derived from flux-tower sites.
description: The ET
 regression function (1) was estimated based on the  method developed by
 (Goulden and Bales 2019) using exponential regression in R language. The
 flux-tower ET observations were summed annually by water year after
 filling data gaps (Rungee et al. 2018). The total 97 site-year ET
 observations also included some site-years that were impacted by drought,
 fire, forest thinning, prescribed fire, reflecting by the significant
 reduction in annual NDVI. The annual NDVI map was calculated as the mean
 of all Landsat scenes for a water year (Oct. to Sept.) after masking for
 shadow, snow or cloud (Zhu and Woodcock 2012). We homogenized Landsat 8
 NDVI (L8, 2014-2019) and Landsat 7 (L7, 2012- 2013) to Landsat 5 NDVI (L5,
 1985-2011) following equation 2 and 3 (Su et al. 2017; Sulla-Menashe et
 al. 2016). The NDVI and ET maps were generated using Google Earth Engine.
 The modeled ET showed a strong correlation to site-level flux-tower
 observations (coefficient of determination,
 R<sup>2</sup>=0.77). Most of the modeled ET fall within ±100
 mm ranges of ET measurements, with a root mean square error (RMSE) at 108
 mm, and mean absolute error (MAE) at 74 mm. The main estimation error is
 observed at site-years with high NDVI and ET values, due to NDVI
 saturation issue. The model’s temporal and spatial sensitivities were
 assessed using leave-one-out cross validation method by removing an
 individual water year or flux-tower site for model building and then
 evaluating on the site-year removed. <i>ET=112.3×</i><i>e</i><sup><i>3.2×NDVI</i></sup>                    (1) <i>L5=0.9883×L7-0.0367      </i>           (2) <i>L5=0.8213×L8-0.0403</i>                 (3)  
description: Each annual ET map is named as “ET water year_expNDVI.tif” in a
 GeoTIFF format under the GCS_WGS_1984 projection system. The ET values is
 calculated in the unit of mm/year. ET values are mapped over the entire
 California, but most accurate in Sierra Nevada. The Google Earth Engine
 code to generate the map can be found in this link:
 https://code.earthengine.google.com/9604115a71d75c57e6bbabf24efcfc9c