**3.3 Data**

Required meteorological data to calculate both ET values (modified GG or GG-NDVI) are air temperature, precipitation, elevation (pressure), net radiation, wind speed, and NDVI. Net radiation was estimated using the equations suggested by [25]. Air temperature and precipitation data are from the PRISM (Parameterelevation Regressions on Independent Slopes Model) climate group (available at http://prism.oregonstate.edu/) at 4-km resolution for the period 2000–2015 covering the CONUS. Wind speed was collected from the Climate Monitoring at NOAA's National Centers for Environmental Information (available at https://www.ncdc. noaa.gov/societal-impacts/wind/). Monthly NDVI data required for the GG-NDVI method are from the NASA Earth Observations (NEO, available at http://neo.sci. gsfc.nasa.gov/).

To assess the capability of EWDI, we used USDM to compare the differences between the two indices during the evolution of drought through time and space. USDM is derived from measurements of climatic, hydrologic, soil conditions, and regional expert comments [26]. USDM is not a forecast instead it assesses the current drought conditions. USDM divides drought severity into five classes: abnormally dry (D0), moderate drought (D1), severe drought (D2), extreme drought (D3), and exceptional drought (D4). All drought indices used in this study were converted to USDM classes as presented in **Table 3**. Additionally, we compared EWDI against PDSI and SPI which were retrieved from the WestWide Drought Tracker (WWDT, available at http://www.wrcc.dri.edu/wwdt/about.html). USDM data from 2000 to 2015 were collected from the USDM website (http://


#### **Table 3.**

*Drought classes of USDM and corresponding threshold value for classifying drought with PDSI, SPI, and EWDI.*

droughtmonitor.unl.edu/Home.aspx), and four indices are resampled to match the 4-km resolution of EWDI using bilinear interpolation in the ArcMap software.

We also used EC flux tower data (in mm/month) from FLUXNET stations to perform a comparison of modified GG and GG-NDVI ET products. The latent heat flux data were collected from the Oak Ridge National Laboratory's AmeriFlux website (http://ameriflux.ornl.gov/, last accessed on November 23, 2016). The tower-measured monthly latent heat flux data were calculated using the equation as *ET* = *LE*/λ, where LE is the latent heat flux (W/m<sup>2</sup> ) and λ is the latent heat of vaporization (2.45 MJ/kg).
