Evapotranspiration
The RS-PMPT ET model
Flowchart of the RS_PMPT model. LAI: Leaf Area Index; TS: MODIS LST; fpar: Fraction of photosynthetic active radiation; Ta: Air temperature; es= saturated vapor pressure; VPD: Vapor pressure deficit; Rl: net long wave radiation; Rs: net shortwave radiation; Rn: net radiation; G: Ground heat flux; Rsoil : Net radiation to the soil.
We introduce a different operational approach to estimate 8-day average daily evapotranspiration (ET) using both routinely available data and the Penman-Monteith (P-M) equation for canopy transpiration and evaporation of intercepted water and Priestley and Taylor for soil evaporation. Our algorithm considered the environmental constraints on canopy resistance and ET by (1) including vapor pressure deficit (VPD), incoming solar radiation, soil moisture, and temperature constraints on stomatal conductance; (2) using leaf area index (LAI) to scale from the leaf to canopy conductance; and (3) calculating canopy resistance as a function of environmental variables such as net radiation and VPD. Remote sensing data from the Moderate Resolution Spectroradiometer (MODIS) and satellite soil moisture data were used to derive the ET model. The algorithm was calibrated and evaluated using measured ET data from 20 AmeriFlux Eddy covariance flux sites for the period of 2003–2012.
The strong relationship between the model estimated ET and the flux tower observations implies that our model has the potential to be applied to different ecosystems and at different temporal scales.
For more information, please check the AFM paper
Model Validation and Calibration
Average 8-day of the RS-PMPT ET estimates compared with average 8-day eddy flux tower ET for the calibration sites. The dashed line is the regression line and the black solid line is the 1:1 line. The dashed line is the regression line and the black solid line is the 1:1 line. The data for each site represent the average 8-day data for the years included in the study
Average 8-day of the RS-PMPT ET estimates compared with average 8-day eddy flux tower ET for the validation sites. The dashed line is the regression line and the black solid line is the 1:1 line. The data for each site represent the average 8-day data for the years included in the study.
Seasonal time series of daily ET for the calibration deciduous sites either at eddy flux tower (open circle) or predicted by the RS-PMPT model (black line), or MODIS ET
Seasonal time series of daily ET for the validation deciduous sites either at eddy flux tower (open circle) or predicted by the RS-PMPT model (black line), or MODIS ET