Forecasting crop evapotranspiration under standard conditions in precision farming

A new methodology is suggested for forecasting crop evapotranspiration under standard conditions (also referred to as potential evapotranspiration ET
p
) by combining VIS-NIR satellite images and numerical weather forecasts. The methodology is proposed for farms where a reliable weather station is operating for assessing current reference evapotranspiration according to FAO Penman-Monteith equation (ET
0-PM
). Measured weather data are exploited for correcting systematic errors in weather forecasts and for locally calibrating Hargreaves-Samani (HS) equation for estimating the reference ET (ET
0-HS
). VIS-NIR crop images and current weather conditions are employed for updating a crop coefficient (K
c
), analytically derived as the ratio of crop potential evapotranspiration computed with the Penman-Monteith equation (ET
P-PM
) and ET
0-PM
. Crop parameters in ET
P-PM
are assessed by means of albedo and LAI, both retrieved from VIS-NIR images. $\mathrm {E}\mathrm {T}_{\mathrm {P}}$ forecasts are then computed with the HS formula by the product ET
P-HS
= K
c
.ET
0-HS
. The methodology was applied to two experimental farms in Southern Italy, by employing COSMO-LEPS numerical weather forecasts, LANDSAT 8 and DEIMOS-1 VIS-NIR images. ET
P-HS
forecasts resulted to be more accurate and precise than ET
P-PM
forecasts. Differently from ET
P-PM
forecasts, ET
P-HS
relies only on temperature forecasts, which are generally less uncertain than the forecasts of the other atmospheric variables, which are required for computing ET
P-PM
.