Analysis of the use of actual evapotranspiration calculated with Landsat imagery and climate forecasts, assessed with an agrohydrologic model for irrigation scheduling in fruit crops

The evapotranspiration of vegetation (ET) is a key component of the hydrological balance. Various tools and models have been proposed to estimate evapotranspiration in fruit crops. Among them, the most widely used approach is that proposed by the Food and Agriculture Organization (FAO), which considers climatic variables included in reference evapotranspiration (ETo), as well as the type of crop and its characteristics represented by a single crop coefficient (Kc). However, there is evidence that in tall and discontinuous canopies, such as citrus orchards, with a high degree of interaction with the environment, Kc can change depending on local environmental conditions and the amount of vegetation.

Other methods, such as measurements of stem water potential, sap flow sensors, and moisture probes, allow for determining the water status of the crop, but only for a limited number of trees, and uncertainties arise when extrapolating values. Remote sensing fills this gap if spatial and temporal resolutions suit the monitored crop. A successful approach in water management is using models that calculate latent heat as a residue of the surface energy balance (SEB).

This study applied an energy balance to calculate ET in an irrigation district. The study site is located in the Valencia region (Spain; 39º22'43'' N, 0º28'20'' W) with localized irrigation, where most crops are citrus. A total of 182 images from the Landsat satellite constellation for the period 2013-2018 were used to estimate instantaneous ET by extrapolating daily actual ET (ETSEBAL) values using climatic data.

These climatic data correspond to predictions the Global Forecast System (GFS) provides. This way, climatic predictions are used for scheduling instead of the classical methodology that uses past data to estimate evapotranspiration. The study's objective is to analyze the results using a dynamic Kc obtained from the actual state of the crops and climatic predictions for each plot, compared to a generic Kc obtained for standard conditions and past climatic data.

The results suggest that, for the studied plots, the relationship between drained water and the actual volume provided by irrigators would be reduced by 20% to -30 %. A point agrohydrological model calibrated with capacitive moisture probes was used to monitor soil water balance.

In the same way, the methodology allows for determining the stress level of crops and maintaining it within recommended limits.