Assessment of remote sensing-based soil water balance and FAO56 dual crop coefficient approach on almond orchards

The irrigated area cultivated with almond trees (Prunus dulcis) has significantly increased in recent years worldwide. In Spain, the extension covered  by irrigated almond orchards has doubled in the past 5 years, currently accounting for about 14% of the harvested almond area. The high water productivity of this crop jointly with the good market perspectives for the almonds have boosted this expansion in many areas, providing a viable alternative to traditional crops, such as cereals and other woody crops. However, the expansion of irrigated almond orchards in water-scarce areas could compromise the water resources and sustainability of agriculture. In this way, precise irrigation management tools are required to adjust the water supply to the actual crop requirements with suitable temporal and spatial resolutions. Accurate estimates of the net irrigation water requirements allow to improve the water use efficiency and therefore, achieve a more profitable and sustainable management. The soil water balance model (SWB) based on the FAO-56 dual crop coefficient approach (Allen et al., 1998) is a well-recognized procedure for the estimation of daily crop water requirements. This approach considers a single-layer soil water balance estimated at the root zone, jointly with soil evaporation in the surface layer.

This work introduces a Remote Sensing (RS) assisted approach to monitor the soil water balance in almond trees. Our experiment aims at comparing this RS-based technique to the traditional FAO-56 methodology. This RS-based approach integrates a basal crop coefficient (K_cb) derived from time series of satellite images into the daily soil water balance. Although well-documented in the literature for other crops, limited information is available for the application of this RS-based methodology to almond orchards.

This study was carried out in two drip irrigated almond commercial fields, located in the semi-arid province of Albacete (Southeastern Spain). Measurements of soil water content, and stem water potential, were available during the campaigns 2020-2022 in the analyzed fields. A continuous sampling of the canopy structure was registered. Thermal infrared radiometers were deployed to model the surface energy balance, and an eddy-covariance tower was placed to monitor the actual evapotranspiration. This instrumentation provides a unique opportunity to assess the performance of the SWB models. The results show an overall similar performance for both approaches in reproducing the temporal evolution of K_cb during the campaigns analyzed. The assessment of the results indicates the potential of both approaches to accurately estimate the temporal evolution of soil water content in irrigated almond fields under different water management, either comfort or water stress.

The operational application derived from the present study will provide the farmer with accurate information on the actual crop water demand and adjust and distribute water supply to the crop’s spatially and temporally varying water requirements. The information obtained is useful for making management decisions aimed at improving irrigation scheduling, developing controlled deficit irrigation (CDI) strategies, and promoting the optimization of water resources and the development of sustainable agriculture.