Mapping Almond Stem Water Potential using Machine Learning

Abstract Almonds are a major crop in California which produces 80% of all the world’s almonds. Widespread drought and strict groundwater regulations pose significant challenges to growers. Irrigation regimes based on observed crop water status can help to optimize water use efficiency, but consistent and accurate measurement of water status can prove challenging. In almonds, crop water status is best represented by midday stem water potential measured using a pressure chamber, which despite its accuracy is impractical for growers to measure on a regular basis. This study aimed to use machine learning (ML) models to predict stem water potential in an almond orchard based on canopy spectral reflectance, soil moisture, and daily evapotranspiration. Both artificial neural network and random forest models were trained and used to produce high resolution spatial maps of stem water potential covering the entire orchard. Also, for each ML model type, one model was trained to predict raw stem water potential values, while another was trained to predict baseline-adjusted values. Together, all models resulted in an average coefficient of correlation of R2=0.73 and an average root mean squared error (RMSE) of 2.5 bars. Prediction accuracy decreased significantly when models were expanded to spatial maps (R2=0.33, RMSE=3.31 [avg]). These results indicate that both artificial neural networks and random forest frameworks can be used to predict stem water potential, but both approaches were unable to fully account for the spatial variability observed throughout the orchard. Random forest models predicting raw stem water potential produced the most accurate maps. Overall, the most accurate maps were produced by the random forest model (raw stem water potential R2=0.47, RMSE=2.71). Being able to predict stem water potential spatially can aid in the implementation of variable rate irrigation. These results indicate that both artificial neural network and random forest frameworks can be used effectively to predict and map stem water potential, but that both approaches are unable to fully account for the spatial variability observed throughout the orchard. Future studies should examine the impact of utilizing stem water potential maps as an irrigation decision guide.