UC Davis

In response to water uncertainties, the almond community has set a goal to reduce the amount of water used to grow a pound of almonds by 20% by the year 2025. The research presented in this paper addresses this goal by developing site-specific irrigation management strategies in different ages and varieties of almond trees using field experiments and data-driven modeling. Crop water use and crop coefficients for young almond trees (1-5 years old) were determined at three adjacent Nonpareil/Monterey almond orchards in the northern Sacramento Valley. Crop water use was determined through a land surface energy balance using eddy covariance. Crop coefficients were determined using the evapotranspiration estimates from each orchard and a short grass reference evapotranspiration from the nearest California Irrigation Management Information System (CIMIS) station. Results showed that crop water use and crop coefficients increased until the 4th year, suggesting that farmers should closely monitor tree development and orchard age and adjust irrigation scheduling as young trees grow. The results led to the conclusion that farmers should use age-specific crop coefficients until the 4th year and then they can start using mature almond crop coefficients. In mature almonds orchards, regulated deficit irrigation (RDI) during hull-split can reduce water use, but limited research has been done on strategies for imposing RDI in almond orchards with multiple varieties with different hull-split schedules. A 2-year study evaluated the impacts of two different regulated deficit irrigation schedules under two levels of crop evapotranspiration irrigation replacement rates in an almond orchard with Butte, Aldrich, and Nonpareil varieties in the Sacramento Valley of California, USA. The two irrigation schedules were (1) regulated deficit irrigation in Butte, Aldrich, and Nonpareil varieties during Nonpareil hull-split timing and (2) regulated deficit irrigation in each variety according to variety-specific hull-split timing. The two levels of irrigation were 50% and 75% of crop evapotranspiration (ETc) replacement during the hull-split period. Results show that the kernel thickness of Aldrich almonds significantly increased under 75% ETc irrigation replacement during Aldrich hull-split period compared to 75% ETc and 50% ETc irrigation replacement during Nonpareil hull-split period. In the Butte almonds, 75% ETc and 50% ETc irrigation replacement during variety-specific hull-split significantly reduced the fraction of sealed shells of the Butte variety compared to 75% ETc and 50% ETc irrigation replacement during Nonpareil hull-split period. This study demonstrated that almond physical quality could change in the Butte and Aldrich varieties when RDI is imposed according to variety-specific hull-split schedules. No marketable kernel yield improvements were achieved by implementing RDI according to variety-specific hull-split after two years. The least labor-intensive strategy of RDI during Nonpareil hull-split in all three varieties is recommended. Advanced RDI regimes should use a sensitive indicator of plant water status to avoid excessive accumulation of plant water stress. In this research, a low-cost site-specific data-driven modeling scheme was developed for estimating midday stem water potential in different varieties of almond trees under various RDI regimes. The available explanatory data for the data-driven model of MSWP included soil water content at 30 cm, 60 cm, 90 cm, 120 cm, and 150 cm, solar radiation, air temperature, relative humidity, soil texture and gravel content at four layers, and fraction of photosynthetically active radiation intercepted by the canopy. Soil water content at 30 cm was the most significant explanatory variable of MSWP and showed a nonlinear relationship with MSWP. The square of soil water content at 30 cm was included in the model to approximate the nonlinear relationship between MSWP and soil water content at 30 cm. When all varieties were combined, the best regression model included the following explanatory variables of MSWP that were significant to enter and exit the model at the 0.001 significance level: soil water content at 30 cm, the square of soil water content at 30 cm, daily minimum air temperature, daily maximum relative humidity, daily minimum relative humidity, fraction of photosynthetically active radiation, and soil texture class between 10 to 86 cm (adjusted R2=0.66, RMSE=0.31). Separate regression models for each variety improved the correlations in the Aldrich and Butte varieties with explanatory inputs selected at the 0.05 significance level to enter and exit the model (adjusted R2=0.74, RMSE=0.27 and adjusted R2=0.73, RMSE=0.28, respectively), but slightly worsened in the Nonpareil variety (R2=0.64, RMSE=0.30). The results from this work indicate that the explanatory variables can vary across almond varieties and that site-specific characteristics, such as soil texture and the fraction of photosynthetically active radiation, are significant in determining MSWP in addition to the meteorological conditions and soil water content. Key results of this research include (1) baseline data on crop water requirements in young almond trees, (2) new data and refined methods on regulated deficit irrigation in multi-variety almond orchards, and (3) a site-specific data-driven model for estimating stem water potential. This research directly addresses pressing issues affecting almond production, including uncertainties in water supplies and labor shortages in agriculture, that merit advanced research on precision irrigation.