Impact of drought and changing water sources on water use and soil salinity of almond and pistachio orchards: 2. Modeling

Authors: HELALIA, SARAH - University Of California; Anderson, Raymond - Ray; Skaggs, Todd; ŠIMUNEK, JIRKA - University Of California

Submitted to: Soil Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2021
Publication Date: 9/24/2021
Citation: Helalia, S.A., Anderson, R.G., Skaggs, T.H., Šimunek, J. 2021. Impact of drought and changing water sources on water use and soil salinity of almond and pistachio orchards: 2. Modeling. Soil Systems. 5(4). Article 58. https://doi.org/10.3390/soilsystems5040058.
DOI: https://doi.org/10.3390/soilsystems5040058

Interpretive Summary: Recent drought in California has forced growers of perennial nut crops to pump groundwater for irrigation when surface supplies are insufficient. This use of groundwater not only unsustainably depletes groundwater reserves, but it also contributes to soil salinization and negative impacts on tree health and yield due to the poorer quality of groundwater found in many tree nut growing regions such as the Western San Joaquin Valley. To understand how changing irrigation water sources impacts established commercial orchards, we conducted an observational and modeling study across five almond and pistachio orchards in the San Joaquin Valley across a gradient of soil salinity and irrigation with different water sources. In the second part of this study, we report the results of modeling studies across the five sites. Modeled results with the HYDRUS model show the largest reduction in root water uptake at the most saline site. The results also show that irrigation water quality has a larger impact on soil salinization than seasonal winter precipitation. The study results emphasize the need to consider irrigation water quality in drought mitigation plans and to minimize the use of lower quality water sources during drought periods for salt sensitive crops. This information will be of interest to farmers, irrigation managers, and policymakers, especially those involved with Groundwater Sustainability Agencies.

Technical Abstract: California is increasingly experiencing drought conditions that restrict irrigation deliveries to perennial nut crops such as almonds and pistachios. During drought, poorer quality groundwater is often used to maintain these crops, but this use often results in secondary salinization that requires skilled management. Process-based models can help improve management guidelines under these challenging circumstances. The main objective of this work was to assess seasonal soil salinity and root water uptake as a function of irrigation water salinity and annual rain amounts. The manuscript presents a comparison of three-year experimental and numerically simulated root zone salinities in and below the root zone of almond and pistachio drip-irrigated orchards at multiple locations in the San Joaquin Valley (SJV), California, with different meteorological characteristics. The HYDRUS-1D numerical model was calibrated and validated using field measurements of soil water contents and soil solute bulk electrical conductivities at four root zone depths and measured soil hydraulic conductivities. The remaining soil hydraulic parameters were estimated inversely. Observations and simulations showed that the effects of rain on root zone salinity were higher in fields with initially low salinities than in fields with high salinities. The maximum reduction in simulated root water uptake (7%) occurred in response to initially high soil salinity conditions and saline irrigation water. The minimum reduction in simulated water uptake (2.5%) occurred in response to initially low soil salinity conditions and a wet rain year. Simulated water uptake reductions and leaching fractions varied at early and late times of the growing season, depending on irrigation water salinity. Root water uptake reduction was highly correlated with the cumulative effects of using saline waters in prior years, more than salt leaching during a particular season, even when rain was sufficient to leach salts during a wet year.