Water quality and alfalfa yield in the Sacramento-San Joaquin River Delta
The Sacramento-San Joaquin River Delta region is a unique agricultural region of California. While the region is named for its waterway configuration, the Delta is also unique for its fertile soils, and of 738K total acres in the Delta, approximately 500K are farmed. Delta farming is challenged by soil salinity, which can stress crops and reduce yields. In general, plants are stressed by saline conditions because they must expend more energy to take up water, leaving less for plant growth. This trade-off is challenging for alfalfa growers because the harvested crop is the vegetative growth, and the extra energy that goes to take up water reduces hay yields. To prevent harmful accumulation of salts, the soil profile must be leached periodically with an amount of water in excess of what is used by the plant. This study was aimed at establishing the leaching fraction for alfalfa in the Delta.
UC Agriculture and Natural Resources
The Sacramento-San Joaquin River Delta of California is a unique agricultural region due to its soil type, climate, and irrigation and groundwater sources. A diversity of crops grow in the Delta region, but alfalfa is a particularly important one. According the Agricultural Commissioners, in 2012 alfalfa was grown on approximately 72,000 acres throughout the region, making it the second most widely grown crop.
As a forage crop, the marketed product of alfalfa is the vegetation, or alfalfa hay. Hay yields are directly related to crop evapotranspiration (ET), or the water transpired by the crop plus the water evaporated from the soil. As crop ET increases, so does alfalfa yield. Nevertheless, agronomic and economic reasons put constraints on this relationship. For example, irrigation must be managed properly due to the susceptibility of alfalfa to Phytophthora root and crown rot disease. This is one of the most common alfalfa diseases and occurs in poorly-drained or over-watered conditions.
In the Delta region, soil salinity can also affect the relationship between evapotranspiration and yield. In general, plants are stressed by saline conditions because they must expend more energy to take up water, leaving less energy for growth. This can cause plant stunting and reduced yields. To prevent harmful accumulation of salts, the soil profile must be leached periodically with an amount of water in excess of what is used by plant ET. The leaching fraction is the fraction of water that passes below the root zone divided by the total applied water. The leaching requirement is defined as the minimum fraction of the total amount of applied water that must pass through the soil root zone to prevent a reduction in crop yield from an excess of salts. Leaching occurs whenever irrigation and rainfall exceed ET.
Two factors establish the leaching requirement: the salt concentration of the applied water (including rainfall) and the salt tolerance of the crop. Some crops are more tolerant of salinity than others; alfalfa is moderately sensitive.
Soil salinity in the Delta is a sporadic problem in the short term – varying with the quality of the surface irrigation water, depth and quality of the groundwater, and volume of winter rainfall. Water tables in the area are typically within two meters of the soil surface, and the ground water quality is near the threshold water quality tolerance for alfalfa. Additionally, many Delta soils growing alfalfa are rated in the slow and very slow permeability category. The leaching requirement considered necessary to maintain alfalfa yields is calculated to be 15% of the total applied water. If a 15% leaching requirement is not possible due to poor soil permeability, proximity of groundwater, or other agronomic considerations, lower salinity irrigation water may be necessary to maintain yields. Thus, salinity will continue to be an issue in the Delta in the long run, especially under conditions of reduced water flows or a higher surface water salinity standard.
The current leaching fraction being achieved in Delta alfalfa soils is the unknown to be learned through this project. This study provided a field assessment of surface water quality effects on the soil leaching fraction and alfalfa yield in the Delta, and offered information on irrigation water management for growers.
Top photo by Terry Prichard, bottom photo by Michelle Leinfelder-Miles