The Sacramento-San Joaquin River Delta region – for its soil type, climate, and irrigation and groundwater sources – is a unique agricultural region of California. Diverse crops are grown in the Delta region, but alfalfa is a particularly important one as the second most widely grown crop. Border check flood irrigation using surface water is the primary method of irrigating Delta alfalfa.
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; although, agronomic and economic reasons put constraints on this relationship.
In the Delta, soil salinity can 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 plant 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 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 quantities 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. Beyond a soil salinity threshold (ECe) of 2.0 dS/m, alfalfa yield reductions are expected (Ayers and Westcot, 1985).
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. Additionally, many Delta soils growing alfalfa are rated in the slow and very slow permeability category. Water tables in the area are typically within 1.5 meters of the soil surface, and the ground water quality is near the threshold water quality tolerance for alfalfa (ECsw) of 1.3 dS/m. At an ECe of 2.0 dS/m and an ECsw of 1.3 dS/m, the leaching requirement 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 of 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 requirement actually being achieved in Delta alfalfa soils is unknown, but a project is being initiated to understand it better. The study will provide a field assessment of surface water quality effects on the soil leaching requirement and alfalfa yield in the Delta, and it will offer information on irrigation water management for growers. Stay tuned for information in the future.