In a blog entry last March, I introduced a study I am conducting to assess the effect of surface water quality on soil salinity of alfalfa fields in the south Delta. This blog revisits that project and describes what was learned in the first year. The objective of this work is to gain knowledge on the leaching fraction being achieved and how surface water quality and rainfall affect leaching. The knowledge to be gained will assist growers with salinity management.
As a forage crop, the marketed product of alfalfa is the vegetation, or alfalfa hay. Hay yields are directly related to crop water use. As crop water use increases, generally so does alfalfa yield. Nevertheless, soil and water salinity can constrain the relationship between water use and yield. Plants that are stressed by salinity show stunting and reduced yields. To prevent a 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. The leaching fraction is the minimum fraction of the total applied water that must pass through the soil root zone to prevent a reduction in crop yield from excess salts. Leaching occurs whenever irrigation and rainfall exceed crop water use.
Two measurements are needed to establish the leaching fraction. One is the average salt concentration of the total applied water, as irrigation and rainfall. The salinity of irrigation water can vary substantially in the Delta based on time of year and location. In the south Delta – an area southwest of Stockton, California that includes approximately 100,000 cultivated acres – the salt concentration of surface water is mandated by the 1978 Delta Plan. The California State Water Resources Control Board established surface water salinity objectives in the Delta Plan, based on the current knowledge of soil types, irrigation practices, and salinity standards of predominant crops. When the 2006 Bay-Delta Plan was adopted, the salinity objectives were not changed due to a lack of scientific information to justify a change. In other words, there have not been any recent studies to evaluate how surface water quality is affecting the crops and soils of the south Delta.
The second measurement needed to establish the leaching fraction is the salinity of the water draining from the root zone. This measurement is made by collecting soil samples from different depths and measuring the electrical conductivity (EC) of the water that drains from the soil when the soil is saturated. Electrical conductivity is the ability of a solution to transmit electrical current because of dissolved solutes. The leaching fraction is calculated from the ECs of the irrigation water and soil-water.
I am conducting the current study in seven south Delta mature alfalfa fields. The study focuses on alfalfa fields because alfalfa is moderately sensitive to salinity. Fields were selected based on similar soil characteristics but differing irrigation water sources. The fields have a high clay content, which is important because drainage is slower in clay soils, making leaching difficult. Irrigation water is sourced from the San Joaquin River, including Old River, Middle River, and connecting sloughs and canals. Soil is sampled in the spring and fall, in one-foot increments, down to five feet. Groundwater is sampled at the time of soil sampling, and irrigation water is sampled from each irrigation throughout the season. Yield measurements are taken from at least three cuttings during the growing season.
Over the course of the 2013 field season, irrigation water salinity (ECw) ranged from 0.37-1.79 dS/m at the seven sites. Four sites had average ECw below 0.7 dS/m, the irrigation season (April-August) salinity objective set by the State Water Board, but the average salinity at three sites exceeded the State Water Board's standard. Previous research suggests that beyond a water salinity threshold (ECw) of 1.3 dS/m, alfalfa yield reductions are expected; although, the 2013 yield results from this project do not suggest that yields were harmed by salinity. The bottom of the root zone was assumed to be the 1-foot layer of soil above the groundwater table. The achieved leaching fraction was calculated for this layer of soil. Across the seven sites, the lowest leaching fraction was three percent, while the highest was 39 percent. The leaching fraction of five sites was less than 15 percent, which is a general recommendation for leaching. In other words, five of the seven sites did not get enough leaching in 2013. This study is continuing in 2014 and will provide current data for understanding the leaching fraction being achieved in south Delta alfalfa fields, a region that would be further challenged by salinity under conditions of reduced rainfall, reduced water flows, or a higher surface water salinity standard.