Salinity level rising in south-central Arizona
Salinity becomes a problem when enough salts accumulate in the root zone to negatively affect plant processes. Excess salts in the root zone inhibit plant roots from withdrawing water from surrounding soil. This lowers the amount of water available to the plant, regardless of the amount of water actually in the root zone. For example, when plant growth is compared in two identical soils with the same moisture levels, one soil receiving salty water and the other receiving salt-free water, plants are able to use more water from the soil receiving salt-free water. Although the water is not held tighter to the soil in saline environments, the presence of salt in the water causes plants to exert more energy extracting water from the soil. The main point is that excess salinity in soil water can decrease plant available water and cause plant stress.
Soil water salinity is dependent on soil type, climate, water use and irrigation schedules. For example, immediately after irrigation, plant available water is at its highest and soil water salinity is at its lowest. However, as plants use soil water, the remaining water is held tighter to the soil and becomes increasingly more difficult for plants to extract. As the water is taken in by plants through transpiration or lost to the atmosphere by evaporation, soil water salinity increases because salts become more concentrated in the remaining soil water. Thus, evapotranspiration (ET°) between irrigation periods can further increase salinity.
Landscape Water Budgets should include an adjustment for periodically increasing run times for deeper irrigation (beyond plant root zone profile) for leaching salts from the soil profile. This is called the leaching fraction. As water quality continues to decline (salinity levels rising) leaching will become more widely used. How much added water is needed is dependent on water quality, usable rainfall and irrigation scheduling.
WatrWise Consulting recommends (counter-intuitively) USING rainfall in arid climates to add additional water during an irrigation event (watering), to leach salts from plant root zones. Simply put – watering shortly before, during or after a rainfall event will facilitate the process of leaching salts in soil profiles without the cost of added irrigation.
Many landscape water managers focus on reducing the number of water applications by stopping irrigation before, during or after a rain event in an effort to conserve water without considering the long term affects on landscape health. See WatrWise Consulting’s Controller Scheduling.
USGS – United States Geological Survey