Leaching Salts from the Rootzone
Leaching is the transport of salts out of the root zone using irrigation water. The proportion of the irrigation water used specifically for this purpose is the leaching fraction (LF). The leaching fraction (LF) is water beyond that required for tree growth and crop development–that is, for evapotranspiration(ET) . It is also beyond that required to refill the rootzone, so it will percolate below the rootzone.
- Required Irrigation = tree growth requirement (ET) + Rootzone refill requirement + requirement for salinity control (LF)
The LF varies, depending on the salinity of the soil and the salinity of the irrigation water. High salinity in either translates into a higher LF to maintain acceptable salinity in the rootzone. The table below provides LF required to sustain a desired rootzone ECe for given irrigation water salinity ECw . Source: Pistachio Production Manual, 2008: adapted from Ayers, RS. D.W. Westcot. Water Quality for Agriculture. FAO Irrigation and Drainage paper 29 (reprinted 1989, 1994).
Table 1. Leaching Fraction (%) Requirement
|Desired Avg. Rootzone ECe|
To computer the LF using Table 1: Needed information is the amount of water consumed annually by the orchard. For the example below: assume a value of 42 inches/year
Required Irrigation water = ET*(1 + LF)
- When ECw is 1 dS/m, for an ECe goal of 3.0: 42*(1 + .05) = 44.1 inches annually
- When ECw is 1 dS/m, for an ECe goal of 1.0: 42*(1 + .33) = 55.9 inches annually
- When ECw is 2 dS/m, for an ECe goal of 3.0: 42*(2 + .05) = 86.1 inches annually
Annual or biannual sampling is advised to adjust a salinity management strategy, due to the factors unique to every orchard. It is also advised to establish an actual distribution uniformity (DU) for the orchard irrigation system by field evaluation (that is, by testing the flows and pressure across the system).
Usually, during the season, it is sufficient to irrigate to meet orchard ET needs plus the extra for system non-uniformity suggested below. With this strategy, salinity may rise during the season, but post harvest irrigation, winter rains and spring irrigation is usually sufficient to return salinity to acceptable levels.
- for border system with tail return: 15 – 25% over ET
- for drip system: 10 – 25% over ET
- for microsprinkler system: 6 – 15% over ET