Field-Scale Monitoring of the Long-Term Impact and Sustainability of Drainage Water Reuse Using ECa-Directed Soil Sampling.

Diminishing freshwater resources have brought attention to the reuse of degraded water as a water resource rather than a disposal problem.  Drainage water from tile-drained, irrigated agricultural land is degraded water that is often in large supply, but the long-term impact and sustainability of its reuse on soil is unknown.  Similarly, nothing is known of the ramifications of terminating drainage water reuse.  The objective of this study is (i) to monitor the long-term impact and sustainability of drainage water reuse on a marginally productive, saline-sodic, 32.4-ha field located on the west side of California’s productive San Joaquin Valley and (ii) to assess spatially what happens to soil when drainage water reuse is terminated. The monitoring and assessment were based on spatial soils data collected during 10 years of irrigation with drainage water followed by 2 years of no applied irrigation water (only rainfall).  Geo-referenced measurements of apparent soil electrical conductivity (EC_a) were used to direct the soil sampling design.  Physical and chemical analyses of soil samples were used (i) to characterize the spatial variability of salinity, Na, B, and Mo, which were identified as critical to the yield and quality of Bermuda grass (Cynodon dactylon (l.) Pers.) grown for livestock consumption, and (ii) to monitor their change during the 12-year study.  Soil samples were taken at 0.3-m increments to a depth of 1.2 m at each of 40 sample sites on five occasions: August 1999, April 2002, November 2004, August 2009, and May 2011.  Drainage water varying in salinity (1.8–16.3 dS m^-1), SAR (5.2-52.4), Mo (80–400 µg L^-1), and B (0.4-15.1 mg L^-1) was applied from July 2000 to June 2009.  Results indicate that salts, Na, Mo, and B were leached from the root zone causing a significant improvement in soil quality from 1999 to 2009.  Salinity and SAR returned to original levels or higher in less than two years after termination of irrigation. Boron and Mo showed significant increases.  Long-term sustainability of drainage water reuse is supported by the results, but once application of irrigation water is terminated, the field quickly returns to its original saline-sodic condition.