Use of em-38 soil surveys in forage fields at a saline drainage water reuse site to calibrate a hydro-salinity model for decision support

Soil salinity is a major factor affecting irrigated agriculture in the western San Joaquin Valley of California. Soil salinity is a spatially and temporally dynamic property, and thus, mapping at the field scale requires a rapid and reliable means of taking geospatial measurements. EM-38 soil salinity surveys were conducted at the SJRIP (San Joaquin River Improvement Project) facility managed by the Panoche Water District (Los Banos, California) where subsurface drainage water is re-used on forages such as ‘Jose’ tall wheatgrass (Thinopyrum ponticum var. ‘Jose’) and alfalfa (Medicago sativa) to reduce drainage discharge and salt loading into the San Joaquin River. Soil samples taken to a depth of 120 cm (4 ft.) in 30 cm (1 ft.) increments for calibration of EM-38 data, were analyzed for pH, ECe, gravimetric water content and saturation percentage. The average ECe for spring and fall 2016 samples was 12.5 to 19.3 dS/m for tall wheatgrass (TWG) fields and 8.9 to 14.4 dS/m for alfalfa (ALF) fields. In 2017, the average ECe ranged from 14.4 to 18.6 ds/m and from 9.5 to 13.3 ds/m for TWG and ALF fields, respectively. GIS maps were developed depicting the spatial variability of salts in the fields. Data will be used to calibrate a computer model (CSUID) developed as a decision support tool to optimize soil leaching requirement guidelines for irrigation water of varying salinity levels, with the overall goal of improving the sustainability of forage production using saline waters in the SJRIP.