Evapotranspiration potential of forages irrigated with saline-sodic drainage water

In the western San Joaquin Valley (SJV) of California, re-use of saline-sodic drainage water (DW) for the irrigation of salt tolerant forages and field crops is an important strategy for drainage water reduction and to extend irrigation water supplies. Drainage reuse designs require estimates of crop water use under saline conditions, so this study employed two methods to estimate the actual evapotranspiration (ETa) of saline-irrigated forages. Using sand-filled drainage lysimeters, ET was measured for ‘Jose’ tall wheatgrass (TWG; Thinopyrum ponticum var. ‘Jose’) creeping wildrye (CWR; Leymus triticoides, var. ‘Rio’) and Paspalum (Paspalum vaginatum, var. ‘SeaIsle’) irrigated with saline DW with an electrical conductivity (EC) averaging 13.2 dS m−1. Annual ET in 2005 (20 January to 28 December) was 1470 mm for TWG, 1376 mm for CWR, and 1275 mm for Paspalum. Comparing to standardized reference evapotranspiration (ETo), the annual (ETa/ETo)lys ratios in 2005 were 0.98 for TWG, 0.92 for CWR, and 0.85 for Paspalum. Forage ET was also estimated by a surface renewal (SR) method in large pastures of TWG and CWR irrigated with saline DW. For TWG, the seasonal (ETa/ETo)SR in 2005 was 0.94, slightly lower than the ratio of 0.98 determined by lysimetry. For CWR, the seasonal (ETa/ETo)SR was only 0.73 in 2005; however, in 2006 it was 1.03 which was closer to the ratio of 0.92 determined by lysimetry in 2005. Conditions were less uniform in the CWR pasture as compared to the TWG pasture which may have caused the greater variability in (ETa/ETo)SR for this forage. These findings, along with earlier work, demonstrate that tall wheatgrass, creeping wildrye and Paspalum are suitable candidates for DW re-use systems for the western SJV due to their high ET under saline irrigation and adequate dry matter production and forage quality.

Surface renewal system and field-measured evapotranspiration (ETa) of salt-stressed tall wheatgrass (cv. ‘Jose’) (Thinopyrum ponticum) in relation to reference evapotranspiration (ETo). Under higher ETo conditions (>4 mm day−1), the ETa/ETo ratio was substantially lower, possibly due to a combination of salinity and moisture stress.Figure optionsDownload as PowerPoint slideHighlights
► The ETa/ETo (actual lysimeter ET vs. reference ET) of salt-stressed ‘Jose’ tall wheatgrass was 0.98.
► The ETa/ETo by surface renewal ET estimation for wheatgrass grown in a saline pasture was 0.95.
► Surface renewal can provide accurate ETa/ETo estimates under non-uniform, saline conditions.
► Both tall wheatgrass and creeping wildrye were deemed suitable as forages for saline (>8 dS/m) irrigation.