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Role of Groundwater Flow in Tile Drain Discharge

U.S. Salinity Laboratory, 450 W. Big Springs Road, Riverside, CA 92507.

^* Corresponding author (pvaughan{at}ussl.ars.usda.gov).

ABSTRACT

Tile systems drain water applied to agricultural fields as irrigation and precipitation but also may intercept regional groundwater flow. Identification and characterization of the potential sources of tile water is essential for informed management of salinity and contaminants. Factors influencing tile discharge including depth of water applied, evapotranspiration, water storage, drain blockage, and interception of regional groundwater flow were evaluated to determine which may be related to a fivefold variation in cumulative tile discharge among six sumps located 100 km west of Fresno, CA. Cumulative depths drained were calculated for 5 yr of weekly irrigation, precipitation, and discharge data. Evapotranspiration and water storage were estimated using the UnsatchemGeo variably-saturated water flow model. Well water levels measured on 19 dates were spatially-averaged providing spatial variation of depth-to-water among the drained areas. Variability in depth of water drained (0.18–0.95 m) was large and was not correlated with either water applied (3.26–4.58 m, r² = 0.03) or with computed water flux from the bottom of the soil column (0.05–0.31 m, r² = 0.00). Groundwater interception by tile drains was a factor because depth-to-water was negatively correlated with discharge (r² = 0.42) and drawdown of groundwater levels by drains was relatively larger for those drained areas encountered first during regional groundwater flow. For all six sumps, drained water is likely derived from locally applied water and interception of regional groundwater flow implying that standard two-dimensional models of flow to drains, representing only water applied locally, would not be applicable to modeling of drain flows or drain-water solute concentrations.