Role of groundwater flow in tile drain discharge

Tile systems drain water applied to agricultural fields as irrigation and p recipitation but also may intercept regional groundwater flow. Identificati on and characterization of the potential sources of the water is essential for informed management of salinity and contaminants. Factors influencing t ile discharge including depth of water applied, evapotranspiration, water s torage, drain blockage, and interception of regional groundwater flow were evaluated to determine which may be related to a fivefold variation in cumu lative tile discharge among six sumps located 100 km west of Fresno, CA. Cu mulative depths drained were calculated for 5 yr of weekly irrigation, prec ipitation, and discharge data. Evapotranspiration and water storage were es timated using the UnsatchemGeo variably-saturated water flow model. Well wa ter levels measured on 19 dates were spatially-averaged providing spatial v ariation 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 w ater applied (3.26-4.58 m, r(2) = 0.03) or with computed water flux from th e bottom of the soil column (0.05-0.31 m, r(2) = 0.00). Groundwater interce ption by tile drains was a factor because depth-to-mater was negatively cor related with discharge (r(2) = 0.42) and drawdown of groundwater levels by drains was relatively larger for those drained areas encountered first duri ng regional groundwater flow. For all six sumps, drained water is likely de rived front locally applied water and interception of regional groundwater flow implying that standard two-dimensional models of flow to drains, repre senting only water applied locally, would not be applicable to modeling of drain flows or drain-water solute concentrations.