Investigating summer flow paths in a Dutch agricultural field using high frequency direct measurements

•We physically separated and measured flow paths to an agricultural ditch.•High frequency measurements revealed dynamic origin of tile drain and ditch outflow.•Tile drains transport the majority of groundwater-derived salts to surface water.•Salinity variations explained by interaction of water velocity and wave celerity.•Surface water flushing demand was found to greatly exceed sprinkling demand. The search for management strategies to cope with projected water scarcity and water quality deterioration calls for a better understanding of the complex interaction between groundwater and surface water in agricultural catchments. We separately measured flow routes to tile drains and an agricultural ditch in a deep polder in the coastal region of the Netherlands, characterized by exfiltration of brackish regional groundwater flow and intake of diverted river water for irrigation and water quality improvement purposes. We simultaneously measured discharge, electrical conductivity and temperature of these separate flow routes at hourly frequencies, disclosing the complex and time-varying patterns and origins of tile drain and ditch exfiltration. Tile drainage could be characterized as a shallow flow system, showing a non-linear response to groundwater level changes. Tile drainage was fed primarily by meteoric water, but still transported the majority (80%) of groundwater-derived salt to surface water. In contrast, deep brackish groundwater exfiltrating directly in the ditch responded linearly to groundwater level variations and is part of a regional groundwater flow system. We could explain the observed salinity of exfiltrating drain and ditch water from the interaction between the fast-responding pressure distribution in the subsurface that determined groundwater flow paths (wave celerity), and the slow-responding groundwater salinity distribution (water velocity). We found water demand for maintaining water levels and diluting salinity through flushing to greatly exceed the actual sprinkling demand. Counterintuitively, flushing demand was found to be largest during precipitation events, suggesting the possibility of water savings by operational flushing control.