Hydrometric and isotopic variability of tile drainage discharge in a small agricultural catchment

Understanding the discharge dynamics of tile drains is crucial to better manage water resources in agricultural areas. This study aimed to compare two tile-drainage systems (Sys4, Frau2) in a small agricultural catchment at the Hydrological Open Air Laboratory in Lower Austria by utilizing six years of high-frequency (10 minutes – 2 hours) hydrometric and isotopic observations, to investigate if their discharge is dominated by new or old water and whether new-water fractions correlate with certain hydrometric characteristics. One tile drain system, Sys4, was a perennial system with a larger drainage area, pipes with larger diameter and a simpler topology. The other tile drain, Frau2, was an ephemeral system with a smaller drainage area, pipes with smaller diameter and a complex topology. The flashiness, time to peak flow, soil moisture and groundwater dynamics were evaluated for the two tile drainage systems. Peak flow new water fractions were estimated by stable isotopes using both two-component (IHS) and ensemble hydrograph separations (EHS). The results indicated clear differences between the discharge dynamics and new water fractions of the two tile drains. Sys4 responded rapidly to even small amounts of rainfall, with 0.9 h median time to peak. The response here was generally independent from the soil moisture state and the depth to the groundwater table. Frau2 had the flashiest behavior with an average Richard-Baker Index of 0.52. Discharge at Frau2 depended rather on rainfall amount than rainfall intensity, and larger discharge peaks occurred only above a soil moisture (0.35 m3/m3) and groundwater level threshold (0.3 m below ground surface). The largest average peak flow new water fractions were obtained for Sys4 (with IHS average 0.54, with EHS 0.66 for δ18O) compared to Frau2 (with IHS average 0.47, with EHS 0.35 for δ18O). The differences in the hydrometric and isotopic characteristics of the drains can be explained by differences in their construction properties, drainage areas and drainage densities. Discharge from drainage systems with a larger area but smaller drainage density, with a simpler topology and pipes with larger diameter had faster response to rainfall and larger new water fractions.