Importance of small geological features for simulated spatial patterns in tile drain flow in Fensholt catchment of Denmark

Abstract

Study region: Fensholt, Denmark. Study focus: Tile drains are commonly used in agricultural fields with loamy soils in Denmark to improve crop yield by removing excessive water. Spatial patterns of drain flows are controlled by the climate, geology, topography, and tile installation. We assessed the combined effect of topography and geology on spatial pattern of tile drain flows in 10 m resolution using numerical modelling. We developed three groundwater models using different geological models by integrating high-resolution data from geophysical methods with field estimated hydraulic conductivity. New insights: The mapping and modelling revealed small geological features of higher hydraulic conductivity in clayey-till. The results showed that the spatial patterns of drain flows to recharge ratio (drainage fraction, DF) are driven by topography; the models had a high DF in local depressions and a low DF in local hills. The DF was related to the Topographical Position Index (TPI), suggesting that the DF is controlled by small-scale topography both upstream and downstream of the study area. We found that geology amplifies the spatial patterns of tile drain flows; a higher hydraulic conductivity relative to a lower hydraulic conductivity increases the change of tile drain flow for a one-unit change in the TPI. This was attributed to a change from small-scale flow systems to field-scale flow systems. The study suggested that topography helps to delineate high and low DF while geology controls the magnitude of DF. The study emphasized the importance of mapping and modeling of geology for managing moraine agricultural areas that can be found in parts of North America and Scandinavia for agricultural water management.