Importance of geological information for assessing drain flow in a Danish till landscape

Abstract

There has, in recent years, been an increasing interest in developing nutrient load mitigation measures focussing on tile drains. To plan the location of such tile drain measures, it is important to know where in the landscape drain flow is generated and to understand the key factors governing drain flow dynamics. In the present study, we test two approaches to assess spatial patterns in drain flow generation and thereby assess the importance of including geological information. The approaches are the widely used topographical wetness index (TWI), based solely on elevation data, and hydrological models that include the subsurface geology. We set‐up an ensemble of 20 hydrological models based on 20 stochastically generated geological models to predict drain flow dynamics in the clay till Norsminde catchment in Denmark and test the results against TWI. We find that the hydrological models predict observed daily drain flow reasonably well. High drain flow volumes were found in stream valleys and in wetlands and lower drain flow volumes in the more hilly parts of the catchment. In spite of the apparent connection to the landscape, there was no statistically significant correlation between TWI and drain flow at grid scale (100 × 100 m). TWI was therefore not found to be a sufficient index on its own to assess where drain flow is generated, especially in the highlands of the catchment. The geology below 3 m was found to have a large impact on the drain flow, and correlations between sand percentage in the subsurface geology and drain flow volume were found to be statistically significant. Geological uncertainty therefore give rise to uncertainty on simulated drain flow, and this uncertainty was found to be high at the model grid scale but decreasing with increasing scale.

Hunting for Information in Streamflow Signatures to Improve Modelled Drainage

“…Tile drain provides a short-cut from the field to surface water bodies, bypassing transport in the deep aquifers. This is crucial to nitrate transport, as nitrate can be reduced only under anaerobic conditions, which primarily are found in the deeper groundwater systems [7][8][9]. Consequently, insight into the amount and dynamics of drain flow is essential to understand and quantify transport of nitrate or other substances.…”

“…(3) Likewise, due to the coarse spatial resolution of large-scale hydrological models, tile drain processes cannot be represented explicitly in the model (for a discussion see [8,12]). The resulting simplified representation of drain processes in large-scale models lead to an aggregation of hydrological processes: typically, drain is described implicitly, and also accounts for underrepresentation of the surface water network in the model.…”

“…In heavily drained agricultural areas, it contributes to a significant share of total stream discharge [5,35,36]. Furthermore, drain flow has specific characteristics, such as quick responses to precipitation events and interactions with the groundwater table dynamics [8,16]. Hence, drain flow will have an effect on the hydrograph [37], even in streams that aggregate contributions from a variety of runoff-generating processes.…”

Mapping of Agricultural Subsurface Drainage Systems Using a Frequency-Domain Ground Penetrating Radar and Evaluating Its Performance Using a Single-Frequency Multi-Receiver Electromagnetic Induction Instrument

“…However, the excessive leaching of nutrients and pesticides through the percolation of solutes through the root zone to drainage pipes is a potential risk for eutrophication and contamination of the surface water bodies [ 5 , 6 , 7 ]. In Denmark, considerable attention is being directed towards the role of drainage systems in the transport and leaching of nutrients and pesticides to the aquatic environment [ 8 ]. Nevertheless, due to limited information on subsurface drainage installations, it is difficult to understand the hydrology and solute dynamics and plan effective mitigation strategies, such as constructed wetlands, saturated buffer zones, bioreactors, and nitrate and phosphate filters [ 9 , 10 , 11 , 12 , 13 ].…”

Mapping of Agricultural Subsurface Drainage Systems Using a Frequency-Domain Ground Penetrating Radar and Evaluating Its Performance Using a Single-Frequency Multi-Receiver Electromagnetic Induction Instrument

“…However, the excessive leaching of nutrients and pesticides through the percolation of solutes through the root zone to drainage pipes is a potential risk for eutrophication and contamination of the surface water bodies [5][6][7]. In Denmark, considerable attention is being directed towards the role of drainage systems in the transport and leaching of nutrients and pesticides to the aquatic environment [8]. Nevertheless, due to limited information on subsurface drainage installations, it is difficult to understand the hydrology and solute dynamics and plan effective mitigation strategies, such as constructed wetlands, saturated buffer zones, bioreactors, and nitrate and phosphate filters [9][10][11][12][13].…”