HGS RESEARCH HIGHLIGHT – Comparing alternative conceptual models for tile drains and soil heterogeneity for the simulation of tile drainage in agricultural catchments
Boico, V. F., Therrien, R., Delottier, H., Young, N. L., & Højberg, A. L. (2022). Comparing alternative conceptual models for tile drains and soil heterogeneity for the simulation of tile drainage in agricultural catchments. In Journal of Hydrology (Vol. 612, p. 128120). Elsevier BV. https://doi.org/10.1016/j.jhydrol.2022.128120
“The drainage discharge, stream generation and heads were simulated with the 3D fully-coupled surface water and groundwater flow model HydroGeoSphere (HGS). HGS has been used for the simulation of complex SW-GW systems at the catchment scale. ”
Fig. 1. Location of (a) the Fensholt catchment in Denmark, (b) the Fensholt catchment showing land use, tile drains, drainage areas and the drainage discharge measurement stations (D1 to D8), and (c) the topography (1.6 m digital elevation model), piezometers, subcatchments delineated for investigation of stream discharge with the correspondent outlet stations (S1, S2, S3 and S4) and gauging stations at the outlet and in the center of the catchment (S2).
A groundbreaking study conducted by Vinicius F. Boico, René Therrien, Hugo Delottier, Nathan L. Young and Anker L. Højberg presents a comprehensive exploration of tile drainage systems within agricultural catchments, with the goal of refining hydrological modeling methodologies.
Tile drains serve as crucial channels for managing water flow in agricultural landscapes, influencing hydrologic pathways and nutrient transport. However, accurately representing these intricate systems in hydrological models poses significant challenges, particularly when detailed data on drain locations are lacking.
This study compares four distinct conceptual models for simulating tile drainage in a Danish agricultural catchment. The first model, referred to as the Main Drain Model, utilizes seepage nodes to explicitly represent main collector drains, extending the representation to all tile-drained fields within the catchment. Conversely, the Distributed Model disperses seepage nodes across agricultural areas without specific drain location considerations, offering a simplified approach.
Additionally, the High-K Layer Model replaces drains with a high-permeability layer, implicitly simulating rapid flow in the drainage system. Lastly, the Benchmark Model explicitly represents all mapped tile drains with seepage nodes, serving as a comprehensive reference.
Using the HydroGeoSphere (HGS) modeling platform, the study evaluates the performance of these models in simulating drainage discharge, stream generation, and hydraulic heads. Surprisingly, all models exhibit satisfactory performance, suggesting their interchangeability for simulating tile drainage dynamics. Moreover, models utilizing seepage nodes demonstrate faster simulation times, making them viable options for large-scale applications and model calibration.
Fig. 4. Location of seepage nodes for the (a) Benchmark, (b) Main Drain, and (c) Distributed Models. The seepage nodes act as sinks extracting water from the subsurface and returning it to the surface at the drainage outlet (linked nodes). The drainage outlets D1 to D8 are labeled in the Figures.
The study explores the impact of soil heterogeneity on model simulations, revealing its significance at smaller scales. Overall, this research offers valuable insights into improving the representation of tile drainage in hydrological models, crucial for sustainable water management in agricultural landscapes.
Plain Language Summary:
This study explored the significance of tile drainage systems in agricultural settings, which are essential for managing water flow and nutrient transport. Despite their importance, accurately integrating these systems into hydrological models presents challenges, especially when detailed data on drain locations are lacking. Researchers compared various methods for representing tile drainage systems in computer models to identify the most effective approach. By improving the representation of tile drains in hydrological models, scientists aim to enhance predictions of water flow patterns and nutrient movement in agricultural areas, crucial for optimizing farming practices, mitigating environmental impacts, and fostering sustainable water management strategies for farming communities.
Abstract:
Tile drains are important water flow paths in agricultural catchments and must be included in hydrological models. However, their locations are rarely known and the explicit incorporation of tile drains in hydrological models requires refined meshes around the drains, which can significantly increase computational times. Although seepage nodes have been used to represent tile drains with satisfactory performance, they have never been applied to represent all tile drainage systems in a catchment. The goal of this study is to compare different conceptual models for tile drains and soil heterogeneity for the numerical simulation of tile drainage in an agricultural catchment in Denmark. The first conceptual model for tile drains uses seepage nodes to represent only the main collector drains in the catchment and the second model uses seepage nodes distributed over all the agricultural areas, without considering the specific locations of tile drains. A third conceptual model, labelled the Benchmark Model, represent all tile drains at their known locations with seepage nodes and a fourth conceptual model implicitly represents tile drains as a high-permeability layer. The four models performed satisfactorily to simulate the observed outlet stream discharge and could be recommended almost interchangeably. The simulation of the water table depth was very satisfactory compared to modeling studies with similar mesh resolution (∼50 m). Results indicated that the three models using seepage nodes i) simulated similar monthly discharges and cumulative discharge volumes for most of the studied tile-drained areas, and ii) simulated surface water flow in tile-drained fields without runoff or ponding water. The shorter simulation times (around 35% faster) of the model representing the main drains and the distributed seepage node model suggest that they are suitable for model calibration, compared to the Benchmark Model. Whenever the location of tile drains is unavailable, using seepage node to represent drains in agricultural areas may satisfactorily simulate catchment-scale stream and drainage discharges. Four alternative soil models were developed to evaluate the effect of soil heterogeneity on the simulations. Our results suggest that at smaller scales (drainage area) soil heterogeneity is more relevant than the drainage conceptualization to improve model results. However, at the subcatchment scale, the opposite was observed and, at the catchment scale, both criteria had a comparable effect.
“Tile drains considerably affect the SW-GW exchange flux patterns reducing surface runoff, lowering the groundwater table level and shortening periods of surface ponding which justify the choice of a fully integrated surface–subsurface model. Recent 3D hydrological models including tile drainage using HydroGeoSphere were published by De Schepper et al., 2015, Thomas et al., 2016, De Schepper et al., 2017, Hwang et al., 2019.”
Fig. 9. Simulated and observed cumulative volumes during the 2014–2015 period at the drainage outlet stations D1 to D8 (red dots), stream stations S1 to S4 and the outlet (black dots).