The importance of subsurface drainage on model performance and water balance in an agricultural catchment using SWAT and SWAT-MODFLOW

Artificial subsurface drainage impacts the hydrological system, but its spatial distribution is typically uncertain, hindering the understanding of the hydrological behaviour of a certain catchment. In this study, the effects of using four different sources to delineate the spatial distribution of subsurface drainage systems on hydrological modelling performance and simulated water balance were assessed. This work was addressed using SWAT and SWAT-MODFLOW (that is, four SWAT models and four SWAT-MODFLOW models were set up and evaluated). The spatial distribution of subsurface drainage systems were based on (1) low electrical resistivity (as a proxy for clay content) in the top 4 m of the subsurface; (2) historical maps from contractors, which conducted the drainage work; (3) settings from previous successful SWAT set-ups in Denmark; and (4) similar settings as in scenario 3, but adjusted according to the national drained area probability map in Denmark. Scenarios 1 and 4 and scenarios 2 and 3, respectively, showed similar fractions of subsurface drained areas out of the total catchment area (large and small, respectively). Scenarios 2 and 3 showed better statistical performance with SWAT, while scenarios 1 and 4 performed better using SWAT-MODFLOW. Best statistical and graphical performances were obtained with the large drain fraction scenarios and when using the coupled model. First, low drain fraction scenarios underestimated subsurface drainage flow to the stream. Besides, SWAT-MODFLOW performed better than SWAT because MODFLOW was able to simulate groundwater flow across the surface catchment, whereas SWAT by default is not capable of doing so. All things considered, impacts of different subsurface drainage scenarios on catchment hydrology are assessed using both SWAT and SWAT-MODFLOW for the first time in this work, revealing the relevance of this input step for satisfactory hydrological modelling and reinforcing previous authors findings regarding the better performance of the coupled model over SWAT, but being even more relevant in this none-zero balance catchment. The proposed method of modelling coupling, and delineating subsurface drained areas using geophysical data, is promising and can be adopted by other researchers to evaluate the water balance impact of land-use and climate changes around the world.