Representation of hydrological processes in a rural lowland catchment in Northern Germany using SWAT and SWAT+

Authors: WAGNER, PAUL - University Of Kiel; BIEGER, KATRIN - Aarhus University; Arnold, Jeffrey; FOHRER, NICOLA - University Of Kiel

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2022
Publication Date: 5/25/2022
Citation: Wagner, P.D., Bieger, K., Arnold, J.G., Fohrer, N. 2022. Representation of hydrological processes in a rural lowland catchment in Northern Germany using SWAT and SWAT+. Hydrological Processes. 36(5). Article 14589. https://doi.org/10.1002/hyp.14589.
DOI: https://doi.org/10.1002/hyp.14589

Interpretive Summary: The Soil and Water Assessment Tool (SWAT) is used extensively in the US and globally to determine the environmental impacts of climate and land use change to determine scientifically-sound conservation policy. One limitation of SWAT is its ability to differentiate hydrological processes in the uplands and flood plains (lowlands) of a watershed. To overcome this limitation, SWAT was restructured into SWAT+, which allows water, sediment, and contaminant transport from the uplands to lowlands. In this study, SWAT+ was set up to delineate and route runoff through flood plains and compared to an earlier version of SWAT that did not. The models were tested on a rural lowland catchment in Northern Germany, characterized by near-surface groundwater tables and extensive tile drainage. Model results showed that the SWAT+ model set up more accurately simulated streamflow, tile flow, and water balance across the landscape. The revised model will provide more accurate model results and improved policy decision making.

Technical Abstract: The hydrology of rural lowland catchments in Northern Germany is characterized by near-surface groundwater tables and extensive tile drainage. The latest version of the Soil and Water Assessment Tool (SWAT+) features several improvements compared to previous versions of the model, e.g. the definition of landscape units that allow for a better representation of spatio-temporal dynamics. To evaluate the new model capabilities for land-water interactions in lowland catchments, we assess the performance of SWAT+ in comparison to previous SWAT model applications in the Kielstau Catchment in Northern Germany. The Kielstau catchment has an area of about 50 km², is dominated by agricultural land use, and has been thoroughly monitored since 2005. We explore the capabilities of SWAT+ in terms of watershed configuration and simulation of landscape processes by comparing two SWAT+ models and a previous SWAT model version (SWAT3S). In both SWAT+ setups, the catchment is divided into upland areas and floodplains, but in the first SWAT+ model setup, runoff from the hydrologic response units is summed up at landscape unit level and added directly to the stream. In the second SWAT+ model, runoff is routed across the landscape before it reaches the streams. Model results are compared with regard to model performance for stream flow at the outlet of the catchment and aggregated as well as temporally and spatially distributed water balance components. All three models show a very good performance at the catchment outlet. In comparison to a previous version of the SWAT model that produced more groundwater flow, the SWAT+ model produced more tile drainage flow and surface runoff. Results from the new SWAT+ model confirm that the representation of routing processes from uplands to floodplains in the model further improved the representation of hydrological processes. Particularly, the stronger spatial heterogeneity that can be related to characteristics of the landscape, is very promising for a better understanding and model representation of hydrological fluxes in lowland areas. The outcomes of this study are expected to further prove the applicability of SWAT+ and provide useful information for future model development.