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Mapping the extent of river valleys and the groundwater-surface water interaction

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

River valleys are significant hydrologically, geomorphologically, and ecologically because they act as buffers for the river against upland nutrient loading from agriculture and other sources. They are generally flat, low-lying areas of the landscape next to the river and separated from the uplands by rising slopes. These regions feature a groundwater table that is close to the soil surface under natural conditions. When researching groundwater-surface water interactions, the river valley is critical. This is because it serves as a link between the upland and the stream adjacent to it. Water and nutrients are carried from the upland to the neighboring stream via the river valley. As a result, it serves a critical role in protecting its stream's quality against polluted water flowing from the uplands.

As a first step towards mapping groundwater-surface water interaction, we used geographic information system techniques to delineate the bottom of river valleys within drainage basins and demonstrated how it could be used to carry out a nationwide delineation. We created a tool that uses a spatial movement algorithm called cost distance accumulation with spatial data inputs that included a river network feature layer and slope raster obtained from a digital elevation model. We then used wetlands adjacent to rivers to determine the river valley bottom boundary from each catchment's cost distance accumulation raster.

We then validated the delineated river valley bottom by first carrying out a visual validation using transects. The cross-section of the transects showed the river valley bottom at the lowest part of the terrain with minor exaggerations for both wide and narrow valleys. We also validated using groundwater-dependent terrestrial ecosystems (GWDTE). In Denmark, these nature types are represented by meadow and bog and have been mapped historically from the 19th century. Our delineated river valley bottom overlaps about half of these areas when we overlap it with a map from the 19th century, and the overlap area decreases as we compare it to newer maps, signaling a loss in GWDTE. We attributed the loss to the reclamation of Denmark's low-lying areas for agricultural purposes. Additionally, mapping the loss of GWDTE for streams with the same stream order reveals that the loss is more prevalent upstream of the river network, indicating that river valleys upstream are losing more of their GWDTE than those located downstream. Since most of the loss currently occurs due to tile drainage resulting from agriculture, and groundwater abstraction, caution should be exercised when designating upstream areas for these purposes.

We developed a geospatial methodology to map elements of the groundwater-surface water interaction using an established typology. The typology framework explains the groundwater-surface water contact and flow paths that are typical within the river valley. Following guidelines from the typology, we mapped three groundwater-surface water contact classes: connected clayey, connected sandy, and disconnected, and three flow path classes: diffuse and/or overland, direct, and artificial drainage. Results revealed that 85% of streams in Denmark are connected to the groundwater, with roughly similar proportions of them flowing in sandy and clayey subsoils. Additionally, around 87% of these streams exchange water with the groundwater, either alone or in conjunction with other flow paths. Additionally, 41% and 19% of stream sections get riparian flow water via artificial drainage and diffuse/overland flow pathways, respectively. These figures correlate to Denmark's land usage, which is around 60% agricultural, therefore limiting runoff capacity, and 50% artificially drained. We believe that the maps will aid policymakers in regulating water and pollution transfer from uplands to streams, thereby ensuring that surface and groundwater bodies maintain a healthy environmental state.
ForlagAarhus Universitet
Antal sider142
StatusUdgivet - okt. 2021

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