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Abstract
It is generally assumed that, in humid climates, the groundwater table is a subdued copy of the surface topography. However, the general groundwater table is unlikely to be affected by the microtopography as seen in high-resolution Digital Elevation Models (DEMs). So far, there has been little guidance on the best resolution DEM to use to determine the shape of the water table or the direction of shallow groundwater flow in headwater catchments. We, therefore, looked at the effects of DEM-smoothing and -aggregation on the calculated flow directions and derived catchment boundaries, and identified areas and landscape features for which the calculated flow directions are particularly sensitive to DEM smoothing or aggregation. For > 40 % of the area of the Krycklan study catchment, the calculated flow directions depend strongly on the degree of smoothing or aggregation of the DEM. The four main landscape features for which DEM smoothing or aggregation strongly affected the calculated flow directions were: local slopes in the opposite direction of the general slope, flat areas, ridges, and incised streambanks. To determine the effects of the changing flow directions on the derived catchment boundaries for the smoothed and aggregated DEMs, we calculated the drainage area for 40 locations, representing the outlets of catchments of varying sizes. The shape and size of the catchments of first-order streams were most affected by the processing of the DEM. These streams were often almost completely smoothed out during the DEM preprocessing steps. These shifts in catchment boundaries and drainage area would have a large effect on the water balance. This study thus highlights the need to carefully consider the effects of DEM smoothing or -aggregation on the calculated flow directions and drainage areas.
Highlights
In humid climates, the groundwater table is often assumed to be a subdued replicate of the surface topography (Condon and Maxwell, 2015; Haitjema and Mitchell-Bruker, 2005; Toth, 1962; Winter, 1999)
In areas where the groundwater table is close to the surface, the flow pathways are highly affected by this microtopography (e.g., Bresciani et al, 2016b; Frei et al, 2010; O’Loughlin and Resources, 1986; van der Ploeg et al, 2012) but at better drained hillslope sites, the general flow direction at the hillslope or catchment scale is unlikely to be affected by the microtopography (Marklund and Worman, 2011; Zijl, 1999)
As a first step, and in the absence of observed groundwater level data to determine the location of the groundwater table and flow directions, we focus here on quantifying the impact of Digital Elevation Models (DEMs) resolution on the calculated groundwater flow directions and catchment areas for an area where shallow groundwater is the main source of runoff
Summary
The groundwater table is often assumed to be a subdued replicate of the surface topography (Condon and Maxwell, 2015; Haitjema and Mitchell-Bruker, 2005; Toth, 1962; Winter, 1999). Digital Elevation Models (DEMs) can, be used to estimate the direction of groundwater flow (Haitjema and Mitchell-Bruker, 2005; Worman et al, 2006). For the identification of the direction of slope parallel groundwater flow and the delineation of the catchment boundaries, preprocessing of highresolution DEMs has, become a common practice. It includes smoothing, aggregation or resampling of the DEM, as well as the removal of sinks
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