Topographic controls on shallow groundwater levels in a steep, prealpine catchment: When are the TWI assumptions valid?

Abstract

Topographic indices like the Topographic Wetness Index (TWI) have been used to predict spatial patterns of average groundwater levels and to model the dynamics of the saturated zone during events (e.g., TOPMODEL). However, the assumptions underlying the use of the TWI in hydrological models, of which the most important is that groundwater level variation can be approximated by a series of steady state situations, are rarely tested. It is also not clear how well findings from existing hillslope studies on sites with transmissive soil can be transferred to entire catchments with less permeable soils. This study, therefore, evaluated the suitability of selected topographic indices to describe spatial groundwater level variations based on time series from 51 groundwater wells in a 20 ha catchment with low-permeability soils in Switzerland. Results showed that median groundwater levels were correlated to slope, curvature, and TWI, but the strength of correlation depended on whether the indices characterized the local topography or the topography of the upslope contributing area. The correlation between TWI and groundwater levels was not constant over time but decreased at the beginning of rainfall events, indicating large spatial differences in groundwater responses, and increased after peak flow, when groundwater levels could be considered to be spatially in a steady state. Our findings indicate that topographic indices are useful to predict median groundwater levels in catchments with low-permeability soils and that the TWI assumptions are best met when groundwater levels change slowly.

Contributing sources to baseflow in pre‐alpine headwaters using spatial snapshot sampling

“…Perennial key locations were chosen such as confluences of different branches (n=65 of which eight samples were taken along an artificial drainage ditch from a wetland (W) within WS04), springs (n=29) and groundwater wells (n=16) (Figure 1a). Groundwater samples were taken from fully-screened wells with an average depth of about 1 m (for details on the groundwater observation network see Rinderer et al, 2014).…”

“…Perennial key locations were chosen such as confluences of different branches (n=65 of which eight samples were taken along an artificial drainage ditch from a wetland (W) within WS04), springs (n=29) and groundwater wells (n=16) (Figure 1a). Groundwater samples were taken from fully-screened wells with an average depth of about 1 m (for details on the groundwater observation network see Rinderer et al, 2014).Similar to the subcatchment characteristics, for every sampling location different upslope controlling landscape features were derived such as the area (km 2 ), altitude (m), slope (°) and, topographic wetness index within a Geographic Information System (GIS) framework. Additionally, for each sampling location the upslope percentage of forest, meadow and wetlands were derived from the land use map.…”

Runoff generation in a pre-alpine catchment: A discussion between a tracer and a shallow groundwater hydrologist

“…The median response frequency of all groundwater monitoring sites for rainfall events between 2010 and 2014 was 84% (Rinderer et al, 2014). Water levels tended to be high at sites with a larger TWI and fluctuated more at sites with a lower TWI; the recession was faster for sites with a lower TWI (Fig.…”

“…5). A rating curve was established for each of the Studibach gauging stations using salt dilution gauging (see Rinderer et al (2014) for a more detailed description of the streamflow measurements). The dry and flowing stream network in the upper parts of WS04, WS07, WS18 and…”

“…This ensured a network of wells that is representative for the different landscape positions and morphological units. For more details on the groundwater measurement network, see Rinderer et al (2014;2016).…”