Catchment-scale non-linear groundwater-surface water interactions in densely drained lowland catchments

Abstract

Abstract. Freely discharging lowland catchments are characterized by a strongly seasonal contracting and expanding system of discharging streams and ditches. Due to this rapidly changing active channel network, discharge and solute transport cannot be modeled by a single characteristic travel path, travel time distribution, unit hydrograph, or linear reservoir. We propose a systematic spatial averaging approach to derive catchment-scale storage and discharge from point-scale water balances. The effects of spatial heterogeneity in soil properties, vegetation, and drainage network are lumped and described by a relation between groundwater storage and the spatial probability distribution of groundwater depths with measurable parameters. The model describes how, in lowland catchments, the catchment-scale flux from groundwater to surface water via various flow routes is affected by a changing active channel network, the unsaturated zone and surface ponding. We used observations of groundwater levels and catchment discharge of a 6.6 km 2 Dutch watershed in combination with a high-resolution spatially distributed hydrological model to test the model approach. Good results were obtained when modeling hourly discharges for a period of eight years. The validity of the underlying assumptions still needs to be tested under different conditions and for catchments of various sizes. Nevertheless, at this stage the model can already improve monitoring efficiency of groundwater-surface water interactions.

Improving catchment discharge predictions by inferring flow route contributions from a nested-scale monitoring and model setup

“…Intensive agriculture in lowland catchments often leads to high nutrient losses and eutrophication of downstream waters (Oenema et al, 2007;Van der Molen, 1998;Vitousek et al, 2009). To identify effective measures to reduce these nutrient loads, the flow routes of water that enter a stream and their nutrient concentrations need to be quantified (Tiemeyer et al, 2010).…”

“…Many field-scale studies identified tube drain effluent as the major source of nitrate (Tiemeyer et al, 2006;Nangia et al, 2010;Rozemeijer et al, 2010c). However, the field scale at which these contributions can be directly measured (De Vos et al, 2000;Van der Velde et al, 2010a) often is not the scale of interest to water management authorities. Extrapolation of fields site results to entire catchments can easily lead to wrong conclusions as field sites can prove nonrepresentative of the patterns and processes that emerge at larger scales (Sivapalan, 2003;Soulsby et al, 2006;Didszun and Uhlenbrook, 2008).…”

“…In Van der Velde et al (2010a), we presented the results of a field-scale measurement setup that separated tile drain flow from overland flow and groundwater flow. We also measured discharges at two larger nested scales and showed that, rather than the actual measured volumes at the field site, the characteristic response of individual flow routes can be used to upscale the field-site flow routes to the catchment scale.…”

“…However, because of very long calculation times and the large number of parameters, groundwater models are inflexible and often inaccurate in simulating fluxes of specific flow routes at catchment scales. Hence, Van der Velde et al (2009) proposed a new upscaling approach for hydrology in lowland catchments (from here on called the Lowland GroundwaterSurface water Interaction model, LGSI-model). We assumed that each flow route (i.e., ditch and stream drainage, overland flow, and tube drain flow) starts to discharge if the groundwater level exceeds a flow route-specific threshold groundwater level at that location, and that the magnitude of the flux depends on the groundwater level.…”

“…The contribution of a flow route to the total catchment discharge is calculated by integration over all groundwater levels in the catchment, described by a groundwater depth distribution. Van der Velde et al (2009) showed that each storage volume of groundwater in the saturated zone corresponds to a unique groundwater depth distribution. They also showed that the relation between storage and groundwater depth distribution can be defined at any spatial scale and thus satisfies Sivapalan's (2003) criterion: it "easily connect scales, and can also be easily scaled".…”