Abstract
The Lowland Groundwater-Surface water Interaction model (LGSI-model) is a lumped model
concept which describes changes in storage via changes in the average groundwater depth
and spatial variation in groundwater depth. These characteristics of the groundwater depth
are used to calculate discharge. The spatial distribution of groundwater depths are described
by means of a normal
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distribution that is related to the average groundwater depth within the
catchment. The LGSI-model was first tested by Van der Velde et al. (2009) in the Hupsel
brook catchment (6km2) and showed very promising results. The goal of the study described
in this report is to apply the theory and model concepts developed by Van der Velde et al.
(2009) to a larger catchment with a more complex geomorphology. For this purpose the
Drentsche Aa catchment was selected.
To build the lumped LGSI-model, the relations between groundwater depth distributions and
fluxes can be calculated based on a spatially distributed groundwater. In this case, the
spatially distributed model MIPWA, that includes the Drentsche Aa catchment, was used.
Based on the MIPWA model results on a daily basis for the period 1989-2001, relations
between storage above the surface (negative groundwater depths) and water fluxes between
groundwater and surface water (e.g. stream discharge, tube drain discharge, saturated
overland flow, evapotranspiration) could be formulated. These relations formed the basis of
the LGSI-model of the Drentsche Aa Catchment. To improve the representation of the
discharge generating processes within the catchment, the LGSI-model of the Drentsche Aa
catchment was calibrated to a measured discharge series of the period 1989-2001 using the
GLUE analysis.
Due to the variations in the geomorphology of the catchment, the Drentsche Aa catchment
had to be divided into two different areas. One low area with shallow groundwater tables
reaching up to the surface and high, very dry areas where the groundwater tables never
reach the surface. These two area types, or reservoirs, could be described by different
statistical distributions: in the low areas, a normal distribution was valid, while in the high
areas groundwater depth distributions were described by a gamma distribution. Both areas
were coupled using a Darcy based equation which was a modification to the original model
concept of Van der Velde et al. (2009).
Results of the LGSI-model of the Drentsche Aa catchment were very promising and the
Nash-Sutcliffe model efficiency for discharge and groundwater depth simulation for both low
and high areas were respectively 0.76 (discharge), 0.78 (groundwater depth low area) and
0.87 (groundwater depth high area) over the calibration period 1989-2001 (all at a daily time
steps). Peak discharges were slightly underestimated by the model, which is the result of
inaccurate simulation of the groundwater depth, whereas recessions and periods with small
discharge were modelled with high accuracy. A detailed analysis of the model stability was
performed, by extending the model simulation of discharge to the period 1980-2010 (Nash-
Sutcliffe discharge 0.64). The LGSI-model was also tested on a hourly time step to determine
if the model could have a good model performance on a smaller timescale (Nash-Sutcliffe
discharge 0.75).
The results of the fast calibration using the GLUE analysis can be useful for evaluation of
model parameters of the spatially distributed groundwater model. Since this type of calibration
cannot be done for spatially calibrated models without very long calculation times, the LGSI-model could be used as a tool for calibration of these models. Another interesting feature of
the LGSI-model concept is that it divides the total discharge of the catchment into its separate
components (groundwater exfiltration in rivers, saturated overland flow, drainage). This offers
the possibility to analyse the composition of peak discharge or determine the relative
contributions of the discharge components throughout the year.
It was concluded that the LGSI-model generated very good simulations of discharge and
groundwater depths in the Drentsche Aa catchment. Due to these simultaneous and rapid
model simulations of groundwater depth, storage and discharge, the LGSI-model concept
provides additional insight in the discharge behaviour for a variety of groundwater depth
distributions and parameter values. Moreover, the LGSI-model shows that knowledge of the
groundwater system and the groundwater-surface water interaction processes that occur in a
catchment is very useful, if not crucial, for good simulation and prediction of stream discharge
of a catchment.
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