Abstract
Calibration of large-scale groundwater models is difficult due to a general lack of groundwater
head measurements. The calibration becomes even more complex if the basin covers several
countries with different observation systems and different base maps, or if calibration must be
done in developing countries with poor data availability.
In this study, we present
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and discuss a novel approach for calibrating a large-scale groundwater
model using various satellite remote sensing products without using groundwater head
data. The study area is the combined Rhine-Meuse basin located in Western Europe and covering
a total area of around 200.000 km2. For this basin an extensive groundwater head database is
available. However, calibration of the groundwater model was carried out using only remote
sensing products while the head data were solely used for model validation. The groundwater
model itself uses as input only global datasets so that the modeling procedure is portable to
other areas on the globe including data poor environments such as developing countries.
Our hydrological model consists of two parts:
(1) the land surface model, which conceptualizes the upper first meter of the unsaturated
zone soil layer; and
(2) the MODFLOW-based groundwater model simulating the saturated horizontal flow in the
deeper layer.
Both model parts together simulate the dynamic interaction between surface water and
groundwater bodies, and between the unsaturated soil and the saturated groundwater zone.
We ran the model for the period 1960 to 2008. Calibration is carried out by adjusting aquifer
characteristics and soil physical parameters determining flow in the unsaturated zone.
State variables used in calibration is the unsaturated zone/soil moisture storage and total
terrestrial water storage. Two promising remote sensing products are considered: for soil moisture
the European Remote Sensing (ERS) scatterometer derived Soil Water Index (SWI) products and
for total basin-average water storage the GRACE monthly gravity field solutions. Time series of
ERS-SWI are available from 1992 onwards. GRACE data are available from 2002 to 2008. The time
series are used because our previous studies showed a strong correlation between timeseries
of remote sensing products and timeseries of groundwater head measurements. We used the
time series of GRACE to constrain anomalies of total basin water storage including groundwater,
unsaturated zone/soil moisture, canopy, surface water, and snow storages. Finally, the resulting
groundwater head of the calibrated model is verified to available groundwater head measurement
data.
Results are very promising and suggest that it is possible to calibrate and constrain
large-scale groundwater models using satellite remote sensing derived products. Comparing
uncalibrated and calibrated model simulations with observed groundwater head data, we
conclude that important model improvements can be made by integrating models with
remote sensing products, particularly for areas with shallow water tables. We argue that, in the
absence of groundwater head measurement data, satellite remote sensing products are useful
products for calibrating groundwater models
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