| Variations of water stocks in the upper Zambezi river basin have
been determined by 2 different hydrological modelling approaches.
The purpose was to provide preliminary terrestrial storage
estimates in the upper Zambezi, which will be compared with estimates
derived from the Gravity Recovery And Climate Experiment (GRACE) in a future study.
The first modelling approach is GIS-based, distributed and
conceptual (STREAM). The second approach uses Lumped Elementary
Watersheds identified and modelled conceptually (LEW). The STREAM
model structure has been assessed using GLUE (Generalized
Likelihood Uncertainty Estimation) a posteriori to
determine parameter identifiability. The LEW approach could, in
addition, be tested for model structure, because computational
efforts of LEW are low.
Both models are threshold models, where the non-linear behaviour
of the Zambezi river basin is explained by a combination of
thresholds and linear reservoirs.
The models were forced by time series of gauged and interpolated
rainfall. Where available, runoff station data was used to
calibrate the models. Ungauged watersheds were generally given the
same parameter sets as their neighbouring calibrated watersheds.
It appeared that the LEW model structure could be improved by
applying GLUE iteratively. Eventually, it led to better
identifiability of parameters and consequently a better model
structure than the STREAM model. Hence, the final model structure
obtained better represents the true hydrology.
After calibration, both models show a comparable efficiency in
representing discharge. However the LEW model shows a far greater
storage amplitude than the STREAM model. This emphasizes the
storage uncertainty related to hydrological modelling in
data-scarce environments such as the Zambezi river basin. It
underlines the need and potential for independent observations of
terrestrial storage to enhance our understanding and modelling
capacity of the hydrological processes. GRACE could provide
orthogonal information that can help to constrain and further
enhance our models. In the near future, other remotely sensed data
sources will be used to force modelling efforts of the Zambezi
(e.g. satellite rainfall estimates) and to identify individual
storage components in the GRACE observations (e.g. altimeter lake
levels and microwave soil moisture). Ultimately, this will create
possibilities for state updating of regional hydrological models
using GRACE. |