Why are long-term storage variations observed but not modelled in the
Luangwa basin?
- Petra Hulsman,
- Hubert H. G. Savenije,
- Markus Hrachowitz
Abstract
In the Luangwa basin, long-term total water storage variations were
observed with GRACE, but not reproduced by a standard conceptual
hydrological model that encapsulates our current understanding of the
dominant regional hydrological processes. The objective of this paper
was to identify potential processes underlying these low-frequency
variations through combined data analysis and model hypothesis testing.
First, we analysed the effect of data uncertainty by contrasting
observed storage variations with multi-annual estimates of precipitation
and evaporation from multiple data sources. Second, we analysed four
different combinations of model forcing and evaluated their skill to
reproduce the observed long-term storage variations. Third, we
formulated alternative model hypotheses for groundwater export to
potentially explain low-frequency storage variations. Overall, the
results suggest that the initial model's inability to reproduce the
observed low-frequency storage variations was partly due to the forcing
data used and partly due to the missing representation of regional
groundwater export. More specifically, the choice of data source
affected the model's ability to reproduce annual maximum storage
fluctuations, whereas the annual minima improved by adapting the model
structure to allow for groundwater export from a deeper groundwater
layer. This suggests that, in contrast to previous research, conceptual
models can reproduce long-term storage fluctuations if a suitable model
structure is used. Overall, the results highlight the value of
alternative data sources and iterative testing of model structural
hypotheses to improve runoff predictions in a poorly gauged basin
leading to enhanced understanding of its hydrological processes.