Hydraulic model calibration using CryoSat-2 observations in the Zambezi
catchment
Abstract
Geodetic altimeters provide unique observations of the river surface
longitudinal profile due to their long repeat periods and densely spaced
ground tracks. This information is valuable for calibrating hydraulic
model parameters, and thus for producing reliable simulations of water
level for flood forecasting and river management, particularly in poorly
instrumented catchments. In this study, we present an efficient
calibration approach for hydraulic models based on a steady-state
hydraulic solver and CryoSat-2 observations. In order to ensure that
only coherent forcing/observation pairs are considered in the
calibration, we first propose an outlier filtering approach for
CryoSat-2 observations in data-scarce regions using simulated runoff
produced by a hydrologic model. In the hydraulic calibration, a
steady-state solver computes the WSE profile along the river for
selected discharges corresponding to the days of CryoSat-2 overpass. In
synthetic calibration experiments, the global search algorithm generally
recovers the true parameter values in portions of the river where
observations are available, illustrating the benefit of dense spatial
sampling from geodetic altimetry. The most sensitive parameters are the
bed elevations. In calibration experiments with real CryoSat-2 data,
validation performance against both Sentinel-3 WSE and in-situ records
is similar to previous studies, with RMSD ranging from 0.43 to 1.14 m
against Sentinel-3 and 0.60 to 0.73 against in-situ WSE observations.
Performance remains similar when transferring parameters to a
one-dimensional hydrodynamic model. Because the approach is
computationally efficient, model parameters can be inverted at high
spatial resolution to fully exploit the information contained in
geodetic CryoSat-2 altimetry.