Using observed soil moisture to constrain the uncertainty of simulated
hydrological fluxes
Abstract
Using data from five long-term field sites measuring soil moisture, we
show the limitations of using soil moisture observations alone to
constrain modelled hydrological fluxes. We test a land surface model,
MESH/CLASS, with two configurations: one where the soil hydraulic
properties are determined using a pedotransfer function (the
texture-based calibration) and one where they are assigned directly (the
hydraulic properties-based calibration). The hydraulic properties-based
calibration outperforms the texture-based calibration in terms of
reproducing changes in soil moisture storage within a 1.6 m deep profile
at each site, but both perform reasonably well, especially in the summer
months. When the models are constrained using observations of changes in
soil moisture, the predicted hydrological fluxes are subject to very
large uncertainties associated with equifinality. The uncertainty is
larger for the hydraulic properties-based calibration, even though the
performance was better. We argue that since the pedotransfer functions
constrain the model parameters in the texture-based calibrations in an
unrealistic way, the texture-based calibration underestimates the
uncertainty in the fluxes. We recommend that reproducing observed
cumulative changes in soil moisture storage should be considered a
necessary but insufficient criterion of model success. Additional
sources of information are needed to reduce uncertainties, and these
could include improved estimation of the soil hydraulic properties and
direct observations of fluxes, particularly evapotranspiration.