Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Significance of Feedback in Land-Use Change Studies
J. Overgaard 1 , M. B. Butts 2 , D. Rosbjerg 1
1 Technical University of Denmark, Department of Hydrodynamics and Water Resources, Lyngby, Denmark, Jeo@er.dtu.dk
2 DHI Water and Environment, Hørsholm, Denmark
Traditionally, hydrological impact assessment of landuse
changes is carried out by forcing hydrological
models with time series of climate observations. The
underlying assumption is that a change in land-surface
properties will not influence on the atmospheric
conditions. However, it can be expected that a change
in land-surface properties will have a significant impact
on the local and down-wind atmospheric conditions.
These feedback mechanisms may work to either
dampen or amplify the sensitivity of the simulated
surface fluxes to changes in the land-surface properties
and hence dampen or amplify the impact of land use
changes.
This study seeks to quantify the effect of feedback on
model sensitivity using a non-hydrostatic atmospheric
meso-scale model coupled to a hydrological model
through a shared Soil-Vegetation-Atmosphere-Transfer
(SVAT) model. The effect of feedback for regional
scale land-use changes is investigated by applying the
modelling system in 1D mode. The simulation period
is 24 hours.
Initially, a sensitivity experiment is carried out for both
the coupled and uncoupled SVAT. Three land-use types
are investigated. In coupled mode, the climate is
dynamically simulated and in uncoupled mode, the
SVAT is forced by time series of simulated climate,
extracted from the coupled model for each land-use
type. This ensures that, for reference land-use
conditions, the coupled and uncoupled model yields
exactly the same results. Sensitivities for each of the
three land-use types are calculated and compared for
the coupled and uncoupled systems by perturbing the
vegetation parameters for each land-use conditions. The
difference in sensitivity indicates the significance of
atmospheric feedback. The sensitivity of latent heat
flux for three different land-use types (Agriculture,
natural vegetation and forest) to changes in stomata
resistance are shown in figure 1. Three different soilmoisture
conditions are (wet, intermediate and dry) are
represented.
Wet Intermediate Dry
Figure 1: Relative sensitivity [-] of latent heat flux to
changes in minimum stomata resistance in the coupled
(RS-c) and uncoupled (RS-uc) model system. The
white bars (F) show the ratio between the coupled and
uncoupled sensitivity.
The results in figure 1 show that atmospheric feedback has
a dampening effect on the sensitivity to a change in stomata
resistance, and reduces the relative sensitivity by up to
60%. Results from similar sensitivity experiments (not
shown here) show that atmospheric feedback increases
sensitivity of latent heat to a change in albedo by up to
70%, and dampens sensitivity to a change in leaf area index
by up to 30%.
To assess the consequences of not allowing atmospheric
feedback in regional land-use change studies, several landuse
change scenarios were carried out in both coupled and
uncoupled mode and the impact compared. Figure 2 shows
the difference in the predicted impact of a agriculture-toforest
land use change study.
Figure 2: The difference in the predicted impact of a
agriculture-to-forest land-use change study. The hatched
bars show the difference in the predicted change, while the
black bars show the absolute difference in the simulated
fluxes.
The results in figure 2 show that the difference in the
predicted change is significant. For example, the simulated
change in total latent heat flux is 54% higher in the
uncoupled system than in the coupled system. The absolute
difference in latent heat flux is 6%. The corresponding
change in total sensible heat flux is 134% higher in the
coupled system than in the uncoupled system.
Due to a strong dependence of the importance of
atmospheric feedback to land-use properties, soil type,
climate and scale, no general conclusions should be made
from the results shown here. However, the results do
indicate that, at least for regional land-use change studies,
feedback can have a significant effect on the predicted
impact and needs to be taken into consideration.