Models Supporting Decision-Making and Policy Evaluation - Environmental Modelling: Finding Simplicity in Complexity - page 329

Figure 20.3 Annual total net soil erosion (erosion minus deposition, hillslope and channel) averaged over subcatchments of Strahler
stream order 6. Indicates high erosion in some basins in Spain and Portugal compared with Morocco and Algeria (Screenshot from
WaterWorld, courtesy of King's College London and AmbioTEK).

scenarios can either be chosen from a range of some 175
IPCC assessment, scenario and GCM combinations - all
downscaled to 1 km 2 resolution or by choosing the mean
or standard deviations of all GCMs for a given IPCC
scenario (the ensemble method), and finally by upload-
ing your own scenario maps or by specifying a scenario
simply as seasonal changes in precipitation and tempera-
ture. All scenarios are applied in equilibrium rather than
transient mode with the 1950-2000 period used as the
baseline against which the scenario is applied using the
delta method (Wilby et al ., 1998) at 1 km 2 resolution as
is necessitated by the spatial variability of climates in
topographically complex regions. Here we choose IPCC
AR4 A2a scenario, 2050s, ensemble mean of all 17 GCMs
available. Figure 20.4 is system output for the change
in mean annual water balance by administrative region
and indicates significant drying expected for southern
Spain and northern Morocco but wetting for other areas
including southern Algeria. The figure also shows the
seasonal progression of water balance for baseline (right)
and scenario (left) and indicates approximately uniform
change in water balance seasonally for the tile as a whole
(though there may be more seasonal change if we were to
look regionally).

20.2.4 Policyoptions

A policy option to the PSS is a policy or intervention that
is under the control of the decision maker and can thus
be applied to change the system for the better (or for
worse). WaterWorld allows users to examine the impacts
of policy options relative to the baseline. Here we zoom in
to the local scale of the 1 ◦ tile centred on 37 5 ◦ W

covering the Guadalentın basin (though we could choose
anywhere) and run a 1 hectare resolution baseline sim-
ulation against which a couple of policy options will be
examined. The first policy option examines deforestation
in the Sierra de Baza protected area (protected area
number 20946 in the World Database of Protected
Areas - WDPA) 15 near to Guadix. In the PSS, we convert
a randomly assigned 75% of land in the protected area
and where the slope gradient < 30 degrees from its
original cover to agriculture (represented as per pixel:
tree cover 10%, herbaceous cover 50%, bare cover 40%)
and examine the implications. This leads to an increase in
tree cover of up to 10% in some parts of the reserve and a
decrease of up to 40% in others whilst herbaceous cover
increases by up to 17% in a few areas but decreases by up to

5 ◦ N,

. − 2 .

15 See www.wdpa.org (accessed 6 April 2010).