Models Supporting Decision-Making and Policy Evaluation - Environmental Modelling: Finding Simplicity in Complexity - page 330

Figure 20.4 Change in annual total water balance under AR4 A2a 17GCM ensemble simulation expressed as a map and through
comparison of seasonal patterns of change (Screenshot from WaterWorld, courtesy of King's College London and AmbioTEK).

50% elsewhere in the reserve. Overall bare cover increases
throughout the reserve by up to 40%. These changes
lead to complex changes in water balance within the
reserve, depending on the change in vegetation cover and
the topographic context. Some areas showed increased
water balance and some showing decreased water balance
(as the balance between evapo-transpiration and fog
interception by vegetation changes). Figure 20.5 shows
the resulting impact on flows in which some tributaries
show increase in flows whilst other neighbouring
tributaries can show decreases. The overall outcome
depends very much on the original vegetation complex,
the nature of vegetation change and the landscape
characteristics, and is thus not simple. Such deforestation
also leads to increases in soil erosion in catchments
draining the protected area with the consequent risk of
sedimentation in downstream reservoirs.
Our final scenario examines the impact on erosion of
bench terracing all land with slope gradient greater than
10 degrees in the Sierra de Baza protected area. The imple-
mentation of bench terracing reduces local slope and thus
decreases erosive power and reduces sediment transport
capacity, leading deposition of soil transported from
upslope. Net soil erosion may thus decrease and even
become negative (net deposition). Figure 20.6 shows the

difference between baseline and terraced net soil erosion
expressed as a percentage of baseline erosion. Where
values are negative soil erosion has decreased, where
values are positive it has increased. Clearly terracing in
this area leads to spatially variable decreases in erosion.
The examples are clearly highly simplistic compared
with most policy problems in which multiple interven-
tions are targeted at multiple (sometimes conflicting)
objectives and the outcomes need to be viewed from
a number of perspectives (for example, environmental,
socio-economic, political). Nevertheless they do give an
indication of the functionality of WaterWorld: the inter-
ested reader is encouraged to work with the PSS them-
selves 16 in order to better understand its potential and
limitations for their own field of enquiry and/or study site.

20.3 Addressing the remaining barriers

A number of barriers to uptake of PSS like these remain.
Perhaps the most substantial is the relative simplicity of
such systems compared with the policy problems that they
might support. We have shown simple climate change,

16 See www.policysupport.org (accessed 6 April 2012).