Models Supporting Decision-Making and Policy Evaluation - Environmental Modelling: Finding Simplicity in Complexity - page 323

a commodity. The capacity of these dams to meet our
needs is highly sensitive to the impacts of climate change
and of land use and land management in the catchments
upstream (see Chapter 11). There is thus great interest
in policy support for understanding the implications of
upstream land-use and climate change on the opera-
tion of dam projects and for developing land-use and
benefit-sharing policies to maintain the effectiveness of
dam operations.

Water quantity is not the only determinant of the
utility of water, since water quality fundamentally deter-
mines the use to which water can be put. Water quality
is often considered in terms of sediment load and of
contamination by organics and inorganic contaminants.
Undisturbed land covers tend to produce lower sediment
loads compared with disturbed land covers especially
where disturbance leads to the presence of bare soil that is
exposed to the elements (see Chapters 11, 15 and 22). In
addition, protected areas or other natural ecosystems will
tend to have lower inputs of fertilizer, pesticide, herbi-
cide, organic manures and other potential contaminants,
in comparison with cropland and pastures. Undisturbed
and protected areas will thus tend to produce higher qual-
ity water for downstream use than would highly disturbed
or intensively cultivated lands.
In many parts of the world, rainfall and evapo-
transpiration vary from month to month to produce
a water balance that is highly seasonal. This seasonality
can lead to water deficits in some seasons and surpluses
in others. Some landscapes and ecosystems are said to
provide a regulation service by storing large volumes of
water during the wet season(s) and releasing it slowly as
baseflow during the dry season(s) (Pena-Arancibia et al .,
2012). Wetlands such as the high mountain Paramos of
the Andes are said to be particularly important seasonal
stores of water (Buytaert et al ., 2006).
There is much interest in policy support for better
maintaining and sharing the benefits of hydrological
ecosystem service production, particularly from the con-
servation and international development communities.
This interest includes support for understanding the
ecosystem service implications of proposed changes to
land use through agricultural subsidy, protected area dec-
laration or expansion, or specific home or foreign direct
investment projects (Mulligan, 2012).

Maintaining ecosystem services

There is also considerable interest in the impact of land-
cover change on downstream water resources and so-
called hydrological 'ecosystem services'. An ecosystem
service is a benefit provided to humanity by the operation
of ecosystem processes (Daily, 1997; Daily et al ., 2000;
MEA, 2005). Such services are provided by nature but not
usually accounted for in economic accounting such that,
for example, the price of water usually reflects the costs
of sourcing, storing, treating and distributing water but
not the costs associated with maintaining a landscape that
can provide sufficient, high quality and well-regulated
water to the point at which it is sourced. This situation
often results in lack of incentive to sustain ecosystem
services and thus degradation of these 'externalities' with
consequent impacts on supply.
Ecosystem services for water are said (Aylward et al .,
2005) to include water-quantity services, water-quality
services and water-regulation services. The presence of
some ecosystems (such as, for example, tropical montane
cloud forests - TMCFs) are considered to provide water
quantity improvements to downstream areas through
additional inputs of passing ground-level cloud (fog)
water. This input is trapped by forests but not by the
grasslands and croplands that tend to replace forests at
the agricultural frontier (Bruijnzeel et al ., 2010). Tropical
montane cloud forests are thus unique amongst forests in
having the potential for increased water flows downstream
when compared with agricultural land uses. Conversely,
the higher transpiration and interception loss of most
other forest types, compared with croplands and pasture,
means that afforestation usually leads to decreases in water
availability downstream and deforestation to increases in
water availability. Given that rivers, agricultural prac-
tices and hydraulic engineering structures are adapted
to current hydrological conditions, any change from the
hydrological status quo ante is potentially problematic
and should be avoided where possible.

Adaptation to the impact of climate change

The previous three sections focus on land-use and land-
management change, which are current, hydrologically
very potent and very rapid compared with climate change.
It is thus imperative to understand and better manage
land use for food, water, energy and the maintenance of
ecosystem services and the biodiversity that underpins it
all. Climate change will, however, become an increasingly
important factor affecting our ability to manage land.
Wherever climate change leads to significant changes
in temperature and especially rainfall we will need to
adapt land use and water management to cope with these