| Abstract: | We present a comparative analysis of projected
impacts of climate change on river runoff from two types of
distributed hydrological model, a global hydrological model
(GHM) and catchment-scale hydrological models (CHM).
Analyses are conducted for six catchments that are global
in coverage and feature strong contrasts in spatial scale as
well as climatic and developmental conditions. These include
the Liard (Canada), Mekong (SE Asia), Okavango (SW
Africa), Rio Grande (Brazil), Xiangxi (China) and Harper’s
Brook (UK). A single GHM (Mac-PDM.09) is applied to
all catchments whilst different CHMs are applied for each
catchment. The CHMs include SLURP v. 12.2 (Liard),
SLURP v. 12.7 (Mekong), Pitman (Okavango), MGB-IPH
(Rio Grande), AV-SWAT-X 2005 (Xiangxi) and Cat-PDM
(Harper’s Brook). The CHMs typically simulate water resource
impacts based on a more explicit representation of
catchment water resources than that available from the GHM
and the CHMs include river routing, whereas the GHM does
not. Simulations of mean annual runoff, mean monthly
runoff and high (Q5) and low (Q95) monthly runoff under
baseline (1961–1990) and climate change scenarios are presented.
We compare the simulated runoff response of each
hydrological model to (1) prescribed increases in global mean
air temperature of 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 deg. C relative
to baseline from the UKMO HadCM3 Global Climate
Model (GCM) to explore response to different amounts of
climate forcing, and (2) a prescribed increase in global-mean
air temperature of 2.0 deg. C relative to baseline for seven GCMs
to explore response to climate model structural uncertainty.
We find that the differences in projected changes of
mean annual runoff between the two types of hydrological
model can be substantial for a given GCM (e.g. an absolute
GHM-CHM difference in mean annual runoff percentage
change for UKMO HadCM3 2 deg. C warming of up to 25%),
and they are generally larger for indicators of high and low
monthly runoff. However, they are relatively small in comparison
to the range of projections across the seven GCMs.
Hence, for the six catchments and seven GCMs we considered,
climate model structural uncertainty is greater than the
uncertainty associated with the type of hydrological model
applied. Moreover, shifts in the seasonal cycle of runoff with
climate change are represented similarly by both hydrological
models, although for some catchments the monthly timing
of high and low flows differs. This implies that for studies
that seek to quantify and assess the role of climate model
uncertainty on catchment-scale runoff, it may be equally as
feasible to apply a GHM (Mac-PDM.09 here) as it is to apply
a CHM, especially when climate modelling uncertainty
across the range of available GCMs is as large as it currently
is. Whilst the GHM is able to represent the broad
climate change signal that is represented by the CHMs, we
find however, that for some catchments there are differences
between GHMs and CHMs in mean annual runoff due to differences
in potential evapotranspiration estimation methods,
in the representation of the seasonality of runoff, and in the
magnitude of changes in extreme (Q5, Q95) monthly runoff,
all of which have implications for future water management
issues. |