Abstract: | Quantitative evaluations of the impacts of climate
change on water resources are primarily constrained by uncertainty
in climate projections from GCMs. In this study we
assess uncertainty in the impacts of climate change on river
discharge in two catchments of the Yangtze and Yellow River
Basins that feature contrasting climate regimes (humid and
semi-arid). Specifically we quantify uncertainty associated
with GCM structure from a subset of CMIP3 AR4 GCMs
(HadCM3, HadGEM1, CCSM3.0, IPSL, ECHAM5, CSIRO,
CGCM3.1), SRES emissions scenarios (A1B, A2, B1, B2)
and prescribed increases in global mean air temperature (1 deg. C
to 6 deg. C). Climate projections, applied to semi-distributed hydrological
models (SWAT 2005) in both catchments, indicate
trends toward warmer and wetter conditions. For prescribed
warming scenarios of 1 deg. C to 6 deg. C, linear increases
in mean annual river discharge, relative to baseline (1961–
1990), for the River Xiangxi and River Huangfuchuan are
+9% and 11% per +1 deg. C respectively. Intra-annual changes
include increases in flood (Q05) discharges for both rivers as
well as a shift in the timing of flood discharges from summer
to autumn and a rise (24 to 93%) in dry season (Q95)
discharge for the River Xiangxi. Differences in projections
of mean annual river discharge between SRES emission scenarios
using HadCM3 are comparatively minor for the River
Xiangxi (13 to 17% rise from baseline) but substantial (73 to
121%) for the River Huangfuchuan. With one minor exception
of a slight (−2%) decrease in river discharge projected
using HadGEM1 for the River Xiangxi, mean annual river
discharge is projected to increase in both catchments under
both the SRES A1B emission scenario and 2 deg. rise in global
mean air temperature using all AR4 GCMs on the CMIP3
subset. For the River Xiangxi, there is substantial uncertainty
associated with GCM structure in the magnitude of the rise
in flood (Q05) discharges (−1 to 41% under SRES A1B and
−3 to 41% under 2 deg. global warming) and dry season (Q95)
discharges (2 to 55% under SRES A1B and 2 to 39% under
2 deg. global warming). For the River Huangfuchuan, all GCMs
project a rise in the Q05 flow but there is substantial uncertainty
in the magnitude of this rise (7 to 70% under SRES
A1B and 2 to 57% under 2 deg. global warming). Differences in
the projected hydrological changes are associated with GCM
structure in both catchments exceed uncertainty in emission
scenarios. Critically, estimated uncertainty in projections of
mean annual flows is less than that calculated for extreme
(Q05, Q95) flows. The common approach of reporting of climate
change impacts on river in terms of mean annual flows
masks the magnitude of uncertainty in flows that are of most
importance to water management. |