4.3.6.2. Effects of Climate Change on River Flows

Figure 4-1: Change in average annual runoff by 2050 under HadCM2 ensemble mean (a) and HadCM3 (b) (Arnell, 1999b).

By far the majority of studies into the effects of climate change on river flows have used GCMs to define changes in climate that are applied to observed climate input data to create perturbed data series. These perturbed data are then fed through a hydrological model and the resulting changes in river flows assessed. Since the SAR, there have been several global-scale assessments and a large number of catchment-scale studies. Confidence in these results is largely a function of confidence in climate change scenarios at the catchment scale, although Boorman and Sefton (1997) show that the use of a physically unrealistic hydrological model could lead to misleading results.

Arnell (1999b) used a macro-scale hydrological model to simulate streamflow across the world at a spatial resolution of 0.5°x0.5°, under the 1961–1990 baseline climate and under several scenarios derived from HadCM2 and HadCM3 experiments. Figure 4-1 shows the absolute change in annual runoff by the 2050s under the HadCM2 and HadCM3 scenarios: Both have an increase in effective CO₂ concentrations of 1% yr^-1. The patterns of change are broadly similar to the change in annual precipitation—increases in high latitudes and many equatorial regions but decreases in mid-latitudes and some subtropical regions—but the general increase in evaporation means that some areas that see an increase in precipitation will experience a reduction in runoff. Alcamo et al. (1997) also simulated the effects of different climate change scenarios on global river flows, showing broadly similar patterns to those in Figure 4-1.

Rather than assess each individual study, this section simply tabulates catchment-scale studies published since the SAR and draws some general conclusions. As in the SAR, the use of different scenarios hinders quantitative spatial comparisons. Table 4-2 summarizes the studies published since the SAR, by continent. All of the studies used a hydrological model to estimate the effects of climate scenarios, and all used scenarios based on GCM output. The table does not include sensitivity studies (showing the effects of, for example, increasing precipitation by 10%) or explore the hydrological implications of past climates. Although such studies provide extremely valuable insights into the sensitivity of hydrological systems to changes in climate, they are not assessments of the potential effects of future global warming.

It is clear from Table 4-2 that there are clear spatial variations in the numbers and types of studies undertaken to date; relatively few studies have been published in Africa, Latin America, and southeast Asia. A general conclusion, consistent across many studies, is that the effects of a given climate change scenario vary with catchment physical and land-cover properties and that small headwater streams may be particularly sensitive to change—as shown in northwestern Ontario, for example, by Schindler et al. (1996).

Table 4-2: Catchment-scale studies since the Second Assessment Report addressing the effect of climate change on hydrological regimes.
Region/Scope	Reference(s)
Africa
– Ethiopia	Hailemariam (1999)
– Nile Basin	Conway and Hulme (1996); Strzepek et al. (1996)
– South Africa	Schulze (1997)
– Southern Africa	Hulme (1996)
Asia
– China	Ying and Zhang (1996); Ying et al. (1997); Liu (1998); Shen and Liang (1998); Kang et al. (1999)
– Himalaya	Mirza and Dixit (1996); Singh and Kumar (1997); Singh (1998)
– Japan	Hanaki et al. (1998)
– Philippines	Jose et al. (1996); Jose and Cruz (1999)
– Yemen	Alderwish and Al-Eryani (1999)
Australasia
– Australia	Bates et al. (1996); Schreider et al. (1996); Viney and Sivapalan (1996)
– New Zealand	Fowler (1999)
Europe
– Albania	Bruci and Bicaj (1998)
– Austria	Behr (1998)
– Belgium	Gellens and Roulin (1998); Gellens et al. (1998)
– Continent	Arnell (1999a)
– Czech Republic	Hladny et al. (1996); Dvorak et al. (1997); Buchtele et al. (1998)
– Danube basin	Starosolszky and Gauzer (1998)
– Estonia	Jaagus (1998); Jarvet (1998); Roosare (1998)
– Finland	Lepisto and Kivinen (1996); Vehviläinen and Huttunen (1997)
– France	Mandelkern et al (1998)
– Germany	Daamen et al. (1998)
– Greece	Panagoulia and Dimou (1996)
– Hungary	Mika et al. (1997)
– Latvia	Butina et al. (1998); Jansons and Butina (1998)
– Nordic region	Saelthun et al. (1998)
– Poland	Kaczmarek et al. (1996, 1997)
– Rhine basin	Grabs (1997)
– Romania	Stanescu et al. (1998)
– Russia	Georgiyevsky et al., (1995, 1996, 1997); Kuchment (1998); Shiklomanov (1998)
– Slovakia	Hlaveova and Eunderlik (1998); Petrovic (1998)
– Spain	Avila et al. (1996); Ayala-Carcedo (1996)
– Sweden	Xu (1998); Bergstrom et al. (2001)
– Switzerland	Seidel et al. (1998)
– UK	Arnell (1996); Holt and Jones (1996); Arnell and Reynard (1996, 2000); Sefton and Boorman (1997); Roberts (1998); Pilling and Jones (1999)
Latin America
– Continent	Yates (1997); Braga and Molion (1999)
– Panama	Espinosa et al. (1997)
North America
– USA	Bobba et al. (1997); Hanratty and Stefan (1998); Chao and Wood (1999); Hamlet and Lettenmaier (1999); Lettenmaier et al. (1999); Leung and Wigmosta (1999); Miller et al. (1999); Najjar (1999); Wolock and McCabe (1999); Miller and Kim (2000); Stonefelt et al. (2000)
– Mexico	Mendoza et al. (1997)

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