Impacts from Climate Change Hydrology

The impacts of climate change on general basin hydrology have been simulated using the SFM hydrological model (Denisov et al., 2002). Since the changes in temperature and precipitation for the time slice 2010-2039 are virtually the same under both the A2 and B2 scenarios, there is nearly no difference between SFM outputs under A2 and B2 scenarios. For 2010-2039, SFM runs do not give any significant change for the inter-annual runoff distribution. However, there is a pronounced tendency that is much more apparent in the modelled changes of runoff distribution for the time slice 2070-2099, for an earlier onset of spring high waters (shifting it by 5-7 days) compared to the baseline period (1961-1990), sharpening the annual runoff peak in spring and increasing its height, while a slight lowering of streamflow (approximately by 10% as compared to the baseline period) is expected to occur from late June till August. Despite an overall increase of annual precipitation (in the range of 1.07-1.08 of the baseline value) and very insignificant increase of annual runoff (in the range of 1.03-1.04 of the baseline value), on average less water will be available in the period of highest demand for irrigation. However, this is not expected to impose any significant impact on agriculture, since currently existing water management mechanisms in transboundary water allocation allow effective adjustments for a much broader range of year-to-year variations in the availability of water resources (Kipshakaev and Sokolov, 2002).

Over the period 2070-2099, there are remarkable differences in the SFM outputs for the scenarios A2 and B2 (Fig. 5.4). The most drastic changes as compared to the current situation are expected to occur under scenario A2. Onset of high waters in spring is expected to start 3-4 weeks earlier compared to the baseline period (1961-1990). The duration of the annual peak is expected to shorten and the maximum specific runoffis expected to be 25% higher. The changes in the hydrolog-ical cycle under the B2 scenario are similar, but less pronounced. The onset of the

Length growing season (days)

Fig. 5.5. Patterns of LGP for the current situation (1960-1990) and expected changes under the HadCm3-A2 scenarios for the period 2070-2099.

82-101

119-180

Fig. 5.5. Patterns of LGP for the current situation (1960-1990) and expected changes under the HadCm3-A2 scenarios for the period 2070-2099.

spring high water period would be 2-3 weeks earlier than at present, its peak approximately 30% higher, but the duration of the high water period will decrease less dramatically than under the A2 scenario.

SFM runs show for the A2 scenario a slight reduction (—4%) of annual flow as compared to baseline period, and a slight increase (+7%) for the B2 scenario. The changes of the inter-annual runoff distribution pattern would lead to serious water shortages over the summer period, when the water is mostly needed for irrigation. While at present 68% of the annual flow occurs during 3 summer months (June, July, August), under the A2 scenario this figure would be nearly halved (35%), and under the B2 scenario it would be only 50%. For the time interval 2070-2099, both scenarios impose a very serious negative impact on agriculture, necessitating a very balanced adaptation strategy to mitigate the risks for food production in the Syr Darya Basin. The impacts on industry and environment are also negative since the risk of spring floods might significantly increase, causing danger for dam security and over-flooding in the lower basin.

Recent patterns of the Length of the Growing Period (LGP) and resulting changes under different climate scenarios are shown in Fig. 5.5 (note different intervals in legend). The numbers in Table 5.3 provide the averages for the basin; more specific conclusions may be drawn based on the analysis of the spatial pattern of modelled LGP changes (Fig. 5.5). Under all scenarios, one can expect positive LGP changes, particularly in the middle reaches in the Fergana Valley and Golodnaya Steppe, where

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