Impact of climate change on runoff

The volume of climatological annual rainfall in China is 6188.9 billion m3, of which 45% is turned to surface water and groundwater, 55% is consumed in evaporation. River runoff accounts for about 94.4% of total amount of water resource in China. It is seen that the runoff could be strongly influenced by rainfall in association with climate anomaly.

Wang et al. (2000) investigated the sensitivity of runoff to climate change by using a monthly hydrological model with hypothetical climate change scenarios. Here the Yellow River basin is shown as an example. In analyzing the sensitivity of runoff to climate change, the spatial-temporal distribution of climatic fields was supposed to remain no change when the rainfall and temperature are varied under different climate change scenarios. Table 11.1 shows the response of runoff to the change of rainfall and temperature. It is seen from Table 11.1 that (1) The runoff increases in association with the increase of rainfall, but decreases with the increase of temperature; (2) The response of runoff to rainfall is more sensitive than to temperature; (3) The influence of temperature on runoff appears to be more significant in the cases with more rainfall, while the opposite situation is found in the cases with less rainfall. (4) The dryer basins (Huang-fuchuan and Hong River) in the four sub-basins are more sensitive to the climate change, in comparison with the wetter ones (Daxia River and Dongnian River); (5) Among the 15 climate change scenarios, the most disadvantageous one is the case with 2°C increase in temperature associated with 20% decrease in the rainfall. Under this condition, the runoff decreases by 35%~43%.

A macro-scale hydrological model was used to investigate the response of land-surface runoff to various climate change scenarios with considering the effects of CO2 and aerosols (You et al. 2002). This advanced model included a climate change scenario model, a potential evaporation model and a modified soil water balance model. The effects of rainfall, evaporation, snow storage, snow melting, water supply of soil, and runoff were considered in the model. In the development of future climate change scenarios, the observed temperature and rainfall data were used to adjust the projected temperature and rainfall changes by the general circulation mod els (GCM). The Thornthwaite Method developed by Mintz et al (1993) was used in the calculation of potential evaporation. The soil moisture, evaporation and runoff were simulated by the soil water moisture budget model according to the status of climate, vegetation, elevation and character of soil. With the support of geographic information system (GIS), the land-surface runoff in every grid cell was calculated under current climate condition and the future scenario. The monthly average runoff was calculated by the accumulation of runoff in every grid cell (0.5 longitude x 0.5 latitude). The results showed that, under the scenarios of global warming, the land-surface runoff of upper Yangtze River basin decreased in spring but increased in summer, while an opposite situation was found in the lower Yangtze River. Aerosols may also influence the land-surface runoff, but the results were inconsistent under different climate change scenarios.

Table 11.1 Runoff within sub-basins of the Yellow River under different climate change scenarios

change rate of rainfall (AP/P)

-20%

-10%

0

+10%

+20%

Daxia River

0

-32.8

-17.0

0.0

18.8

38.2

AT/°C

1

-36.1

-28.1

-6.3

11.2

29.5

2

-38.9

-25.6

-11.4

4.8

22.2

Huangfuchuan

0

-34.4

-18.0

0.0

19.6

40.8

AT/°C

1

-37.9

-22.0

-4.5

15.1

35.2

2

-40.2

-24.8

-7.7

10.8

30.9

Hong River

0

-36.6

-19.4

0.0

23.1

49.1

AT/°C

1

-39.8

-23.6

-5.5

15.5

39.6

2

-42.6

-27.4

-10.1

9.2

31.5

Dongnian

0

-31.9

-16.5

0.0

17.6

36.1

River AT/°C

1

-34.3

-19.3

-3.3

13.8

31.8

2

-36.5

-21.9

-6.3

10.3

27.9

Zhang (2006) evaluated the potential effects of global climate change on the mean annual runoff in the Yellow River Basin under different climate scenarios of HadCM3 GCM based on the evaporation ratio function of the aridity index. Simulations under HadCM3 A2 and B2 scenarios indicated that the changes in annual runoff varied from region to region within the range of -48.0% to more than 203%. In general, the potential changes in annual runoff decreased from east to west. For the Yellow River Basin, the mean annual runoff increased by 5.0%, 11.7%, and 8.1% for the A2

scenario, and the changes were 7.2%, -3.1%, and 2.6% for B2 scenario by the year of 2020, 2050 and 2080, respectively.

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