The main consequences of climatic changes to inland waters include the following (da Cunha 1988): (a) changes in the global amount of water resources and in the spatial and temporal distribution of these resources; (b) changes in soil moisture; (c) changes in extreme phenomena related to water resources, i.e., floods and droughts; (d) changes in water quality; (e) changes in sedimentation processes; and (f) changes in water demand.
The consequences of climatic change on water quality include possible changes in the precipitation regime and the occurrence of acid rain. Direct consequences of climatic changes on water quality occur. For example, temperature increases can decrease levels of dissolved oxygen in the water. Second, the biochemical oxygen demand (BOD) also increases with temperature. These two effects can decrease the dissolved oxygen concentration in a surface water system. Also, climatic changes can have indirect consequences on water quality since a decrease of river discharges, particularly during the dry season, can increase the concentration of pollutants in water bodies.
Climatic changes influence not only water availability but also water demand. For example, water demand for irrigation is largely affected by climatic change which conditions evapotranspiration. Water demands for domestic or industrial use are also affected by climatic change, for example, as a result of temperature increases that influence water consumption for cooling systems, bathing, washing, and gardening.
The simplest way to view the implications of global climate change on water resources is to consider the relationship between increasing atmospheric CO2 and the hy-drologic cycle; this relationship is shown in Figure 5.5.1. The following comments relate to the implications of Figure 5.5.1 (Waggoner and Revelle 1990):
1. When the atmosphere reaches a new and warmer equilibrium, more precipitation balances faster evaporation. This faster hydrologic cycle is predicted to raise the global averages of the up-and-down arrows of precipitation and evaporation in Figure 5.5.1 by 7 to 15%. However, the predicted change in precipitation is not a uniform increase. The net of precipitation and evaporation, that is, soil moisture, is predicted to fall in some places. Also, precipitation is predicted to increase in some seasons and decrease in others. Although global climatic models disagree on where precipitation will decrease and although they do not simulate the present seasonal change in precipitation correctly, disregarding the warning of a drier climate is unwise.
2. To be most usable for water resource considerations, frequency distributions of precipitation (and flood and drought projections) are needed. Models to develop such information are in their infancy.
3. A dimensionless elasticity for runoff and precipitation is:
[Percentage change in runoff/percentage change in precipitation] 5.5(4)
If the elasticity for runoff and precipitation is greater than 1, the percentage change in runoff is greater than the percentage change in precipitation that causes it. Therefore, a general conclusion about the transformation of climate change into runoff change can be stated. Over diverse climates, the elasticities of percentage change in runoff to percentage change in precipitation and evaporation are 1 to 4. The percentage change in runoff is greater than the percentage change in the forcing factor.
Based on this brief review of water resource issues related to global climate change, the following summary comments can be made:
1. Global climate change caused by the greenhouse effect appears to be a reality although scientific opinion differs as to the rate of change.
2. Numerous inland and coastal water resource management issues are impacted when temperature and precipitation patterns change.
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