SR-21 : Impact of global change on hydrological and water resources parameters in arid and semi-arid areas

Abstract

Global warming resulting from increasing concentration of greenhouse gases could alter hydrologic processes and cycles within many parts of the world. Current consensus is that a doubling of CO concentration will cause a 1. 6 C to 4. 5 C global warming and that in turn, may cause major changes in regional precipitation, evapotranspiration, annual rainfall regime and may effect water availability in some areas. A majority of scientific community believes that an abnormal planetary warming Is likely. There is insufficient evidence at the moment to prove that a climate change caused by human activities is underway. Observed changes could be merely the result of normal variability. Nor is there agreement about the possible rate of change and what its impact might be on the people and nations of the world. Nor is it very much clear what effect it may have on the hydrological cycle.

A large number of research studies have been carried out on climatic change and its impact on various hydrological and meteorological aspects. The conclusions drawn from various studies are not very precise and refer only to the long-term averaged characteristics of hydrological, meteorological and water resources parameters. While reviewing the state of art on the subject, methodological approaches are considered as well as basic information/data and pre-requisites used by different scientists to evaluate and predict the effect of climatic changes on water resources and on hydrological regimes. The results obtained for different regions of the world have been discussed, including the river runoff forecasts subjected to climate changes.

There is little certainty about now meteorological and hydrological droughts will be affected by climate change. But the results from various models point towards the possibility of more frequent and severe weather events (floods, droughts, typhoons etc.). Though not uniform throughout the zone, increase in precipitation in most parts of semi-arid tropics is predicted by some models. The possibility of precipitation decrease in some areas is also indicated. Increased precipitation is expected to be in the form of convective rainfall, implying high intensity but not necessarily increased frequency of rains. Thus the seasonally dry tropics would have potentially high rainfall, high runoff and high evaporation, without necessarily having enhanced growing seasons. Some parts of semi-arid tropics already face with large variability in seasonal and annual rainfall and a trend toward decrease in precipitation could be highly sensitive even to small negative impact of greenhouse warming. As an impact of global warming, more winter precipitation and drier summers could be expected for arid and semi-arid regions of India and Africa. The western world felt more winter precipitation as rain instead of snow and winter runoff increased while spring snowmelt runoff decreased. Increased rainfall could bring significant moisturing in arid and semi-arid regions. This may benefit agricultural practices in some regions.

This report documents the present status of knowledge in respect of climate changes and its probable effect on various hydrological and water resources characteristics. It also discusses the impact of climate changes for various regions of the world. Emphasis has been given to review the relevant studies for arid and semi-arid regions. The impact of climate changes specially on precipitation, evaporation and runoff for different regions has been reviewed.