1. INTRODUCTION
Water is an essential resource for human life on Earth. Water resources are widely used for agriculture, human consumption, recreation, industry and power generation. As a result of changing weather patterns and expanding population water resources are becoming increasingly scarce. Today many parts of the world are facing serious water resources problems, such as water stress and water scarcity. Water resources are the most important factor regulating ecosystem processes in semi-arid climates because these environments are frequently water stressed (IPCC 2007; USGCRP 2009).
Water resources can be affected by climate change. Numerous researchers have demonstrated that climate change intensifies the variations in precipitation and temperature