**Acknowledgement**

Authors are thankful to the various Journals for allowing the use of published materials for this chapter. Figures used in this chapter are considered after taking permission from the respective Journals. SNT acknowledges the support under the program, Changing Water Cycle funded jointly by the Ministry of Earth Sciences, India and Natural Environment Research Council, UK. AKS thanks to Prof. B. N. Goswami, Director, IITM, Pune and Dr. P. C. S. Devara for their encouragement and support.
