With the growing population and technological innovations, the water needs for drinking, industry & agriculture and as well wastages are growing rapidly.For appropriate planning &management of water resources use, one needs realistic estimates of water availability in space and time under changing climate which is beyond human control; and increased levels of man-made water contamination from different sources. The principal objective of this book is to put the water availability under the current climate change scenario; and help government planning. To achieve this goal, one need reliable and unbiased water availability estimates; and also one must understand the limitations of methods-data used in the estimation of water availability. This book discussed these issues in practical way.