The water cycle is presented, and water balance equations are discussed. Different water balance terms are described. Detailed descriptions of water surface processes are beyond the scope of this book, but particular attention is given to recharge assessments and drainable groundwater reserves. The precipitation term is described with an emphasis on temporally and spatially averaged assessments. Evapotranspiration estimates by measurements and empirical equations are described, including distinctions among potential, reference, and actual evapotranspiration. For assessment of the recharge, the water table fluctuation method and the chloride mass balance method are discussed with descriptions of their advantages and drawbacks. Base flow, particularly the interpretation of recession hydrograph curves, is addressed in regard to the deduction of the groundwater reserves that can be drained in a watershed.