Introduction

The classical hydrological approach is to confine a regional water resources system to only the land hydrological cycle: precipitation P, evaporation E, infiltration G, and runoff R, assuming fixed atmospheric conditions (see Fig. 1.1 ). However, measurements and modeling studies in many river basins (e.g., Amazon, Mississippi) have confirmed a significant contribution of local evaporation to the atmospheric moisture, and hence to the regional precipitation ( Eltahir and Bras, 1996 ; Bosilovich and Schubert, 2001 ). Therefore, the atmospheric component of moisture recycling should also be accounted for when aiming at a complete and thorough analysis of the regional scale water resources. Fig. 1.1 shows the components of the water cycle in a region of domain length L. Q in and Q out are the incoming and outgoing atmospheric moisture fluxes, and W is the atmospheric moisture content.