Validation is a central issue to future model design in environmental science. This book is the first to provide a critical appraisal of today's validation needs, capabilities, and required changes in philosophy. It takes examples from four different scales: hillslope and river channel, catchment, regional, and global.This timely book offers unique, multifaceted coverage of model validation in hydrological science today. Topics covered include calibration procedures, data assimilation, scaling, critical future need in validation, and evidence of field data.* State-of-the-art research book on an important new topic* End-of-section discussion chapters written by leading international researchers