This volume will focus on the general theories and approaches that serve as the foundation for the field of ecohydrology. It examines the relationship between water and our ecosystems and discusses the drivers for water sustainability under intensifying global changes. And the other three volumes are "Observation and Measurement" by Xin Li and Harry Vereecken, "Water-Limited Environments""by Yi Zheng and Bradford P. Wilcox, "Integrated Ecohydrological Modeling" by Dawen Yang, Dave Goodrich.