**1. Introduction**

Human life requires clear, clean, and adequate water to physically survive [1]. Besides direct consumption, water is needed to grow and prepare food. The source of the water for human use comes from the sky and then takes varying paths into the human body, plants, and aquatic life. Fifty percent of the world population obtains their water from groundwater; however, the number is larger when considering that 40% of streamflow is derived from groundwater discharge into a channel during baseflow [1].

Aquatic life is 100% dependent upon water, and when water disappears or changes temperature or becomes contaminated, fish and other organisms die or move or adapt. When a watershed changes vegetation or the amount of impervious surface, the hydrologic pathways shift from infiltration-evapotranspiration to subsurface/ interflow to overland runoff. Aquatic life tends to degrade and even disappear as a watershed loses the sustained steady discharge of groundwater into fluvial habitat [2].

 Whether human, plant, or aquatic life, groundwater is life-giving; without groundwater our quality of life and the quality of rare plants and aquatic life are less than optimal! Apart from connate water (water held in storage from a different climatic era [3]), groundwater is renewable—the question we address is how we

sustain groundwater to meet the current and future demands of human, plant, and aquatic life. The answer is embedded in watershed management; land use decisions in both space and time greatly influence the hydrologic pathways and processes, which also influence human, plant, and aquatic life.

 This chapter uses examples of hydrogeologic landscape settings in North America, Europe, Asia, and Africa to illustrate the theoretical movement of water from the sky upon, over, into, and through a watershed. We will address a range of settings and scales to elucidate systems' understanding. It is our hope that this approach will help the reader see critical thresholds that sustain human, plant, and aquatic life in a changing environment. Future groundwater managers will need to grasp the ramifications of their decisions, because like a large ship in the ocean, we can turn or stop the vessel before it may be too late. Well thought-out management decisions about future groundwater supply and demands are needed more now than ever before.
