Water is found in three states naturally i.e solid,liquid gas .
he world’s water exists naturally in different forms and locations: in the air, on the surface, below the ground and in the oceans (Figure 4.1).
Although a large volume of freshwater exists ‘in storage’, it is more important to evaluate the renewable annual water flows, taking into account where and how they move through/ the hydrological cycle (Figure 4.2).
This schematic of the hydrological cycle illustrates how elements can be grouped as part of a conceptual model that has emerged from the new discipline of ecohydrology, which stresses the important relationships and pathways shared among hydrological and ecological systems (Zalewski et al., 1997). This conceptual model takes into consideration the detail of the fluxes of all waters and their pathways while differentiating between two components: ‘blue water’ and ‘green water’. Blue waters are directly associated with aquatic ecosystems and flow in surface water bodies and aquifers. Green water is what supplies terrestrial ecosystems and rain-fed crops from the soil moisture zone, and it is green water that evaporates from plants and water surfaces into atmosphere as water vapour. This concept was developed by Falkenmark and Rockström (2004) who contend that the introduction of the concepts of ‘green water’and ‘blue water’, to the extent that they simplify the discussion for non-technical policy-makers and planners, may help to focus attention and resources on the often neglected areas of rain-fed agriculture, grazing, grassland, forest and wetland areas of terrestrial ecosystems and landscape management.