The hydrological cycle
The hydrological cycle describes the constant movement of water above, on, and below the Earth's surface. The cycle operates across all scales, from the global to the smallest stream catchment (Smith 1998) and involves the movement of water along evapotranspiration, precipitation, surface runoff, subsurface flow and groundwater pathways. In essence, water is evaporated from the land, oceans and vegetation to the atmosphere, using the radiant energy from the Sun, and is recycled back in the form of rain or snow. When moisture from the atmosphere falls to the Earth's surface it becomes subdivided into different interconnected pathways. Precipitation (excluding snow and hail) wets vegetation, directly enters surface water bodies or begins to infiltrate into the ground to replenish soil moisture. Excess water percolates to the zone of saturation, or groundwater, from where it moves downward and laterally to sites of groundwater discharge. The rate of infiltration varies with land use, soil characteristics and the duration and intensity of the rainfall event. If the rate of precipitation exceeds the rate of infiltration this leads to overland flow. Water reaching streams, both by surface runoff and groundwater discharge eventually moves to the sea where it is again evaporated to perpetuate the hydrological cycle.
The hydrological cycle and connectivity
Groundwater is fed by rain and surface water. Groundwater ultimately discharges to surface water or the sea. Surface water is fed by groundwater and feeds groundwater. Surface and groundwater form parts of one interlinked system. (Nevill et al. 2001)
Groundwater and surface water are not isolated components of the hydrological cycle. They interact in a range of topographic, geologic and climatic landscapes (Winter et al. 1998). Effective management of water requires an understanding of the components of the hydrological cycle as well as the linkages between those components. One important connection that has been traditionally overlooked in water resource management in Australia is the interaction between surface water and groundwater resources. This is largely due to the hidden nature of groundwater. Direct observation and measurement of groundwater processes is also hindered by the differing timescales associated with the movement of water in the landscape. Whereas the movement of water above ground can be quite rapid in response to rainfall events, the movement of water beneath the land surface can be sluggish and more difficult to predict.
Relevant Links
US Geological Survey Water Cycle
References
Nevill J, Maher M, Nichols P, 2001. Water law, COAG, and the environment, In Proceedings of the 3rd Australasian Natural Resources Law and Policy Conference, Adelaide, 22-23 March, pp. 93-107.
Smith DI, 1998. Water in Australia: resources and management, Oxford University Press, Melbourne.
Winter TC, Harvey JW, Franke OL, Alley WM. 1998. Ground water and surface water - a single resource', U.S. Geological Survey Circular 1139, Denver, Colorado. http://pubs.usgs.gov/circ/circ1139/