Water perpetually circulates on the earth from the oceans to the atmosphere to land and
back to the oceans; this is called water cycle or hydrologic cycle. Note that the term
„hydrologic cycle‟ literally means “Water-Science Cycle”, and hence the correct term to
describe this cyclic movement of water in nature is water cycle, which should be used
instead of widely-used term „hydrologic cycle‟. Thus, the water cycle describes how water moves into and out of various domains viz., atmosphere, land surface, subsurface (underground) andoceans. The main components of water cycle are precipitation, evaporation, transpiration,infiltration, surface runoff (overland flow and streamflow), and subsurface runoff (interflow,vadose-water flow and groundwater flow).
Of the water falling on the land, a proportion quickly evaporates, some flows into streams
or lakes as overland flow, and some infiltrates into the subsurface. Of the water entering the
subsurface, some is transpired back into the atmosphere by plants, some is retained in the
vadose zone, some reaches saturated zone (aquifer) as groundwater recharge, and the
remaining water follows a subsurface pathway back to the land surface and oceans
Note that water moving in the water cycle is neither gained nor lost, i.e., it is conserved
(Input – Output = Change in Storage). Thus, the water cycle follows the principle of
continuity. Consult: Perforacion de pozos profundos de agua
Groundwater is found in aquifers (water-bearing geologic formations), which act as conduits
for water transmission and as underground reservoirs for water storage. Practically, all
groundwater originates as surface water. Water enters aquifers from the land surface or
from surface water bodies through the vadose zone, and then it travels slowly within the
aquifer for varying distances until it finally returns to the land surface by natural flow,
plants, or humans. The residence time of groundwater in the subsurface can vary
from days to thousands of years (centuries or millennia) depending on the length of the flow
path and the transmissivity of porous media.
Principal sources of natural groundwater recharge are precipitation, streamflow, lakes and
reservoirs, while the artificial sources of recharge are seepage from canals, return flow from
irrigation, recharge from storage and percolation tanks, recharge due to check dams, and the
water purposely applied to augment groundwater. The discharge of groundwater occurs
when water emerges from underground (subsurface) as flow into streams, lakes or oceans
(called „baseflow‟), or as springs. Very shallow groundwater may return directly to the
atmosphere by evapotranspiration. Pumping of wells constitutes the major artificial
discharge of groundwater..
