Chapter two: Literature review
Resources and Hydrologic Cycle
All organisms that live in the earth, including
humans, require water for their existence. Therefore, ensuring that adequate
supplies of water are available is essential for human well-being. Water
circulates, forming closed hydrologic cycles. The amount of water will not
diminish on shorter than geological time scales (Oki, 2005).
Water the most powerful substance for living things
has a 1.36 Billion Km3 of water resources globally, from this 97.2% is salt
water mainly in oceans and 2.8% is available as freshwater (Raghunath, 2006).
Although the water available in water bodies, such as ocean and great lakes
stores plenty of water in amount, it is not directly useful for human beings. The
immediate use of water for human being is the one stored as groundwater and the
remaining water found in land surfaces, lakes and streams as fresh water.
Ethiopia is quite rich in water resources and its
drainage pattern is of great importance for its neighboring countries. It has
twelve river basins with a total annual water resources estimated at 111
Billion m3 of that 75.5 Billion m3 is in the Nile basin (Yazew, 2005).
Furthermore, the country releases an annual runoff volume of 122 Billion m3 of
water (Awulachew et al., 2007). Water by nature is renewable natural resources
found in three phases as liquid, solid and vapor which are mostly explained by
hydrologic cycle. The hydrologic cycle is the conceptual model which stats the
storage and movements of water between the biosphere, atmosphere, lithosphere
and hydrosphere. Water in this planet can be stored in any of the following
reservoirs: atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields and
groundwater. water moves from one reservoir to another by ways of processes
like evaporation, condensation, precipitation, deposition, runoff,
infiltration, sublimation, transpiration, melting and groundwater flow. The
oceans supply most of the evaporated water found in the atmosphere. From this
evaporated water, only 91% of it is returned to the ocean basins by way of
precipitation. The remaining 9% is transport to areas over landmass where
climatological factors induce the formation of precipitation. (National
Research Council, 1991) cited in (Karamouz, 2003).
Water is a naturally circulating resource that is
continuously recharged. Consequently, even if the stocks of water in natural
and artificial reservoirs are helpful to increase the available water resources
for human society, the flow of water should be the main focus in water
resources valuations. Climate change is expected to accelerate water cycles and
thereby increase the available fresh water. This would slow down the increase
of people living under water stress; however, changes in seasonal patterns and
increasing probability of extreme events may balance this effect. Reducing
current vulnerability will be the first step to prepare for such anticipated
changes (Oki, 2005).
The hydrological cycle also defined as a water
transfer cycle occurs continuously in nature; at
which the phenomena of evaporation and
evapotranspiration, precipitation and runoff takes place during the water
transfer system (Raghunath, 2006). Water first evaporates from the surfaces of
water bodies and transpires from surface vegetation as a vapor. Then the vapor
rises up to the atmosphere, condenses and form clouds and then through process
of condensation, results precipitation back to the earth surface. This
precipitation flows as runoff to the oceans or infiltrates into the soil to be
a groundwater. This system of water circulation starts its cycle again and
again and will not be stop at one time.
of groundwater recharge
of WetSpass Model
of GIS and Remote Sensing for groundwater