The+Hydrosphere

= __**Key Terms**__ =


 * Hydrosphere** - the water component of the earth's environment including oceans, rivers, lakes, glaciers and ice caps.
 * Drainage basin** - the are from which a river collects its water, sometimes referred to as a catchment area
 * Watershed** - the boundary of a drainage basin, usually defined by ridge lines that determine into which catchment water may flow
 * Tributary** - a stream / river that enters a larger stream / river
 * Source** - the point at where a stream / river begins
 * Mouth** - the point at where the river enters the ocean
 * Confluence** - point where two or more tributaries meet to form a bigger stream / river
 * Evaporation** - process whereby water becomes water vapour and rises into the atmosphere
 * Traspiration -** water returned to the atmospher in a gaseous state through the leaves of plants
 * Evapotranspiration** - combination of both evaporation and transpiration
 * Condensation** - Process whereby water cools and returns from a gaseous state to a liquid or liquid crystal state, usually forming clouds
 * Precipitation** - Water that falls to the ground as rain, hail, sleet or snow
 * Interception** - where precipitation is prevented from reaching the ground owing to the presence of vegetation
 * Infiltration** - water soaking into the ground
 * Infiltration rate** - the speed at which water can soak into the ground
 * Infiltration capacity** - the amount of water that can enter the ground at any given time
 * Surface run-off / overland flow** - water that flows over the ground surface into rivers, streams, lakes and oceans
 * Throughflow** - water that flows through the soil into rivers, streams, lakes and oceans
 * Percolation** - water that flows from the soil into the bedrock beneath
 * Groundwater flow** - water that flows through the bedrock into rivers, streams, lakes and oceans
 * Channel flow** - water flowing within the confines of a river or stream
 * Discharge** - the volume of water in river passing a given point over a period of time. Discharge is measured in cubic metres of water per second (cumecs)
 * Hydrograph** - a graph that show the discharge in a river over a period of time, often used to show the discharge of a river during and after a storm event
 * River regulation** - controlling the flow of rivers to avoid problems such as floods and droughts
 * Dam** - a barrier, usually a wall made of cement, brick etc, built across a river to hold back water
 * Reservoir** - the lake formed behind a dam wall

= __ I**ntroduction to the Hydrosphere** __ =

The //**hydrosphere**// is the water realm of our earth encompassing water that is stored in a liquid, gaseous or solid state in the lithosphere (on the earth's surface and beneath it) and in the atmosphere.

Did you know that less than three percent of the Earth's water is freshwater? In fact, approximately 97.2% of the world's water is saltwater found in the ocean stores.


 * //**Storage**// || **//Percentage of Freshwater//** ||
 * Ice Caps and Glaciers || 76.5 ||
 * Groundwater 800m deep || 10.1 ||
 * Groundwater 800-4000m deep || 12.8 ||
 * Other - lakes, soil moisture, atmospheric water vapour, rivers and marshes || 0.6 ||

= __**The hydrological cycle**__ =



The hydrological cycle shows the various stores and transfers of water through drainage basins. All drainge basins have inputs, stores, throughputs and and outputs and can be represented as a system. The main input is precipitation and most water is held in stores in the drainage basin via vegetation, streams, rivers, lakes, ice caps / glaciers and ground stores. Movements of water within the drainage basin may occur over the surface of the ground as surface run-off, into the soil as infiltration, through the soil as throughflow, into rock via percolation and through the rock as groundwater. Water exits the drainage basin system either via channel flow, which ends up in the ocean or via evapotranspiration back to the atmopshere. Given the water ends up in oceans or in the atmopshere, this allows for presipitation to develop providing inputs to the system once again. The diagram below is a systems model of a drainage basin.



Source: BBC Scotland (2009)

= __**Drainage Basins**__ =



A //**drainage basin**// is the area where a river collects its water from. Drainage basins are usually named after the major river in the drainage basin e.g Colorado, Murray-Darling and Amazon. Drainage basins will have several streams / rivers, known as //**tributaries**//, which all connect in some way to the largest river in the drainage basin. The point at where two or more tributaries meet is known as the //**confluence**//. Towns or cities that are located at the confluence of two or more major rivers can have issues with flooding owing to the additional water build-up from two rivers joing together, for example St Louis in the USA is at the confluence of the Missourri and Missippi rivers and has invested significant amounts of money into flood protection. All rivers and streams have a //**source**//, where they begin, and have a //**mouth**// , where they enter the ocean. Drainage basins a delineated by their //**watershed**//, a boundary which detremines the direction of water flow between drainage basins. Ridge lines are the most common topographic features to look for in determining the watershed on maps and in real life. = __**Storm Hydrographs**__ =

Storm hydrographs show the discharge of a river during and after a storm event. By analysing the storm hydrograph it is possible to examine the features of the drainage basin that may contribute to the behaviour of the river during and after a storm event. A typical hydrograph is show below.

Source: University of Wisconsin (2009)


 * Note the discharge is represented in cubic feet per second (cfs), as opposed to cumecs

The //**rising limb**// of the graph illustrates the speed of the increase in dicharge from the storm event. The faster water may enter the river the steeper the rsing limb. This is significant in terms of flooding as very quick rises in the discharge of a river lead to //**flash flooding**// events. The //**peak discharge**// is the point at which the river reaches its maximum discharge from the water added by the storm event. The //**lag time**// is the time taken for the river to reach its peak discharge. A high peak discharge along with a short lag time, would leave people with little warning of flooding and catastrophic flash floods result. If the rising limb is steep but not enough to cause flash flooding, providing there is enough water from the storm, //**rapid onset flooding**// may occur. On the other hand, if the rising limb is shallow and water takes a long time to reach the river, then //**slow-onset flooding**// results. The total amount of water from the storm event will determine the extent of the flooding.

The //**falling limb**// of the graph illustrates the speed at which the discharge of the river returns back to normal after a storm event. The steeper the falling limb the quicker the water is exiting the system allowing the river to return to its //**baseflow (normal flow)**// level. Some hydrographs show a shallow falling limb, as water can still ber entering the river via the soil or rock even after the precipitation from the storm event has subsided. This continual input of water prevents the river from reaching its baseflow quickly, as opposed to a storm event whereby the majority of water entering the sytem has come from surface run-off.

Some hydrographs may show the //**bankfull discharge**// of the river, if this point is exceeded the river will overflow its banks causing flooding.

= __**Factors influencing the shape of storm hydrographs**__ =



The features of a drainage basin along with the nature of the storm event greatly influence the shape of the hydrograph. In fact, it is possible to look at the shape of a storm hydrograph and make assumptions about the storm event and the drainage basin.

**River Nile**


[|Recent news article about Egypt] rejecting a new agreement about water supplies in the River Nile - situation similar to SA and the River Murray!