Rivers, Floods & Management

The drainage basin is the catchment area from which a river system obtains its water from. An imaginary line called the watershed delimits one drainage basin from another. The watershed generally follows a ridge of high land; any rain falling on the other side of the ridge will eventually flow into another river in the adjacent drainage basin.

The drainage basin hydrological cycle is an open system with inputs and outputs - water and energy from the sun are introduced into the drainage basin from outside and water can be lost from the drainage basin in a number of ways.

The drainage basin hydrological cycle can be studied using a system approach:

• Inputs to the drainage basin include: energy from the sun for evaporation, percipitation (rain, snow).
• Outputs move moisture out of the drainage basin and include: evaporation and transpiration from plants known as evapotranspiration, runoff into the sea, water percolating deep into underground stores where it can be effectively lost from the system.

• Many stores of water within the drainage basin (such as glaciers, lakes, rivers and puddles) occur on the surface. Other stores are less odvious, for example, vegetation stores water by interception and plants contain a significant amount of water taken up from the soil tthrough thier roots. The soil itself holds water and groundwater is stored in permeable rocks.
• Transfers and flows move water through the system and enable inputs of water to be processed from one store to another. Transfers include: throughflow, throughfall, stemfloe, inflitration and groundwater flow.

Withing the drainage basin, the balance between inputs and outputs is known as the water balance or budget. Rivers are present on the surface only is the stores are capable of releasing water and if there is direct surface runoff. This is a dynamic relationship; river levels rise and fall over the short term following heavy rainfall and often show an annual pattern (called the river's regime) in terms of their discharge. The water balance can be shown using the fromula:

percipitation = streamflow + evapotranspiration + changes in storage

(p) = (Q) + (E) + (S)

When percipitation is greater than evapotranspiration, at first the pores of the soil are refilled with water. When the soil beomes saturated, excess water has difficulty infiltrating into the ground and may then flow over the surface.

The water balance of a particular location can be studied using a water budget graph. This shows the relationship between temperature, percipitation, and evaporation rates over a year. During the months of the year when percipitation exceeds evapotranspiration, once the soil has been recharged there will be a water surplus available to supply river s & streams.