River discharge is defined as the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs). The overall discharge from the drainage basin depends on the relationship between percipitation, evpotranspiration and storage factors and can be summarised as follows:
drainage basin discharge = percipitation - evapotranspiration +- changes in storage
Discharge can be illustrated using hydrographs. These can show annual patterns of flow in reponse to climate. Short term variations in discharge are shown using a flood or storm hydrograph.
The storm hydrograph
The storm hydrograph shows variations in the river's discharge over a short period of time, usually during a rainstorm. The starting and finishing level show the base flow of the river. As storm water enters the drainage basin the discharge rises, showing the rising limb, to reach the peak discharge, which indicates the highest flow in the channel. The receding limb shows the fall in discharge back to the base level. The time delay between maximum rainfall amount and peak discharge is the lag time. The shape of the hydrohraph is influenced by a number of factors:
- the intensity & duration of the storm - if both are high they produce a steep rising limb as the infiltration capacityof soil is exceeded.
- the antecedent rainfall - heavy rain falling on soil which is saturated from a previous period of wet weather will produce a steep rising limb.
- snow - heavy snowfall may not initally show on a hydrograph since the water is being 'stored' in snow on the ground. Indeed, water levels in the river may fall during a prolonged period of snowfall and cold weather. When temperatures rise and melting occurs, massive amounts of water are released, greatly increasing discharge. This water may reach the river channel even quicker if the ground remains frozen and restricts infiltration.
The storm hydrograph 2
- porous soil types and/or permable rock types - such as limestone - these produce less steep hydrographs because water is regulated more slowly through natural systems.
- impermeable rock types such as granite and clay - these tend to have higher densities of surface streams (higher drainage intensities). The higher the deinsity the faster the water reaches the main river channel, causing rapid increases in discharge.
- size of the drainage basin - a small drainage basin tends to respond more rapidly to a storm than a larger one, so the lag time is shorter.
- shape of the drainage basin - rainfall reaches the river more quickly from a round basin than a enlongated basin.
- slope angle - in steep-sided upland river basins the water reaches the channels much more quickly than in gently sloping lowland river basins, producing a steeper rising limb and shorter lag times.
- temperature - high temperatures increase the rate of evapotransipration, thereby reducing discharge. Cold temperatures may freeze the ground, restrict infiltration, increase overland flow and increase river discharge.
- vegetation- this varies with season. In summer there are more leaves on deciduous trees, so interception is higher and peak discharge is lower. Plantations of conifers have a less variable effect.
The storm hydrographs 3
- land use - water runs more quickly over imperable surfaces such as caravan parks or agricultural land which has been trampled by cattle. Lag time is reduced and peak discharge is increased.
- urbanisation - this is the main human impact on a storm hydrograph. The following processes combine to alter the shape of the hydrograph by reducing the lag time and increasing peak discharge:
1 - removal of topsoil and compaction of the ground with earth-moving machinery during building work.
2- building of roads which increases the impermeable surface area
3- building of drains and sewers that transport water rapidly to river channels, reducing the lag time.
4- starightening of river channels and lining with concrete. This also leads to the faster delivery of water downstreams of the irban areas and increases the risk of flooding downstream areas.
Less water reaches the channel by throughflow and base flow, and more gets there by overland flow. Local authorities abd water companies may need to responds to these changes to prevent damaging flooding in thier areas.