River Basin can vary over time

Both the inputs to and outputs from a river basin can vary both spatially (in different areas) and temporally (over time), due to daily, seasonal and annual occurrences. Precipitation mainly reaches the grounds surface from the atmosphere as snow and rain. Heavy snow is often intercepted and stored on the tops of hills and mountains and may lead to a decrease in river levels. Some rainfall may fall directly into the stream or river and some may fall onto the land and flow into the river basin through a variety of routes including throughflow, stemflow and overland flow.

The intensity and duration of precipitation has a short term but regular effect upon both the inputs and outputs of a river basin due to the weather patterns. Over a short period of time, for example a week, the precipitation levels over the river basin can significantly vary, as on some days of the week the total precipitation can be extremely high and the next it could be virtually zero.

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Long periods of rainfall are the main cause of flooding as the soil has become saturated it reaches its infiltration capacity and infiltration is reduced, therefore increasing the frequency of surface run off and the risk of flooding. In addition to the weather having a large impact on the inputs over a short period of time in and around the catchment area of the river basin the seasons also influence the levels of precipitation. For example in winter precipitation can fall in the form of snow and in summer the precipitation levels often decrease dramatically, due to the vast heat imposed on the surface of the land.

High temperatures especially in summer increase evapotranspiration rates which in turn can reduce the river discharge but also can have an effect upon the stored water. This stored water is in the form of snow and icecaps on mountains and high temperatures cause the snow to melt and the discharge to increase rapidly at a dangerous rate. On a daily basis, this meltwater can fluctuate because at the hottest time of the day, the increase in temperature accelerates the rate at which the ice melts and creates more meltwater.

In spring and summer these increased temperatures result in greater melting of glacial areas which come to a halt in autumn and winter due to the decrease in temperature. Cold temperatures in winter may increase the levels of surface runoff and total discharge as the ground may become frozen and restrict the frequency of infiltration into underwater stores, but also may decrease the amount of melting in glacial areas due to the decrease in temperature.

Between spring and autumn there is a period of adjustment where the precipitation can vary widely and can be extremely increased or decreased. Climatic change and wind patterns can have a long term effect upon the inputs to the river basin as they also influence the precipitation levels. For example the smaller amount of precipitation a catchment area receives, the less input to the river basin. The wind patterns would determine the positioning of clouds and the possibility of relief rainfall.

The size, shape and relief of a river basin also has a role in a rivers discharge, because in a large river basin, the water has much further to travel (lag period) than that in a small river basin before it reaches the main river channel, and therefore compared to a river with a small river basin, the total river discharge is smaller. Circular rather than elongated river basins have a tendency for shorter lag periods and higher peak flows and a steep sided upland river basin due to the force of gravity acting upon the flow of water will have a higher discharge than that of gently sloped river basin.

Lithology of river basins can also affect the output of a river as those river valleys which are made up of permeable or porous rocks such as chalk and sandstone allow the water to pass through them into underground water stores for instance aquifers. Limestone also allows water to pass through it into bedding planes and joints. Due to this, areas of the river basin which are made up of this type of rock will have low rates of surface run off and no surface streams due to high rates of infiltration through the rocks and soil, reducing or moderating the input into the river’s main channel.

Impermeable rocks such as granite however, will not allow any water whatsoever to infiltrate through them and areas of land which contain these rock types in their lithology will be subject to a higher drainage density (surface streams) and a higher level of runoff into the main river channel, decreasing the overall lag period of the river basin and increasing the total output, as well as causing the discharge to fluctuate rapidly over a short period of time and can be therefore more prone to flooding.

The rate of infiltration, throughflow and amount of soil moisture storage is controlled by the type of soil. Sandy soils encourage infiltration due to the large pore spaces which can be found between each particle, whereas soils which are made up with clay encourage very low rates of infiltration as clay soils are not porous as the particles are very close together and therefore encourage surface runoff and therefore the output increases. There are three main land use factors which can affect a river basins discharge or output.

These are natural vegetation cover, agricultural landuse and whether the area is urban or rural. The vegetation of an area intercepts most of the rainfall it receives, whereas on bare, open ground there is no interception at all and the precipitation can flow into the rivers main channel through a variety of routes. Trees and plants help to reduce the amount and rate at which water enters the river channel through interception, root uptake and evapotranspiration. As a result of this, deforestation can increase overland flow, severe erosion and flooding inside the river basin itself.

Agricultural landuse has a direct effect upon the vegetation on the land. Permanent crops such as grassland slow down the water flow compared to annual crops such as cereals. Urbanisation has created more impermeable surfaces such as roofs and roads and rainwater now flows into drains. The impermeable surfaces reduce the rate of infiltration, but the drains however increase the transfer of water into the main river channel and therefore increase the total discharge and risk of floods in urban areas.

The level of output from a river basin from transpiration varies considerably over time. This is because it relies upon the temperatures and humidity of the atmosphere as it occurs when the vegetation releases the moisture it has intercepted via root uptake. As the humidity of the atmosphere increases, the rate at which the vegetation releases moisture decreases and this fluctuates on a daily basis. In the morning when the temperatures are low, the rate of release is low, but as the day goes on and the temperature increases, so does the rate of evapotranspiration.

Seasonally, the rate of transpiration is higher in the summer months as the temperatures are traditionally higher during this period. Man has had an influence in afforestation and deforestation which also affects the transpiration levels and river basin outputs over time. The variation in levels of evaporation can also have an effect upon a river basins output. Evaporation like transpiration is influenced by humidity and temperature.

Rates of evaporation are highest when the humidity of the atmosphere is low and increases as the temperature of the area with the river basin increases. In the summer, evaporation is more frequent due to the higher temperatures and low levels of humidity and in winter, evaporation rates are very low as the wetter and colder periods decrease the rate of evaporation. Evaporation has also been reduced by man building amenities such as drainage systems where the surface water is channelled underground.

As an increased demand for water through industry, homes and expanding populations also contributes to a river basins overall output, fresh water stores, demand by man, global warming and precipitation levels are only a few of the short term effects placed upon the rivers overall output total. Groundwater stores and levels of precipitation are linked to the stream flow, baseflow and throughflow found in a river basin, all of which have a large impact upon the total output of the basin itself.

An increase in the levels of groundwater can lead to a decrease in overland flow and the output of the river. The numerous inputs and outputs of a river can vary dramatically over time due to changes in all aspects of life including climate, urbanisation, relief and precipitation levels which fluctuate both diurnally and seasonally. As the weather and atmospheric conditions are now unpredictable, no one can determine how these inputs and outputs will continue to change over time.