The Hydrological Cycle
This describes the process whereby water in its various forms is continually cycled between the land, sea and atmosphere. It also makes its way into the biosphere to influence animal and plant ecosystems around the globe.
This is a common approach in geography and the two main examples in this topic are:
The hydrological cycle: a closed system.
The drainage basin system: an open system.
Both consist of transfers, stores, inputs of water but the hydrological cycle is a closed system as no gains or losses from outside are added to the system.
The drainage basin system is said to be open as both inputs and outputs of energy and material occur. All systems in their natural state aim to be in a state of balance (dynamic equilibrium) as this is when they function best. Heavy rainfall, drought and human activity such as deforestation can easily upset the balance.
Within the hydrological cycle, four main processes operate:
1. Interception
This is when plants prevent some rainfall from directly reaching the ground, for example, water on leaves or foliage. It may later reach the floor via stem flow (water flows down the stem to the ground) or through-fall, where water drips to the ground. Secondary interception occurs at ground level where water hits undergrowth. Some water returns to the atmosphere via evapotranspiration.
2. Evapotranspiration
Water lost from vegetation via both evaporation and transpiration. Click on the boxes below to reveal definitions of both terms:
Potential Evapotranspiration:
The amount of water that could be lost by evapotranspiration. For example, this is potentially high in deserts, but the amount that can take place is limited due to the minimal moisture available. Actual evapotranspiration is what actually occurs. In the UK there is more water available for evapotranspiration than takes place.
3. Infiltration
Where water slowly soaks into the soil from the ground. The maximum rate at which this can occur is known asinfiltration capacity (mm/hour) and it is dependent on the amount of water already present in soil structure and vegetation.
4. Precipitation
The most important input into the system forms includes snow, hail, rain, and fog.
Percolation:
Water in the soil does not remain there but moves down slowly into the lower layers of soil and rock. It creates groundwater storage found in rocks and this may later be moved sideways through the rock via groundwater flow.
Flow:
Water flows through the hydrological cycle in various ways:
Throughflow: where water moves downwards through layers of soil.
Channel flow: downhill movement of water in rivers.
Groundwater flow: Lateral movement of water from the water table.
Drainage basins
The drainage basin - as stated earlier - is an example of an open system, and many of the terms above are central to it. Below is a flow diagram of the drainage basin system:
All rivers receive a water supply and the area of land this comes from is known as a drainage basin. The boundaries of the basin are known as the watershed and will usually be marked by areas of higher land.
Drainage basins have many different characteristics that influence how quickly or slowly the main river within them responds to a period of intense rainfall, these are outlined in more detail in the section relating to storm hydrographs.
Land is drained by rivers in a variety of ways these are exhibited as drainage patterns.
This is shown in the diagram below:
This relates to the number of streams in a particular drainage basin and can be measured by dividing total length of all streams in a basin (L) by its area (A). As a rule, the higher the drainage density (D) the more quickly water drains to a river.
D = total L/A
Characteristics of high and low-density drainage basins:
| High density (+2km per km2) | Impermeable land surface, steep slopes, limited vegetation cover, limited rainfall, gentle slopes, large channel frequency (tributaries). |
| Low density (-2km per km) | Permeable rock, for example, chalk, much vegetation cover, limited rainfall, gentle sloes, lower channel frequency. |
Storm hydrographs and river discharge
Storm hydrographs are graphs that show how a drainage basin responds to a period of rainfall. They are useful in planning for flood situations and times of drought as they show the discharge (amount of water reaching channel via surface run-off, throughflow, and base flow) that originated as precipitation.
A great deal of information can be gleaned from a hydrograph and the interpretation of them is often tested in exam questions. The diagram below shows the main points:
Drainage basins all have a variety of characteristics in terms of vegetation, geology, soil type and so on, all of which interact to influence how quickly or slowly river discharge increases after a storm. The table below outlines the major influences on hydrographs and drainage basins:
| A | Size of basin, shape and relief | Size - the smaller the basin the less time it takes for water to drain to the river, resulting in a shorter lag time. Shape - the shape of basin that lends itself to most rapid drainage is circular. In a long, narrow basin water takes longer to reach the river. Relief - the steeper the basin the more quickly it drains. |
| B | Forms of precipitation | Heavy Storms - in such a situation, rainfall is often far in excess of the infiltration capacity of the soil leading to much overland flow, and rapid rises in river levels. Lengthy rainfall - leads to the ground being saturated and overland flow. Snowfall - until snow melts, potential discharge for a river is held in storage. Rapid melting can lead to flooding. |
| C | Temperature | High rates of evapotranspiration reduce amounts of discharge, and low temperatures can store water in the form of ice and snow. |
| D | Land Use | Vegetation - Important in reducing discharge as it intercepts precipitation and adds to rates of evapotranspiration. Roots of plants take up water reducing throughflow. Interception is less in winter in the UK due the shedding of leaves from deciduous trees. Flooding is more likely in deforested areas. |
| E | Geology | Rock type varies within drainage basins and can be permeable (allowing water through) orimpermeable (not allowing water through). Permeable rocks can be porous such as chalk that store water within them or pervious, such as limestone where water flows along bedding plains. Impermeable rocks encourage grater amounts of surface run-off and a more rapid increase in discharge than permeable rocks. |
| F | Soil | A control on the rate of infiltration, amount of soil moisture storage and rate of throughflow. Larger pore spaces as found in sand, allow for greater water storage and limit the risk of flooding. |
| G | Drainage density | As stated earlier, the higher the density the greater the risk of flooding. |
| H | Tides and storms | High spring tides (illustrated by the Severn Bore) prevent water from entering the sea and increase the risk of flooding. |
| I | Urbanisation | A major impact because of its alteration on the hydrological process. The main affects are shown on the diagram below. Click on the magnifying glass to see the graph in more detail. |
The water balance
This is the balance between inputs into a drainage basin and outputs. It is important for understanding the processes operating in a drainage basin and water balances throughout the year.
It is expressed as follows:
P = Q + E (+/- change in storage)
P = precipitation
Q = run-off
E = evapotranspiration
The diagram below illustrates the main features of the water balance:
Here are some questions and answers that will help you to learn to read the graph accurately:
In which months is there a water surplus?
Jan, Feb, Mar, Apr, Nov, Dec.
Why is there soil moisture recharge in October?
Due to the excess of evapotranspiration over precipitation in May - Sept.
When is field capacity attained?
November.
Why is a water deficit not shown on the graph?
Precipitation always far exceeds evapotranspiration.
Soil moisture
Surplus: If precipitation exceeds evapotranspiration and the excess is not been used by plants.
Deficiency: Evapotranspiration exceeds precipitation.
Recharge: Replacement of water lost during drier periods.
Field capacity
The maximum amount of water soil can hold.
A water surplus can result in wet soils, high river levels and run-off whereas a deficit leads to dry soil, falling river levels and possibly drought. Management is shown in the example at the end of this topic.
Water Deficit
Evapotranspiration is in excess of precipitation and any previously available moisture has been used, in soil moisture utilisation.
The regime of a river is expected to have a seasonal pattern of discharge during the year. This is due to factors such as climate, local geology and human interaction. Equatorial rivers have regular regimes but in the UK where seasons exist one or two peaks may be recognisable.
These show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.
If a river has more than one period of high water levels and/or low water levels, a more complex regime results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, The Rhine.
A river has two main functions: one, to transport water and two, to transport sediment. The type of flow that occurs depends on factors such as gradient, volume of water, channel shape, and friction.
There are two types of flow:
Laminar Flow: This rarely occurs, water flows smoothly in a straight channel. It is most common in the lower parts of a river. It is shown in the diagram below:
Turbulent flow: This is far more common, it occurs where the shape of the rivers channel is varied with pools, meanders, and rapids. A great deal of turbulence results in sediment being disturbed. The greater the velocity the larger the quantity and size of particles that can be transported. Turbulent flow is illustrated in the diagram below:
Channels
It is often thought that the velocity of a river is greatest near its start. This is not the case, as large angular boulders create a rough channel shape and therefore, a large amount of its bed friction. This creates more resistance to flow than a river with smooth clays and silt forming its banks. The roughness coefficient is measured using Manning's 'n', which shows the relationship between channel roughness and velocity. The equation is as follows:
v = velocity
R = hydraulic radius
S = slope
n = roughness
A high-value indicates a rough bed.
The efficiency of a rivers channel is measured by finding its Hydraulic radius. It is the ratio between the length of wetted perimeter and cross section of a river channel.
Wetted perimeter: the entire length of the riverbed bank and sides in contact with water.
The examples below show the difference between an efficient river channel and an inefficient river channel:
| Cross-section area: | Wetted perimeters: | Hydraulic radius: |
|---|---|---|
| Stream A = 400m2 | Stream A = 18m | Stream A 40 / 18 = 2.22m |
| Stream B = 40m2 | Stream B = 24m | Stream B 40 /24 = 1.66m |
Channel slope
Steeper gradients usually lead to greater velocities, because of the influence of gravity.
The long profile
The characteristics of a river and its valley found in this course include vertical erosion, lakes, waterfalls, potholes, rapids and gorges. Overland flow is found in depressions making lakes. Eventually, the channel gets deeper and waterfalls become rapids.
Characteristics include, lateral erosion, transportation, floodplain, meanders, truncated spurs, and river cliffs. The gradient of the river is increasingly even and smooth, and the flood plain begins to develop.
Here, transportation and deposition are found. The channel is large and features such as Deltas, levees, bluffs and meanders are found. The flood plain increases in size as meanders migrate downstream.
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