Topic: Drainage Basins As Open Systems 3.1.1.2 Runoff, Hydrographs & Changes in the Water Cycle Over Time

Topic: Drainage basins as open systems 3.1.1.2 Runoff, hydrographs & changes in the water cycle over time

What you need to know How runoff varies within the water cycle. How to analyse a flood hydrograph How the water cycle changes over time

Introduction:

Runoff (the flow of water over the Earth’s surface) can vary depending upon a range of physical and human factors.

These include: • Time of year. • Storm conditions. • Vegetation cover. • Soil saturation levels. • Topography & relief. • Agricultural land use. • Urban land use.

Physical factors affecting runoff:

Time of year

In temperate climates, where seasonal change is evident, runoff levels can vary greatly throughout the year. In summer, runoff levels can be low due to a reduction in rainfall. Soil saturation levels will be low and therefore any rainfall at this point can easily infiltrate into the ground. However, intense baking of the soil by the sun can lead to the soil becoming effectively impermeable and summer storms can lead to high levels of runoff as the rain is unable to soak in. This can lead to flash flooSAMPLEds. In winter, precipitation may be in the form of snow and the water may be stored on the ground due to low temperatures. Warmer temperatures in spring may lead to snowmelt and this can lead to the soil reaching field capacity quickly. Further meltwater will therefore run over the surface.

Storm conditions

Intense storms with heavy rainfall can lead to soils quickly becoming saturated. This can happen in two ways:

• Prolonged rainfall – moderate to high volumes of rainfall over a sustained period can eventually saturate the soil leading to runoff. • Intense rainfall – heavy rain in a short period can bounce and then flow over the Earth’s surface as runoff.

Vegetation cover

Vegetation can intercept precipitation and reduce runoff. Leaves and stems can capture rain and prevent it from reaching the ground. It will eventually reach the ground but the process will have been slowed by the vegetation and therefore water will infiltrate into the ground rather than runoff.

Dense vegetation with proteoid roots (hairy roots with a large surface area) can absorb large volumes of water even in storm conditions, which will prevent runoff.

Soil saturation levels

Where field capacity is reached in the soil, no more infiltration can take place and therefore runoff will occur.

Topography & relief

In “v” shaped valleys on steep slopes, runoff will increase due to gravity. On flatter surfaces runoff is less likely to happen as water will be able to infiltrate into the soil more easily. Equally, depressions in the Earth’s surface and an undulating relief can allow water to collect and reduce runoff.

Human factors affecting runoff:

Agricultural land use SAMPLE Initially, agricultural land use can have the same impact as vegetation cover, in that crops can intercept precipitation and reduce runoff. However, intensive agriculture where irrigation may be used, can cause waterlogged soils and therefore lead to runoff. Heavy use of agricultural machinery can compact the soil and reduce its infiltration capacity, making runoff more likely.

Urban land use

Changing greenfield surfaces to impermeable concrete and tarmac as construction takes place can increase the level of runoff in an area.

Flood hydrographs

These are graphs that show how a drainage basin responds to a period of rainfall. They are used to plan for flood situations and times of drought. They show the river discharge that occurs as a result of precipitation from an earlier storm.

Key terms on the hydrograph are:

• Peak rainfall – time of the highest rainfall level. • Peak discharge – time of the highest river channel level. • Lag time – difference in time between the peak rainfall and peak discharge. • Rising limb – the increase in river discharge. • Falling limb – the fall in river discharge. • Base flow – normal river level.

SAMPLE

Physical factors affecting flood hydrograph dynamics

Drainage basins all have a variety of characteristics. The range of characteristics can affect how likely a river within the basin will flood.

Size: the smaller the drainage basin the less time it takes for water to reach the river, resulting in a shorter lag time. Large river basins will have a larger lag time.

Shape: a circular shaped drainage basin leads to rapid drainage whereas a long drainage basin will take time for the water to reach the river.

Topography & relief: the steeper the basin the more quickly it drains. Indented landscapes will collect water and reduce runoff rates, reducing the amount of water reaching the river channel.

Heavy Storms: runoff will increase after soil field capacity is met which means water will reach the channel quicker.

Lengthy rainfall: leads to the ground being saturated and runoff will increase which means water will reach the channel more quickly once soil capacity has been reached.

Snowfall: until the snow melts, the water is held in storage but when the snow melts this can lead to flooding.

Vegetation: this can reduce discharge as it intercepts precipitation. Roots of plants can also take up water that goes into the soil. Seasonally, in the UK the vegetation will reduce discharge in the summer whereas in the winter it will have less of an impact due to less foliage being present on trees.

Rock type: the underlying geology varies within drainage basins and can be permeable (allowing water through) or impermeable (not allowing water through). Impermeable rocks encourage greater amounts of surface run-off and a more rapid increase in discharge than permeable rocks. SAMPLE How the water cycle changes over time:

The inputs, transfers, flows, stores and outputs of the water cycle change over time for a variety of reasons. Some may be natural but others are anthropogenic (human-factors).

Natural changes over time affecting water cycles:

• Storm events: these lead to an increase in both channel flow and surface runoff. Depending upon the drainage basin, flood events can occur.

• Seasonal changes: seasons with high levels of precipitation lead to increased surface runoff and channel flow. In contrast, drier seasons will lead to reduced river discharge and no runoff. In mountainous regions, increased channel flow and run off can occur due to ice melt. Countries to the south of the Himalayas will face flooding during this time but also will utilise the extra water for agricultural and domestic use.

• Ecosystem changes: plant successions may change the dominant type of vegetation in an area. If vegetation dies off due to natural events there will be less absorption of water by plant roots, and less transpiration – which may reduce precipitation.

Human changes over time affecting water cycles:

• Climate change: increasing global temperature is leading to a reduction in size of mountain glaciers and therefore future dependency on this water will become more of an issue as this input declines after a period of increased discharge. Potential drought conditions and associated economic and social impacts will be likely consequences.

• Farming practices: particularly in hotter climates, farming can have a significant effect on the water cycle. Irrigation for plants can lower channel levels in rivers together with groundwater levels if wells are the source for the irrigation.

• Deforestation: removing vegetation for agriculture, urbanisation or – most frequently in many rural developing countries – for fuel supply, an important water store and water-transfer capacity is lost. Soil moisture reduces, transpiration declines and micro-climates give less precipitation, leading to local river systems drying up.

• Land use change: change from natural landscape to urbanised landscape increases impermeable surfaces. This leads to an increase in runoff and a reduction in infiltration. Cities create drainage systems to take water quickly away from the urban environment. However, this can lead to flooding as river SAMPLE levels then receive this water too quickly in a large amount.

• Water abstraction: growth of global population and in particular in countries where climates are drier has resulted in increased water demand. Where precipitation levels are low, an alternative supply is ground water. This supply of water in porous rocks underground is known as an aquifer. Excessive abstraction of this water, where it is taken too quickly and does not recharge naturally, is unsustainable and can lead to groundwater stores being depleted.

River water is also used for water supplies for both domestic and industrial uses and that can affect river levels. In conjunction with limited groundwater reaching the river, the river can ultimately dry up.

Specification topic: Runoff, hydrographs & changes in the water cycle over time 3.1.1.2 Case study: Mississippi drainage basin The Mississippi drainage basin is the fourth largest in the world and covers approximately 1.2 million square miles. The transfers and stores within this complex system change naturally and involve human impact across a year.

Natural changes and the impacts upon runoff:

The flow of the Mississippi River is maintained by runoff from precipitation or snowmelt. However, not all rainfall or snowmelt runs off into the river channels. Some infiltrates the soil and is eventually transpired into the atmosphere by plants or evaporates directly into the atmosphere from land and water surfaces within the drainage basin. In total over the basin, evapotranspiration losses account for more than three quarters of the annual precipitation. However, these losses vary regionally. In the arid western section of the drainage basin, very little water is left to run off after evapotranspiration occurs. Most of the river's flow is runoff from the wetter eastern part of the drainage basin.

Flow also varies over time corresponding to variability in precipitation and evapotranspiration. Seasonally, there is an increased amount of precipitation during winter months compared to the drier summer months. The simple seasonal pattern is far too simplistic, however, and there is a variability in precipitation levels within the seasons. One of these variable times is when the north west region receives lower rainfall than the south east corner, particularly for the months of November to February. The uneven nature of the rainfall distribution as well as the amount of rainfall significantly alters runoff levels throughout the year.

Weather events such as storms and drought conditions can also impact upon the amount of runoff within the basin and the flow shape on the hydrograph. During storms, high levels of precipitation can lead to soil water storage limits being reached and the soil becoming saturated. This then leads to increased surface run off which increases discharge within the river channel. Droughts can have the opposite effect at first, with the limited precipitation, reducing river discharge. However, over a longer time period, drought conditiSAMPLEons can harden and bake the soil, especially in summer months - which converts the ground into an impermeable surface. Subsequent rainfall following a period of sustained drought can then lead to rapid runoff and flooding.

Human impact upon runoff:

There are many human impacts affecting runoff; notably increased urbanisation, agricultural land use and over-abstraction. Within the Mississippi basin there are two major human factors which interfere with the natural systems within the basin. The main human activity which

significantly affects runoff is agriculture. In the arid western part of the drainage basin, where agriculture is a key economic activity (producing 92% of the USA’s total agricultural export) large volumes of runoff are collected in man-made reservoirs. These store significant volumes of water, which is then extracted and - through irrigation - waters crops. The change in the water cycle here, whereby the water is diverted from flowing across the surface or infiltrating and flowing as throughflow towards the river impacts greatly upon the amount of water reaching the river channel. Storage and then mass use of irrigation results in higher levels of evapotranspiration from crops; so this water is then returned to the atmosphere.

The second key human impact is related to the operation of reservoirs within the basin. Along the course of the Mississippi, there are 43 dams which hold back the water. Approximately 6% of total runoff is transferred to the atmosphere by the higher levels of evaporation occurring during summer months from the surface of reservoirs. Increasing population and demands for domestic, urban and industrial uses of water has led to an increase in the number of these reservoirs.

Unsurprisingly, the biggest factor likely to affect flow patterns is climate change. Climate change predictions for the USA estimate that the amount of available water within the Mississippi drainage basin is set to change initially due to changing precipitation patterns - with an expected reduction of 5 inches (12.7 cm) of rainfall a year in the lower Mississippi basin. With less rainfall, runoff and channel flow will both decrease. Combined with the increasing potential evapotranspiration rates and increased demand for irrigation water by agricultural uses this could cause major disruption to the systems within the basin with many tributaries drying up, at least during the drier months.

Exam style questions:

1. Examine the view that human activity is having a greater impact than natural factors on the water cycle (9 marks)

2. Using an example, describe and comment on the effects of human activities on a river SAMPLE system. (6 marks)

1. Examine the view that human activity is having a greater impact than natural factors on the water cycle (9 marks)

This question requires the candidate to demonstrate an understanding of how both natural and human factors can affect the water cycle and its components. Throughout the response the candidate needs to examine the extent to which both natural and human factors affect the processes in the cycle.

The key focus of this question is to show understanding of:

1. The natural factors that can affect the water cycle: seasonal changes in precipitation and evapotranspiration alongside other natural factors such as rock type, vegetation cover etc. 2. The human activities that can affect the water cycle: demand for water (reservoir construction & over-abstraction from groundwater stores), agricultural use and irrigation as well as the impacts of climate change. 3. Overall conclusion: is human activity having a greater impact? - support with evidence from within the points that have been made.

2. Using an example, describe and comment on the effects of human activities on a river system. (6 marks)

Refer to how increasing use of water in the Mississippi drainage basin is affecting the river system.

Detailed description of what is being done in the drainage basin and how this is changing the natural water system and its processes:

• Surface runoff reduced due to increased evaporation from large scale reservoirs created for the needs of the USAs population. • Reduced natural runoff as many dams and water storage facilities prevent the natural overland flow from occurring. Linking this to the water demand factor, as population demand increases, will secure higher marks. • Extraction of water from groundwater supplies will reduce storage and underground flows (throughflow, percolation and groundwater flow). • Construction of impermeable surfaces can increase surface runoff after intense rainfall leading SAMPLE to increased river discharge and possibly localised flooding. • Abstraction of water from rivers for a range of human needs can reduce channel volumes.

You could also mention the indirect effect that climate change caused by the enhanced greenhouse effect can have on changing rainfall patterns and therefore other processes within the basin.

Q1 True or False? A Runoff depends entirely on the level of precipitation B The velocity of runoff is affected by the gradient of land C The same soil can vary in terms of its infiltration capacity at different times D It’s not just the amount of rainfall but its distribution which affects runoff E A flood hydrograph predicts when a river will flood

Q2 Match each term to the correct description A The entire area from which a drop of rainfall eventually reaches a river B Number of hours between maximum rainfall and peak river discharge C The shape of the land surface D Rock quality permitting water to flow through it by means of fissures & joints E Standard level of water in a river

Select from: Topography Base flow Lag time Drainage basin Pervious

Q3 Tick which is the odd one out from each group of 6 terms A Drizzle Hail Sleet Evaporation Snow Rain B Rising limb Base flow Peak discharge Falling limb Infiltration Lag time C Confluence Drainage pattern Watershed SAMPLESource Tributary Impermeable rock D Flooding Water vapour Precipitation Evaporation Condensation Solar energy E Permeable Pervious Evapotranspiration Porous Impervious Infiltration capacity

Q4 Decide which factors will lead to a long lag time/small peak discharge flood hydrograph and which will result in a short lag time/high peak discharge flood hydrograph Long lag time / small peak discharge Short lag time / high peak discharge

Impermeable surface Intense prolonged rain Small river basin

Steep topography Long drought before rainfall Urban growth on farmland

Deep soil layer Porous rock Rapid snow melt Afforestation

Q5 Suggest ways in which human activity can affect the hydrological cycle over time A Amplifying the hydrological cycle

B Reducing the hydrological cycleSAMPLE

Q1 True or False? A Runoff depends entirely on the level of precipitation False B The velocity of runoff is affected by the gradient of land True C The same soil can vary in terms of its infiltration capacity at different times True D It’s not just the amount of rainfall but its distribution which affects runoff True E A flood hydrograph predicts when a river will flood False

Q2 Match each term to the correct description A The entire area from which a drop of rainfall eventually reaches a river Drainage basin B Number of hours between maximum rainfall and peak river discharge Lag time C The shape of the land surface Topography D Rock quality permitting water to flow through it by means of fissures & joints Pervious E Standard level of water in a river Base flow

Select from: Topography Base flow Lag time Drainage basin Pervious

Q3 Tick which is the odd one out from each group of 6 terms A Drizzle Hail Sleet Evaporation ü Snow Rain All the other 5 are forms of Precipitation. Evaporation is a process that contributes to precipitation. B Rising limb Base flow Peak discharge Falling limb Infiltration ü Lag time All the other 5 are features of a flood hydrograph. Infiltration is a preceding process. C Confluence Drainage pattern Watershed Source Tributary SAMPLEImpermeable rock ü All the other 5 are features of a drainage system. The rock type is independent of this. D Flooding ü Water vapour Precipitation Evaporation Condensation Solar energy All the other 5 are features of the hydrological cycle. Flooding may or may not be a consequence. E Permeable Pervious Evapotranspiration ü Porous Impervious Infiltration capacity All the other 5 are qualities of surfaces that allow determine whether water penetrates the ground.

Deep soil layer Impermeable surface Porous rock Intense prolonged rain Afforestation Small river basin Steep topography Long drought before rainfall Urban growth on farmland Rapid snow melt

Impermeable surface Intense prolonged rain Small river basin

Steep topography Long drought before rainfall Urban growth on farmland

Deep soil layer Porous rock Rapid snow melt Afforestation

Q5 Suggest ways in which human activity can affect the hydrological cycle over time A Amplifying the hydrological cycle

Contributing to global warming à more evaporation of water vapour à more rainfall

Contributing to global warming à faster glacier melt à greater river discharge

More impermeable surfaces (cities) à faster runoff à shorter time lag for discharge

Increased surface water storage à more evaporation à increased precipitation SAMPLE

B Reducing the hydrological cycle

Removal of natural vegetation à reduced transpiration à less precipitation

Reduction of surface water à reduced evaporation à less precipitation (Aral Sea)

Greater river abstraction à reduced flow à less evaporation from river channel