Hydrological Cycl1 Notes 28-2-09

HYDROLOGICAL CYCLE: 1. Hydrology: study of water. 2. Hydrological cycle: movement of water from land, sea and air, back to land and it continues. Draw a well label diagram showing hydrological cycle:

 The Hydrological cycle is a system because it has a set of components which are links to one another; input, outputs and process (transfer) and storage.  Is the hydrological cycle a closed or an opened system? Why? (Draw diagram fig. 2.1, Study and Revise Geography As/A2 p: 9)

It is a closed system because: (i) Water circulates continuously and the processes is fuel by energy fro the sun. (ii) There is no gain or loses in the cycle and there is a fixed amount of water. Refer to diagram of a hydrological cycle page 1, and define the terms below: 1. Precipitation: 2. infiltration: 1

3. Percolation: 4. Throughflow: 5. Interflow: 6. Groundwater: 7. Water table: 8. Evaporation: 9. Throughfall: 10. Stemflow: 11. Transpiration: 12. Interception: 13. Overland flow: 14. Seepage: 15. Evapotranspiration: Drainage Basin 1. At the local scale the drainage basin is part of a hydrological cycle. What is a hydrological cycle? The area that is drained by a river and its tributaries. 2. Is a drainage basin an opened or a closed system? Why? It is an opened system because it has a series of inputs, processes and outputs. 3. Draw a flow diagram to show water which flow within a drainage basin.

 Can you identify the ‘input’, ‘output’, ‘flow’ and ‘stores’.  Use different colour pen to show all these and draw keys.

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4. Write briefly in your own words how water flow throughout the drainage basin.

WATER BALANCE: 1. What is water balance? The relationship between precipitation, evapotranspiration and storage (soil moisture and groundwater). 2. Water balance equation: Input = output Precipitation (P) = streamflow (Q) + evapotranspiration (E) + change in storage (S) It can be shown using a graph called a water budget graph/soil moisture budget. 3. Precipitation and evapotranspiration rates are potted on to a single graph. (Draw the graph below and label terms, ref: Geography An Integrated Approach, Waugh, D. p: 60)

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4. Define the terms: 1. Soil moisture budget: 2. Soil moisture surplus: 3. Field capacity: 4. Soil moisture deficit: 5. Why do we have to know the water balance in hydrological cycle? To control flood, provide water supply when deficit 6. When does this condition occur? (a) Positive water balance: (b) Negative water balance: STORM HYDROGRAPH: 1. Why are some areas in Brunei flooded?  Prolonged heavy rainfall and sudden and intense rainfall  Human interference when they cut down trees thus increase run-off which later join river as discharge. 2. What is river discharge? Discharge is the amount of water originating as precipitation which reaches the channel by surface run-off, throughflow and baseflow. It is not the water store in drainage basin by interception, as surface storage, soil moisture storage or groundwater storage or lost through evatranspiration. 3. The discharge of a river at a given point over a short period of time can be shown using a graph. Name the graph: A storm or flood hydrograph. 4. What is a storm hydrograph? A graph to show how the discharge of a river responds to an individual rainfall event. * Watch video clip-animation showing storm hydrograph. 5. Since each storm hydrograph shows respond of a river toward a rainfall event so this graph can help you predict flooding. When? When it is fill up or it has a high peak discharge. It is important for planning flood prevention measures since they know how a river behave in time of intense rainfall. Draw a well-labelled diagram of a storm hydrograph. (refer to Waugh, D, p: 61)

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Define terms: 1. Approach segment 2. Rising limb: 3. Peak discharge 4. Lag time 5. Recession limb/falling limb 6. Baseflow 7. Stormflow 8. Bankfull discharge 9. Discharge Factors contributing to changes in the shape of the storm hydrograph. (Read your textbook, Waugh, D, p: 62-63 draw and write briefly how these factors influence the shapes of the storm hydrograph) (* when it takes time for water to travel to reach stream-lag time longer, lower peak discharge & low run-off & high infiltration rate & vice versa) 1. Basin size, shape and relief Basin size: Discharge increases with the size of drainage basin. It has longer lag time as water has to travel further to reach the main channel. Basin shape: A circular basin will have a higher peak flow and shorter lag time than an elongated basin.

2. Relief: Water will reach a river more quickly where there are steep-sided valleys in a basin. (Shorter lag time and high peak discharge) 3. Climate Rainfall: Rainfall intensity; prolonged, steady rainfall allows water stores to fill up gradually and efficiently. However when the ground because saturated, further rainfall will result in rapid runoff and flooding. Intense rainfall can exceed soil infiltration and vegetation interception capacities and so causes rapid runoff. Temperature: Low temperature is low enough to cause precipitation to fall as snow, much water will be stored on the surface. When temperature rises, river will be swollen by meltwater (especially if ground remains frozen so restrict infiltration)-shorter lag time and high peak discharge Very high temperature: result in baked ground and restrict infiltration. –longer lag time & low peak discharge.

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On the otherhand high temperature, increase evapotranspiration rates (longer lag time and low peak discharge) & effects combination with other influencing factors on hydrological processes such as vegetation cover. 4. Vegetation cover. Vegetation intercepts rainfall and stores moisture on its leaves. Some of this water is evaporated and the rest is delivered to soil more slowly than by direct rainfall. (longer lag time, low peak discharge) Plant roots take up water from soil thus reduce throughflow. It effect restrict run-off and reduce flooding in forested areas. (longer lag time and lower peak discharge, less run-off) 5. Rock types (geology) Permeable rocks: (i) Porous: rocks with pores able to store water e.g. sandstones (ii) Pervious: rocks which have joints and bedding planes that allow the passage of water, e.g. Carboniferous limestone. Impermeable rocks: rocks that do not allow water to pass through thus little infiltration and greater run-off (shorter lag time and high peak discharge) 6. Soil type Sandy & clayey soil : sand has large pore spaces allows quick infiltration & greater volume of throughflow & thus low run-off. (longer lag time & low peak discharge) Soil depth: In deeper soil more water can be stored or flow through. 7. Drainage density- sum of the lengths of all streams in a drainage basin divided by area of the basin. High drainage density (developed on impermeable rocks): causes a more rapid response in river discharge to rainfall events. (shorter lag time & high peak discharge) Low drainage density: less efficient in moving water through and so produces lower peak discharge, but extended falling limb on a hydrogaph. 8. Human influences: (i) Agriculture and land drainage *Agriculture involves changes to vegetation cover thus affect interception, runoff, infiltration and percolation. Ploughing & other activities alter soil texture & thus infiltration rates and storage capacities. -Soil conservation measures such as contour ploughing-aim to reduce runoff/soil erosion. (high infiltration, longer lag time & low peak discharge) -irrigation –e.g. land drainage aims to lower water table and prevent waterlogging by use pipes laid below surface & deepened ditches. Result in rapid throughflow, shorter lagtime and higher peak flows. (ii) Deforestation and afforestation: Deforestation: accelerates runoff and increase risk of flooding. Greater runoff flows accelerates soil erosion, and result in greater sediment loads thus increase flood levels further. E.g. Deforestation in Himalayas where the Ganges and its

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tributaries have their headwaters has resulted in greater flood frequency and intensity in Bangladesh. (shorter lagtime & high peak discharge, risk flooding)

Afforestation has become important method used to reduce runoff (& erosion) and even out river discharges.

(iii)

Urbanisation: Urban development replaces vegetation & permeable surfaces with impermeable tarmac & concrete. Thus runoff volumes grow and flood risk increases. (shorter lagtime & high peak discharge). Flood risk aggravated by gutters & drains increasing flow velocities & reduce lag time. Less vegetation cover & reduce surface water storage lead to lower evapotranspiration thus more water available for runoff.

(iv)

River engineering  There is direct human impact on river flows through various engineering works to reduce flooding, generate HEP, provide water for irrigation and supply water for industrial and domestic uses.  Therefore dams are built, river channels altered and major diversions of water flows are made. ( less run-off, longer lag time & low peak discharge)

Draw a mind map or concept map to show how physical and human factors influence hydrological processes.

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Find a case study-Flood control management –Uk & Bangladesh. 1. Draw basic sketch map of basin 2. What are the main physical and human influences on basin hydrograph? 3. Have there been major flooding incidents? 4. What are the main priorities of management? 5. Are there political considerations affecting management (e.g. cross boundaries) 6. What management strategies have taken place in the following in the following areas? (e.g. HEP, flood prevention, recreation etc)

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