Topic On Hydrology Revision)

Hydrology 1. 2. 3. 4. 5.

Hydrological cycle Drainage Basin Water Balance Storm Hydrograph Flooding

Hydrological cycle What is hydrology? What is hydrological Cycle? Define all the terms Is a hydrological cycle an Opened or closed system? Why? Why is it called a system?

Hydrology and Hydrological cycle •

Hydrology : study of water

(2) Hydrological cycle: continuous movement of water from land, sea and air, back to land and it continues.

Why is it called a system? The Hydrological cycle is called a system because it has a set of components which are links to one another, inputs, outputs and process (transfer) and storage.

ATMOSPHERE (ATMOSPHERE STORE) E HYDROSPHERE (OCEAN STORE)

Key: E-Evaporation P-Precipitation T-Transpiration

P HYDROSPHERE SURFACE STORE (LAKE, RIVERS & ICE)

T BIOSPHERE (VEGETATION STORE)

LITHOSPHERE (GROUNDWATER STORE)

THE HYDROLOGICAL CYCLE AS A CLOSED SYSTEM

Hydrological cycle is a closed system. Why? 2. Water circulates continuously & the processes is fuel by energy from the sun 2. There is no gain or loses in the cycle 3. There is a fixed amount of water.

Water movement in H.C http://www.nwlg.org/pages/resources/geog/hydro_cycle/hydro/cycle.htm

PRECIPITATION Evapotranspiration interception transpiration

evaporation evaporation

Aeration zone Zone A Water table

stemflow Infiltration Percolation

BEDROCK Zone B Groundwater zone

throughfall Surface storage

Run-off

SOIL

Through flow seepage

baseflow

GROUNDWATER

Stream flow

Terms Aeration zone: zone between soil moisture zone and capillary zone above water table. Water-table: it separates Ground water zones that lies below and Aeration zone /capillay fringe lines above. Ground water zone: Precipitation that succeeds in moving from the soil layer down into the underlying bedrock will at some point reach an area of permanent saturation that is known as the groundwater zone

Terms: • Precipitation: Form major inputs into the system, for example rainfall and snow. • Groundwater flow/baseflow: water transferred slowly laterally below water table. • Infiltration: The maximum rate at which water can pass through the soil. • Percolation: Water reaches underlying soil or rock layers, which tend to be more compact (slow movement) create groundwater storage. • Throughflow: Water that flow laterally, parallel to the earth’s surface.

Terms • Interflow: The water, derived from precipitation, that infiltrates the soil surface and then moves laterally through the upper layers of soil above the water table until it reaches a stream channel or returns to the surface at some point downslope from its point of infiltration. • Evaporation: physical process by which moisture loss into atmosphere from water surfaces, including vegetation, soil due to sun’s heat and effects of air movement. • Throughfall: Water reaches the ground when it drops off the leaves. • Stemflow: Water reaches the ground when it flows down the trunk.

Terms: • Transpiration: biological process by which water is lost from a plant through stomata in its leaves. • Interception: the first raindrops of a rainfall event will fall on vegetation which shelters the underlying ground. (interception storage) • Overlandflow: Excess water which flow over earth surface when precipitation is very heavy or when soil becomes saturated. • Seepage: slow movement of water through the soil. • Evapotranspiration: Moisture which is loss directly into atmosphere from water surface by process of evaporation and by process of transpiration from vegetation.

Water movement throughout drainage basin. • Look at the diagram: 1. List down input & output 2.Which are the storages & flow. 3. How does water from surface storage reaches groundwater storage? 4. Name the flow marked A and B. 5. Define terms shown in diagrams.

Inputs: Precipitation Outputs: evapotranspiration, river runoff. Transfers: infiltration, percolation, throughflow, baseflow, overlandflow, baseflow, throughfall and channel flow. Storage: groundwater storage, soil water storage, Interception storage,surface storage, vegetation storage & channel storage.

Terms: •

Soil moisture storage: amount of water held in the soil at one time.

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Interception storage: first raindrops of a rainfall event that fall on vegetation which shelters the underlying ground. (water collected in leaves of trees)

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Surface storage storage: rainfall that are not intercepted by vegetation reaches the ground surface and fill in small surface depression and excess water will overflow as run-off.

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Ground water storage: water which is held in pores and cervices and its underlying bedrock.

1. Name the flows shown as A, B & C. 2. Describe what is meant by percolation. 3.Describe and explain the occurrence of the flows A, B and C.

Flow of water in a cross section of soil and bedrock

Infiltration occurs: 2. Soil is less saturated 3. Porous soil, permeable 4. If precipitation exceed infiltration rate, run-off will occur. (depend on antecedent pp-water already in soil, porosity, Vegetation cover and soil surface; ploughed, crusted, cracked)

Percolation occurs: 3. After water infiltrates into soil 4. Deep in the soil where soil is compact (as it reaches Underlying soil/rock layers) create groundwater storage) -permeability decrease thus slow movement.

Overland flow/surface run-off occurs: 2. Heavy rainfall 3. Soil is saturated 3. Water excess, thus overflow on surface Baseflow: (water slowly transferred laterally as baseflow/groundwater flow) Occurs: 4.water-table rises above streambed 5.Groundwater discharged or escaped into stream by seepage.

Throughflow: (Horizontal or lateral water movement above water table-later join stream) Occurs: 5.Ground become saturated 6.Soil permeability decreased with increase in depth.

Drainage basin A. What is a drainage basin? B. Is a drainage basin a closed/opened system?

H.C & Drainage basin Drainage basin is part of the hydrological cycle (local scale) Drainage basin is used to describe the water movement for hydrological cycle

Drainage basin is an extent of land where water from rain or snow melt drains downhill into a body of water, such as a river, lake, reservoir, estuary, wetland, sea or ocean. Each drainage basin is separated topographically from adjacent basins by a geographical barrier such as a ridge, hill or mountain, which is known as a water divide.

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Other terms that are used to describe a drainage basin are catchment, catchment area, catchment basin, drainage area, river basin, water basin and watershed

Drainage basin Drainage basin: the area that is drained by a river and its tributaries. It is an opened system. Because: 5. It has a series of inputs, processes and outputs.

Water balance River discharge: amount of water originating from precipitation that reaches channel by direct pp, overland flow, throughflow and baseflow. Equation: Discharge : Velocity X Area (cross section area) (Cumec = cubic metre per second) River discharge in drainage basin is a product of relationship between precipitation and evapotranspiration and other factors that control amount of stores in river basin. This is called water balance.

Water balance • What? State of equilibrium in Drainage basin between inputs & outputs.: (Drainage basin discharge : Input = Outputs) -soil moisture budget graph/water balance graph

Equation: P = Q + E +/- change in storage P: precipitation Q: run-off, E = Evapotranspiration

Water balance graph (soil moisture budget)

1. Actual evapotranspiration (AET): the loss of moisture to the atmosphere by the processes of evapotration and transpiration which actually takes place. 2. Potential evapotranspiration (PET): the maximum amount of evapotranspiration which occur if an adequate supply of water were continuously available. (In Desert PET >AET amount of water is limited) 3. Soil moisture surplus: occurs when the soil water store is full and thus there is surplus of water for plants, runoff and groundwater recharge. (PP>PET) 4. Soil moisture utilisation: Plants (and people) use moisture stored in the soil, leaving it depleted. (AET >PP) 5. Soil moisture deficiency: Equivalent to the extra water which would be needed to maintain maximum plant growth. There is little or no water available for plant growth (irrigation could make good this deficit) (PET > AET) 6. Soil moisture recharge: The soil water store starts to fill again after a period of deficiency 7. Field capacity: The moisture a freely drained soil can hold after all free or gravity water has drained away. Such moisture is held by tension around soil particles, mainly as capillary water.

PE & AE • Potential Evapotranspiration: Evapotranspiration which occur when unlimited water is available. • Actual Evapotranspiration: Evapotranspiration occurs when limited amount of water is available.

Soil moisture budget

Look at the diagram (Waugh, p:60) 1. In which months is there a water surplus? J, F, M, A, N, D (PP>ET, excess not used by plant; surface run-off & rise in river level) 2. Why is there soil moisture recharge in October? Soil moisture utilisation-May-Sept (autumn-first surplus water recharged by soil) 3. When is field capacity attained? (The maximum amount of water soil can hold) November 4. Why is a water deficit not shown on the graph? (In summer ET>p thus water utilised by plant & humans – Water from soil (store) depleted, river level fall) P>ET

Importances: When: Water surplus: wet soil, increase river level, runoff. Water Deficit: Dry soil, fall in river’s level, Drought. Importances: 8. Period of water deficit/drought & flood 10. Changes in storage 3.

Implementation for irrigation, flood control, pollution control

Occurance of +ve & -ve water balance Positive water balance: PP(input) >ET (loss) Negative water balance: ET > PP

Storm hydrograph Bankfull discharge

Rising limb

Falling limb Stormflow/ runoff Time of rise

Approach segment/Antecedent flow

Base flow

Storm hydrograph 1. Line graph-discharge in cumec 2. Rising limb: rising flood water in a river 3. Peak flow: maximum discharge in the river 4. Recession/falling limb: falling flood water in a river 5. Lag time: time difference between peak rain storm & peak flow of river 6. Base flow: normal discharge of the river 7. Storm flow (overland flow and throughflow)

How to interpret storm hydrograph (A) Rising limb (B) Recession limb (C) Lag time (D) Rainfall intensity (E) Peak flow compare to base flow (F) Recovery rate, back to baseflow (overland flow, throughflow and base flow)

Interpretation of graph Approach segment/ Antecedent flow rate Time to rise

= (Discharge of river before storm)

Storm begins (negligible response) Rising limbs Storm flow

=rapid increase in discharge (surface runoff & throughflow reaches stream

Lag time (max pp & peak discharge)

Peak discharge

=river reaches maximum level

Bankfull discharge Falling/recession limb Return to Baseflow

River level reaches top-further increase-flood.

=Segment of graph-discharge decrease & river falls. (less steep than R.L-throughflow release into stream) =slow respond to storm, continuous released G.W to Maintain river flow during low Pp.

Factors influencing shape of Hydrographs: 4. Size of drainage basin 2. Soil types-sandy/clayey • Geology-permeable or impermeable 4. Gradient-steep/gentle 5. Vegetation cover 6. Urbanisation

•clue-look at infiltration rate, run-off lead to high/low discharge & shorter or longer lag time (time taken for The river to fill up with water)

Basin size / shape

(i) Elongated & circular drainage basin Elongated shape basin has lower peak Flow and longer lag time than circular one Of the same size. (since it is narrower Width & takes longer time for water to reach Stream) BASIN SHAPE/SIZE (ii) Large & small size basin. Large basin receive much precipitation water than small size basin thus much Water overflow on surface. But it takes longer distance for water to travel to reach Trunk of river (stream) thus has longer lag time

Relief In steep slope basin, channel flow faster down steep slope therefore it has steep rising limb & shorter lagtime compare to gentle slope basin.

Drainage Density Density (no. of tributaries)

Drainage basin with high drainage density allows rapid run-off/overland flow. (shorter lag time & high peak discharge)

Land use Urbanisation: surface covered with concrete & tarmac thus form impermeable surface thus create steep rising limb & shorter lag time. Gutter or drains carry water quickly to nearest river. Small streams canalised so water flows away more quickly or culverted which limit amount of water to pass through at one time. Afforestation: intercepts precipitation, creates shallow rising limb & longer lag time.(less run-off)* (Forest intercept 80% compare to arable land only intercept 10%)

*Vegetation cover Vegetation help prevent flooding by intercepting rainfall (store in leaves before evaporates) –less run-off occur. i.e. T. Rainforests (80%, 30% evaporates) & Arable land (20%) Interception less in winter deciduous trees shed its leaves & crops harvested to expose bare earth. Plant roots, expecially trees, reduce throughflow by taking up water from the soil.

Deforestation • Flooding occur in deforested areas. E.g. Flooding in Bangladesh due to removal of trees in Nepal & other Himalayan areas.

Climate Precipitation: Short intense rainstorms produce rapid overland flow & steep rising limb. Temperature: Extreme temperature (low/high) Ground hard (frozen/baked) thus cause rapid run-off (shorter lag time) Snow on the ground acts as store produce longer lag time & shallow rising limb but once thaw rising limb become steeper

Tidal condition What is time & when occurs? Rise & fall in sea level due to gravitational pull between moon and the sun. Spring tide: when moon and sun are aligned thus create strong gravitational pull which causes rise or very low tides. (full moon, new moon)

How spring tide effect storm hydrograph During high spring tides it blocks normal exit of water flow thus extending length of time for river basin to return to base flow. Prevent flood water to escape into sea. Floodwater built up in lower part of valley. Storm surge occurs when high tides coincide with gale-force winds blowing onshore And a narrow estuary.

Soil types: Sandy soil with large pore spaces, allow infiltration and do not encourage flooding. Clays have much smaller pore spaces &less well connected, reduce infiltration and throughflow, but encourages surface runoff & increase risk of flooding

Rock types Rocks that allow water to pass through are said to be permeable. • Porous e.g. sandstone and chalk, contain numerous pores able to fill with water & store water. • Pervious e.g. Carboniferous limestone, which allow water to flow along bedding planes and down joints within rock, although the rock itself is impervious.

Rock type Both porous and pervious rocks permit rapid infiltration, thus there is little run-off and limited number of surface stream. In contrast impermeable rocks e.g. granite do not allow water to pass through them & so produce more surface runoff and greater number of streams.

Catchments of rivers X & Y

hydrographs over 24 hours of the two rivers.

Using both diagram explain why the discharge of the two rivers are different Explain how each of these influence storm hydrograph: Drainage basin shape, geology, rainfall intensity, drainage density

Describe the differences between the discharges of rivers X and Y in response to the rainfall

Types of soil & rocks * Soil: Clayey soil acts impermeable with small pore space compare to sandy soil but stored much water. Clayey soil has shorter lag time (much run-off)

Rocks: Permeable rocks –rapid infiltration & little overland flow therefore shows shallow & gentle rising limb (longer lag time & low peak discharge) e.g. Chalk Impermeable e.g. shale, limestone. *impermeable can be permeable-pervious (joints & bedding planes)

Flood 

What is flooding?

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Why occur?

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How to predict & Method of prevention?

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Students to read and find out from internet / textbook case studies ; UK and Bangladesh:

1. Cause; Physical &human causes 2. Flood management & problems

What is flooding? A body of water that rises to overflow land not normally submerged. Why does it occur? 1. During & after torrential rainfall. •Very heavy rainfall, often produced by convectional thunderstorms, tropical cyclones or monsoons, lead to flash flooding. •But also inability of drainage system to with huge amount of water. (natural/unnatural) 2. Costal flooding: • Low lying areas vulnerable to flooding. •Heavy rainfall and storm surges. •Tsunami (sudden displacement of ocean floor which sends waves out in all directions).

3. River flooding: River channel no longer able to contain the water flowing within it. Excess water sweeps over banks and onto adjacent floodplain.

Factors that can increase river flooding: 1. Heavy & prolonged rainfall 3. A reduction in channel size. 5. The silting of channels, by excessive soil erosion 7. Action of landslips in displacing water in lakes or blocking channels. 9. An increase in storm flow due to urbanisation

Factors that can increase river flooding: 6. A reduction in size of floodplain by development. 7. Blocking of bridges & culvert entrances by debris transported by river. 8. Failure of flood defence structures such as dams & embankments. 9. Drainage basin: steep slope, impermeable bedrock, Saturated or frozen soil, and deforestation encourage rapid transfer of water to river channel.

Flood • Explain how river floods might be predicted. Giving examples, describe the methods which may be used to reduce the effects of flooding. • Describe the main features of river flood plains and explain why flood plains may present problems for human settlements.

Human activities of flow • Suggest how human activities might affect flows within a river channel. • How can changes in land use affect flows and stores in a drainage basin? • How can the abstraction (removal) and the storage of water by humans affect flows and stores within a drainage basin. • Explain how urbanisation can affect river channel flows.