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 (1) 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? 1. 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 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.

Terms: •

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

•

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

• Flow A – overland flow (water that flow over the earth surface. Condition to occur: Heavy rainfall / the soil becomes saturated, excess water flow away the surface. • Flow B –throughflow (horizontal movement of water within soil layer. Condition to occur:permeability of soil decreases with increase in depth of soil (base of Aeration zone) and when the ground is saturated. • Flow C – baseflow/groundwater flow.(water that is transferred laterally. Condition to occur: when the water table (top of groundwater saturation) rises above the streambed, thus groundwater discharged into stream.

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

Drainage basin Drainage basin: the area that is drained by a river and its tributaries. It is an opened system. Because: 1. It has a series of inputs, processes and outputs.

Water balance 1. Briefly define the term water balance. 2. Write an equation for water balance. 4. Label A, B, C & D and define terms; soil moisture budget, soil moisture surplus, field capacity, soil moisture deficit. 4. Importance of water balance 5. When do positive/negative water balance occur?

Storm hydrograph

1. Why are some areas in Brunei flooded? 2. What is river discharge? 3. Name the graph drawn to a river discharge at a given point & time. 3. What is a storm Hydrograph? 4. Importance of a storm Hydrograph. 5. Define terms; lag time, Baseflow, falling/recession Limb, rising limb, throughflow etc.

Factors influencing shape of Hydrographs: 1. 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)

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

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 2. A reduction in channel size. 3. The silting of channels, by excessive soil erosion 4. Action of landslips in displacing water in lakes or blocking channels. 5. 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.