hydrology 1

School of the Built Environment - BEng/MEng Civil Engineering

A. J. Adeloye: Hydrology (Level 2) Lesson I -1-

AIM The aim of this module is to achieve an understanding of the basic principles of hydrology and the importance of hydrology in water resources assessment, design and management. HYDROLOGY Hydrology is a multidisciplinary subject dealing with the occurrence, circulation and the distribution of the water of the earth in the atmosphere, surface rivers, springs and lakes; seas and oceans; and within the ground. As will be shown later on, this water exists in a series of compartments that are linked together in what is generally known as the Hydrological Cycle or Water Cycle. Understanding hydrology inevitably therefore implies understanding the processes and linkages in the hydrological cycle. EXAMPLES OF DIRECT APPLICATION OF HYDROLOGY Design of water supply schemes - estimation of amount of water that can be

reliably made available Design of hydro-electric projects Irrigation schemes Flood protection/urban drainage Navigation schemes Bridge crossings and culverts Water pollution control

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LEARNING OUTCOMES On completion of this module, the students should know and understand: the different physical processes in the hydrological cycle and how they interact; the concept of catchment and river basin; how to calculate a catchment annual water balance; contemporary institutional framework for managing the water cycle in the UK; raw water sources and their treatment requirements to make them potable; how to measure rainfall; how to calculate catchment average rainfall; how to measure river flow and analyse the data. THE HYDROLOGICAL CYCLE The hydrological cycle is a graphical illustration of the natural circulation of water near the surface of the earth (see Fig. 1).

Fig. 1: The Hydrological Cycle

Precipitation

Snow Glacier

Ground

Water

Flow

Lake River

Aquifer

Sea

EVAPORATION from Vegetation soil Lakes streams And the sea

Swamp

Transpiration

Surface water run-off

Spring Percolation

Rain Hail Snow Dew

The Hydrological Cycle

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Essentially, the water can be considered to exist in three distinct compartments or sub-systems. These compartments and the relevant processes affecting water circulation in them are as follows: Atmospheric Water Compartment precipitation - e.g. rain, snow, sleet, dew. Precipitation is the main input to the hydrological cycle system. Precipitation drives the entire system since without it, the other processes cannot occur, in theory. Rain is the most common form of precipitation, although snow can be as equally important in cold regions such as the UK. The main difference between rain and snow is that water is immediately available from the former whereas there is often a time-delay between snowfall and water availability. Dew may be the significant form of water in arid environments. evaporation and evapotranspiration (E & E) - the transfer of water from the liquid to the gaseous state to form part of the atmosphere is known as evaporation. When this water loss takes place from plants and the soil, the process is known as evapotranspiration. While rainfall is the main input, E & E constitute the main output of water from the earth surface. Surface Water Compartment infiltration - the gentle movement of water through the soil surface and into the ground. depression storage - the water collected in surface depressions and subsequently evaporated or infiltrated. interception - the water collected by the leaves of vegetation and subsequently evaporated. surface runoff - the balance from precipitation which runs off in rivers. Sub-surface Water Compartment percolation - the downward movement of water in the soil to replenish ground water storage interflow - the lateral movement of water in the soil which eventually emerges as springs

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groundwater flow - the water existing in aquifers and rocks that is contributed to by percolating rainfall. THE RIVER BASIN OR CATCHMENT The catchment or river basin is the smallest hydrological unit representation incorporating all of the compartments of the hydrological cycle. More specifically, it is the area contributing water to a given point on a river or stream and is separated from adjacent catchments by a divide, or ridge, that can be traced on topographic maps (see Fig.2). Within this catchment, movement of water into the atmospheric and subsurface compartments can also take place.

Indeed, each river catchment (or basin or watershed) is a kind of system, with its distinct input and output and there exists a natural BALANCE between all these inputs and outputs. Table 1 shows estimated world water quantities and Table 2 presents the balance between the various components of the hydrological cycle on a global scale. It is important to note from Table 2 that a large proportion of available water is lost annually through evaporation and this is a major problem for water resources conservation and management.

Catchment Outfall

Mainstream (River)

Catchment Boundary Catchment Area

Fig.2 River Catchment

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Table 1: Estimated world water quantities (UNESCO, 1978)* Item Area (Mkm2) Volume (km3) % of total % of fresh total Oceans 361.1 1,338,000,000 96.5 Fresh g.water 134.8 10,530,000 0.76 30.1 Saline g.water 134.8 12,870,000 0.93 Soil moisture 82.0 16,500 0.0012 0.005 Polar ice 16.0 24,023,500 1.7 68.6 Other ice & snow

0.3 340,600 0.025 1.0

Fresh lakes 1.2 91,000 0.007 0.26 Saline lakes 0.8 85,400 0.006 Marshes 2.7 11,470 0.0008 0.03 Rivers 148.8 2,120 0.0002 0.006 Biological H2O 510.0 1,120 0.0001 0.003 Atmospheric 510.0 12,900 0.001 0.04 Total water 510.0 1,385,984,610 100 Total fresh 148.8 35,029,210 2.5 100 *UNESCO (1978): World water balance and water resources of the Earth, Studies & reports in Hydrology, vol. 25, UNESCO, Paris. Table 2: Global annual water balance (UNESCO, 1978)* Unit Ocean Land Area km2 361,300,000 148,800,000 Precipitation km3/yr 458,000 119,000 mm/yr

1270 800

Evaporation km3/yr 505,000 72,000 mm/yr

1400 484

Runoff to ocean from:

Rivers km3/yr - 44,700 Groundwater km3/yr - 2200 Total km3 - 47,000 Total mm/yr - 316 *UNESCO (1978): World water balance and water resources of the Earth, Studies & reports in Hydrology, vol. 25, UNESCO, Paris.



WATER BALANCE EQUATION Whether it is the globe or a simple catchment, there is always a balance between all the inputs and outputs of water. This balance is often represented by a water balance equation written in the general form: P R E S= + +

where P = precipitation (in a given time), mm; R = runoff in same period, mm; E = evaporation volume in same period, mm; S= change in volume of storage (in lakes, reservoirs or groundwater), mm. Notes i. All factors in the water balance equation must have the same unit for the equation to be valid. ii. The importance of S increases as the time period becomes shorter. For example

over an annual cycle, this term is very small that, for all practical purposes, it can be ignored. This is obviously not case for monthly or shorter time scales.

iii. The water balance equation is very important in hydrology since, with it, one could

estimate with sufficient degree of accuracy, any element of the equation if unavailable. One practical example is the estimation of runoff at ungauged sites from concurrent precipitation and evaporation data.

iv. In hydrology, the sum total of E and S is termed the loss L, i.e. L = E + S v. Also, the ratio R/P is termed the Runoff Coefficient, often denoted by C. In

general 1C0 . The runoff coefficient is one simple way for characterising losses in hydrology. As the loss increases, the runoff coefficient tends towards zero; for very little loss, the runoff coefficient will tend towards one!

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THE UK WATER INDUSTRY England & Wales The water industry in England & Wales has seen many changes in recent times. Some notable events post 1973 are briefly discussed below Water Act 1973: This established 10 Regional Water Authorities for : River Pollution Control Land Drainage Fisheries Water Supply Sewerage and Sewage Treatment Water Conservation Recreation & Amenity Water Act 1989: Privatised Water Service Companies Water Only Companies National Rivers Authority (NRA) Provisions for Regulation of Water Industry Water Resources Act 1991: Environment Agency - formally came into being in 1996 Implementation of EU Water Directives Water Resources Act 2003: This is the current legislation guiding the protection and improvement of water

environment in England & Wales. Specifically, the Act reflects the requirements



of the EU Water Framework Directive to protect the ecology of each river catchment.

Act makes it mandatory for water suppliers to have a 25-year water resources plan,

covering how they intend to provide sufficient water to meet their customers needs.

Scotland 1967 Water (Scotland) Act: 13 source to Tap Regional Water Board One Bulk Water Supply Board - The central Water Development Board

(CSWDB) 1975 Following Local Government re-organisation, the water supply and sewerage

functions were transferred to 9 REGIONAL and 3 ISLAND councils. The CSWDB retained its traditional functions of bulk supply

7 River Purification Boards (RPBs) 1996 3 New Water Authorities : West, East and North of Scotland Water Authorities.

These three authorities were later merged in 2002 into a single entity known as Scottish Water

Scottish Environment Protection Agency, SEPA came into being. Water Environment and Water Services (Scotland) Act 2003: This is the current legislation in Scotland similar to the Water Resources Act 2003

in England & Wales. Act also reflects the provisions of the EU Water Framework Directive and

empowers Scottish ministers to introduce regulatory controls over activities in order to protect and improve Scotlands water environment.

The necessary regulatory controls were passed as the Water Environment

(Controlled Activities) (Scotland) Regulations 2006 (or Controlled Activities



Regulations, CAR, for short). CAR came into force on 1 April 2006 and from this date CAR authorisation is required for:

1. Discharges to all surface waters, including wetlands, and

groundwaters; 2. Disposal to land; 3. Abstractions from all surface and groundwaters; 4. Impoundments of rivers, lochs, wetlands, etc., 5. Engineering works in inland waters and wetlands.

Northern Ireland Water and Sewerage functions are carried out by the DEPARTMENT of ENVIRONMENT (NI).



EU Water Directives- The Water Framework Directive (WFD) The WFD incorporates the main requirements for water management in Europe into a single, holistic system based on river basins. Thus the WFD provides an umbrella to previously issued directives, e.g. the quality of water intended for drinking directive; the fish and shell fish directive, which will all be repealed. Some other existing directives are judged to contribute to achieving the objectives of the WFD and these will be retained. Examples of the latter are the Urban wastewater treatment directive and the Nitrates directive.

Objectives/Key guiding goal of the WFD To achieve good status of both ground and surface waters; good meaning that the water meets the standards (mostly chemical standards) established in existing water directives and, additionally, new ecological quality standards. Thus good is specified in terms of both chemical and ecological quality standards: Good chemical status- means compliance with all the quality standards established for chemical substances at the EU level. Good ecological quality- means there is only a slight departure from the biological community that would be expected in conditions of minimum anthropogenic (i.e. human) impacts.

Provisions of the WFD Formation of new or re-organised river basin district authorities, each with a

management plan for achieving the goals of the directive; Emission standards to be set using a combination of fixed standards and river

quality objectives; A new mechanism for controlling the discharge of dangerous substances; Derogations from good status- chemical and ecological- will be allowed in

emergency situations, e.g. floods, droughts, accidents; Basin authorities to designate vulnerable protection zones for specific purposes

(e.g. bathing, drinking water; protected natural areas); where necessary, more stringent quality standards may be set for such zones;

All rivers must achieve good chemical and ecological status as a minimum; Prohibits all direct discharges to groundwater aquifers. It also provides for regular

monitoring of groundwater level and quality in order to detect any unusual changes in characteristics and also implement mitigating measures.



Implementation Timetable 15 years- 9 years to prepare management plans and a further 6 years to achieve

targets. Where derogations/delays are granted, and additional deferment of up to two

periods of 6 years is allowed but such must be justified.



SOURCES AND TREATMENT REQUIREMENTS OF WATER Two Main Sources There are two main sources of raw water: SURFACE WATER - streams; rivers; ponds; lakes and reservoirs GROUND WATER - usually water held in aquifers within the earth crust. In general, most water supply facilities use a combination of ground and surface water, albeit in varying proportions. For example, in the UK, the national average proportions are: Groundwater (25%); Surface Water (75%). However, Scotland tends to use a lower proportion of groundwater while England & Wales may use a higher proportion of surface water in some areas. Treatment Requirements Groundwater: Relatively purer and hence only requires very minimal treatment, usually disinfection Surface Water: Surface water contains a large number of impurities which impart colour, odour, taste, to the water as well as a variety of pathogenic disease-causing organisms. Extensive treatment, involving physical, chemical and biological processes, often required to remove the very many impurities in surface water before it becomes potable. WATER DEMAND AND DEMAND FORECASTING Demand Total UK Demand (Gross 20 x 109 litres/day) comprises: DOMESTIC (43 %) COMMERCIAL & INDUSTRIAL (28 %) AGRICULTURAL (2 %) LOSSES (27 %)

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Typical Domestic per capita Demand: 140 litres/day comprising TOILET FLUSHING (32%) BATHING & SHOWERING (17%) CLOTHES WASHING (12% DISH WASHERS (1%) Demand Forecasting The objective of water demand forecasting is to obtain best estimates of future trends in demand growth so that allowances could be made for this in the planning of water supply facilities. Demand forecasting is an imprecise activity; as a result, most forecasters prefer to err on the side of caution by over-designing for predicted increases in demand, which often do not materialise. Demand forecasting can be accomplished in two basic ways: (Linear) Extrapolation based on past trend - this could be highly unreliable as

illustrated in Fig. 3, because past trend is often not a good indicator of future demand patterns. Extrapolation does not take into account emerging environmental factors such as climate change, for example.

Fig. 3: Illustration of significant error of demand forecasting by past trends



Analytical techniques - involve relating water demand to those factors known to influence it, e.g. price, living standards, environmental factors, etc., e.g.

)()()( TdlsicpbaD +++= (1) where D is the demand, p is price, lsi is a living standard index and T is temperature (which is an indicator of environmental condition) and a, b, c and d are coefficients or parameters of the model. In eq. (1), D is the dependent variable and p, lsi and T are the independent (or explanatory) variables. If sufficient data about the dependent and corresponding independent variables are available, then the parameters can be determined by regression analysis. However, some of the independent variables are difficult to measure for current and future conditions, thus making the practical application of analytical demand forecasting methods somewhat problematic.

hydrology 1

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