## Soil moisture deficit

The calculation of potential evaporation (Ep) from readily available meteorological data is seen to be a much simpler operation than the computation or measurement of actual evapotranspiration (Et) from a vegetated surface. However, water loss from a catchment area does not always proceed at the potential rate, since this is dependent on a continuous water supply. When the vegetation is unable to abstract water from the soil, then the actual evaporation becomes less than potential. Thus the...

## Stage

The water level at a gauging station, the most important measurement in river hydrom-etry, is generally known as the stage. It is measured with respect to a datum, either a local bench mark or the crest level of the control, which in turn should be levelled into the geodetic survey datum of the country (Ordnance Survey datum in the UK). All continuous estimates of the discharge derived from a continuous stage record depend on the accuracy of the stage values. The instruments and installations...

## Measurement of velocity

The simplest method for determining a velocity of flow is by timing the movement of a float over a known distance (sometimes called float gauging). Surface floats comprising any available floating object are often used in rough preliminary surveys these measurements give only the surface velocity and a correction factor must be applied to give the average velocity over a depth. A factor of 0.7 is recommended for a river of 1 m depth with a factor of 0.8 for 6 m or greater (BS EN ISO 748, 2007)....

## River routing

For a river channel reach where the water surface cannot be assumed horizontal, the stored volume becomes a function of the stages at both ends of the reach, and not just at the downstream (outflow) end only. In a typical reach, the different components of storage may be defined for a given instant in time as in Fig. 14.10. Again, the continuity equation (14.2) holds at any Fig. 14.8 Auxiliary curve for outflow O v. G for At 2h. Table 14.3 Calculations for level-pool routing Table 14.3...

## River gauging methods

As in the measurement of precipitation, measurement of river discharge is a sampling procedure. For springs and very small streams, accurate volumetric quantities over timed intervals can be measured, and is called volumetric gauging. For a large stream, a continuous measure of one variable, river level (Section 7.3), is related to the spot measurements of discharge collected by dilution gauging methods (Section 7.5) or calculated from sampled values of the variables, velocity and area...

## The complementary concept

In 1963 Bouchet suggested a complementary concept for estimating large-scale evapotranspiration rates. His idea, since developed by Fred Morton and others, was that advection of air over a surface would mean that the humidity of the air would come into a dynamic equilibrium with the latent heat fluxes from the surface. What is measured as local evaporation and transpiration locally will therefore depend on what is happening up-wind. Thus, if the surface was dry, the humidity of the air would be...

## Q Av

Within the range of the measured stages. The cross-sectional area curve can be extrapolated reliably from the survey data (and a change of slope will indicate any change in cross-section shape). The mean velocity curve can normally be extended with little error. Then the higher values of discharge can be calculated for the required stage from the product of the corresponding V and A. Extrapolation of the rating curve can also be made using the empirical Chezy formula (or other friction formula)...

## Energy budget analysis vapotranspiration and snowmelt

Hydrological processes depend not only on the water budget but also on the surface energy budget, which affects, in particular, the transfer of water back to the atmosphere as evaporation and transpiration and also the way in which precipitation as snow builds up into a snow pack and later melts. Evapotranspiration requires energy to change the phase state of water from liquid to vapour snowmelt requires energy to change the phase state of water from ice to liquid (or sometimes directly to...

## Hydrological regime modifications

Studies in the UK have failed to relate annual and seasonal rainfall differences to urban development (Tabony, 1980). In the London area, rainfall differences are influenced mainly by altitudinal differences. On a short timescale, the proportionally greater incidence of severe thunderstorms in built-up areas compared with rural areas is well noted and can be ascribed to greater concentrations of condensation nuclei in the air, increased turbulence and urban overheating. Information on such...

## Factors affecting evaporation

The physical process in the change of state from liquid to vapour operates in both Eo and Et, and thus the general physical conditions influencing evaporation rates are common to both. (a) Latent heat is required to change a liquid into its gaseous form and, in nature, this is provided primarily by energy from the Sun. The latent heat of vaporisation comes from solar (short-wave) and terrestrial (long-wave) radiation. The incoming solar radiation is the dominant source of heat and affects...

## Flood routing

One of the most common problems facing a practising hydrologist or hydraulic engineer is the estimation of the hydrograph of the rise and fall of a river at any given point on the river during the course of a flood event. The problem is solved by the technique of flood routing, which is the process of following the behaviour of a flood water upstream or downstream along the river and over the flood plain. There are two primary uses of flood routing models. The first is to provide maps of areas...

## Weirs

Weirs constitute a more versatile group of structures providing restriction to the depth rather than the width of the flow in a river or stream channel. A distinct sharp break in the bed profile is constructed and this creates a raised upstream sub-critical flow, a critical flow over the weir and super-critical flow downstream. The wide variety of weir types can provide for the measurement of discharges ranging from a few litres per second to many hundreds of cubic metres per second. In each...

## Numerical groundwater models 1551 Assumptions and boundary conditions

Complex and transient (non-steady) flow conditions require an approximate numerical solution to the groundwater flow equations. There are a variety of numerical solution methods available, including finite difference, finite element, finite volume and boundary element methods. All are general methods for solving partial differential equation, such as that based on Darcy's law used to describe groundwater flows. Similar solvers are now available in many general mathematics packages such as...

## Advection and dispersion in groundwater systems

Transport of solutes through porous media is generally described by the advection-dispersion equation (ADE). The ADE can be formulated by looking at the mass fluxes of a solute into and out of a control volume in the same way as the subsurface flow equation (see also the derivation of the ADE for transport in a river in Section 14.5). For the transport equation, we have to consider two types of flux, advection and dispersion. The joint effects of advection and dispersion on a point source of...

## Snow and the energy budget

In many parts of the world snow forms an important part of the hydrological cycle. Where it is the major input of water in the annual water balance, it can be critical in water resources assessment, while the snowmelt period can be the most critical periodfor flooding in some areas. In California, for example, up to 80 per cent of the annual discharge of some rivers is generated by snowmelt. Thus the build-up and (a) Soil moisture in top metre of soil (fraction of saturation) (b) Rainfall and...

## Areal reduction factors and depthareaduration analysis

In designing hydraulic structures for controlling river flow, a hydrologist needs to know the areal rainfall of the area draining to the control point. Sometimes it is only the average river flow being considered, but more often the works are intended to control flood flows and knowledge of heavy rainfalls is required. There is then an issue that the rainfall depths at a point will not be the same as the average depths over a catchment area estimated by one of the interpolation techniques...

## References

Archer, D. (2003) Scale effects on the hydrological impact of upland afforestation and drainage using indices of flow variability the River Irthing, England. Hydrology and Earth System Sciences 7, 325-338. Archer, D. and Newson, M. D. (2002) The use of indices of flow variability in assessing the hydrological and instream habitat impacts of upland afforestation and drainage. Journal of Hydrology 268, 244-258. Arnell, N. W. (2003) Relative effects of multi-decadal climatic variability and...

## Pressure potential pressure head and hydraulic head

There are two principal forms of energy kinetic and potential. Because subsurface water flow is relatively slow (with the exception of that within natural soil pipes), its kinetic energy, which is proportional to velocity squared, is considered negligible. In contrast, the potential energy, which is due to position or internal condition, is of critical importance in determining the state and movement of water in the soil. Water, like other forms of matter, flows from areas of high potential to...

## SPR J2 SPRtHOSTt i1

Where HOSTi is the proportion of the catchment mapped to HOST class i. For the other components of (13.3) This dynamic adjustment requires a value of the catchment wetness index (CWI). This changes over time as the catchment wets and dries, although for design purposes the FEH also gives a relationship between a design CWI and the standard annual average rainfall in a catchment (Fig. 13.5). For particular events, a more complex calculation of the initial CWI is carried out, depending on the...

## Measurement of meteorological variables for evaporation estimation

The factors affecting evaporation have already been described. The principal source of energy, the Sun, transmits its radiation through the atmosphere. This is measured by solarimeters maintained by the UK Met Office at observatories and major meteorological stations, and these data are available from the British Atmospheric Data Centre. However, the measurement of net radiation, the difference between incoming radiation (short and long wave) and outgoing radiation (reflected short wave and...

## Economic

Property damages Agricultural damages CFMParea 5 AEP 1.30 942747 6613 10341 I 1341 12995 ISS73 170 2484 397 677561 585 593 231 618 389 500 729 702 915 988 358 087 1757912 2059529 2 COMAH, control of major accident hazards SAC, special area of conservation SAM, scheduled ancient monument SPA, special protection areas SSSI, site of special scientific interest. banks or walls, offering varying standards of protection, predominantly around the 1 5 to 1 30 AEP level. There are also reservoirs in the...

## The importance of groundwater

In many catchments, there are groundwater bodies that are important for water supply and the maintenance of low flows in rivers. The proper management of groundwater is therefore an important topic in hydrology and all hydrologists need to understand the basic principles that govern the analysis and prediction of groundwater. Some of the necessary theory has already been covered in Chapter 5 where the basic principles, parameters and terminology were introduced in considering the monitoring of...

## Subsurface moisture content

The moisture content can be expressed as the mass of water (mw) within a mass of dry soil, regolith or rock (ms), This is called the mass wetness (0m, gg-1, also called gravimetric wetness the term avoided here to prevent confusion with the gravimetric method discussed later). This measure is rarely used by hydrologists because of the influence of unmeasured variations in the mass of dry soil, regolith or rock (ms) within undisturbed volumes of soil, regolith or rock (Vs), i.e. the dry bulk...

## UK environmental standards for river water quality

The Water Framework Directive, or WFD (European Commission, 2000 2000 60 EC) came into force across the European Union on 22 December 2000. This Directive establishes new environmental objectives for the water environment. Two key objectives Table 8.1 Typical physical, chemical and biological characteristics of water sources in the UK. (Adapted from Tebbutt, 1998) Upland basin Lowland basin Chalk aquifer Upland basin Lowland basin Chalk aquifer are (1) to prevent deterioration of the status of...

## The hydrological cycle and hydrometeorology

The history of the evolution of hydrology as a multi-disciplinary subject, dealing with the occurrence, circulation and distribution of the waters of the Earth, has been presented by Biswas (1970). Man's need for water to sustain life and grow food crops was well appreciated throughout the world wherever early civilization developed. Detailed knowledge of the water management practices of the Sumarians and Egyptians in the Middle East, of the Chinese along the banks of the Hwang-Ho and of the...

## Measurement of open water evaporation

An indirect measurement of evaporation from open water can be made by taking the difference in storage of a body of water measured at two known times, which gives a measure of the evaporated water over the time interval. If rain has fallen during the time period, then the rainfall quantity must be taken into account. In practice, this water budget method is used on two widely differing spatial scales, by measurements at reservoirs and by measurements with specially designed instruments...

## Integrated urban drainage pilots

As noted in Chapter 16, there is a complex mix of responsibilities, institutional and funding arrangements that divide responsibilities between water companies, the Environment Agency, planning departments in local government, housing developers, householders and internal drainage boards. An integrated approach is seen as necessary to ensure that these different stakeholders communicate and work together to manage flood risk and water quality in the urban environment. The Government's 'Making...

## Critiques of the historical arrangements for flood management

16.2.4.1 Institutional arrangements The institutional arrangements in the UK mean that numerous organisations have responsibility for different aspects of flood management. There are, broadly speaking, four types of flood event that can be distinguished. They are river (fluvial) flooding, coastal flooding, groundwater flooding and surface water (sometimes called 'pluvial') flooding. To date most flood management organisations have concentrated on river and coastal flood. In England, the lead...

## Precipitation

Of all the components of the hydrological cycle, the elements of precipitation, particularly rain and snow, are the most commonly measured. Sevruk and Klemm (1989) have estimated that there are 150 000 storage rain gauges in use worldwide. It would appear to be a straightforward procedure to catch rain as it falls and the depth of snow lying can be determined easily by readings on a graduated rod. People have been making these simple measurements for more than 2000 years indeed, the first...

## Q Qo Q

Where Q is flow, C is concentration, the subscript o indicates old or pre-event water, and the subscript n represents new or event water. These are two simultaneous equations in two unknowns (Qo and Qn) and can therefore be solved easily when it can be assumed that the concentrations for the two sources are constant in time and distinctive. Substituting (11.1) into (11.2) and rearranging gives the result Qo Q(C - Cn) (C0 - Cn) (11.3) The need for a difference in the old and new water...

## S I O dt141

Where I and O are the corresponding rates of inflow and outflow. An alternative statement of this equation is that the rate of change of storage within the reach at any instant is given by This, the continuity equation, forms the basis of all the storage routing methods. The routing problem consists of finding O as a function of time, given I as a function of time, and having information or making assumptions about S. Equation (14.2) cannot be solved directly. Any procedure for routing a...

## Water resources management policy in the UK 1721 European legislative framework

The Water Resources Act of 1963 can be seen as the beginning of river basin management in the UK. Now, in the early twenty-first century, water resources management falls under the over-arching framework of European legislation that takes this concept further. The European Water Framework Directive 2000 60 EC (WFD) established an integrated approach to the protection, improvement and sustainable use of rivers, lakes, estuaries, coastal waters and groundwater. It introduced two significant...

## A k i1 2k

13.3.3 Fitting frequency distributions The FEH recommends fitting the GL and GP distributions by the method of L-moments (Hosking and Wallis, 1997). This is the method used in the WINFAP software. WINFAP also provides estimates of the uncertainty in the parameter values for the fitted distributions and confidence limits for the estimated extremes similar to those shown in Table 11.9 and Fig. 11.16. Where a three-parameter distribution is fitted and the k parameter (as defined above for the GEV,...

## The Transport and Road Research Laboratory hydrograph method

In designing storm water sewerage systems for towns, city suburbs and new developments of around 200-400 ha with varied surface characteristics, a method is required that also takes into account differences in storm rainfall over the catchment area. Developed from the time-area concept of catchment response (Chapter 12), the TRRL hydrograph method (Watkins, 1962) was applied widely in the UK. In the time-area method, the total catchment area is deemed to be contributing to the flow after the...

## P 1T161

That the discharge Q is equalled or exceeded in any given year (see Chapters 11 and 13). For a flood of annual probability p, assume that a corresponding value of flood damage D(p) can be estimated. In the definition of methods used by the USACE, this was based on the depth of inundation of the floodplain and on the value of the inundated structures. Regardless of exactly how the damages are calculated, the EAD is the notional long-run average value of such damages taken over floods of all...

## The Transport and Road Research Laboratory rational method

Hydrologists are often concerned with evaluating the runoff from the subareas to be drained in order to design the necessary storm water sewers. The peak runoff from the selected design storm determines the size of sewer pipes, which are dependent on the extent of each sub-area to be drained. At the head of a catchment sub-area, the required pipe size may be quite small, but downstream, as the sewer receives water from a growing area through a series of junctions, the pipe size gradually needs...

## Info

Where N is the number of annual maxima in the sample. Then the upper and lower limits are calculated from where ta v are values of the t distribution obtained from standard statistical tables with a the probability limit required and v the degree of freedom. The calculations for the 95 per cent confidence limits for the plotted example in Fig. 11.16 are set out in Table 11.9. The value of the t statistic is 2.06 for a 100 95 per cent 5 per cent and v n 1 23. The curves of the 95 per cent...

## Waterquality sampling 831 Sampling river water

Choice of sampling site for river water may be governed by an abstraction point or a discharge point associated with an industrial user or waste water treatment works. However, it is often most useful to take water-quality samples at a river gauging station. Ideally, a single sample from the well-mixed waters downstream of a weir would suffice to give a good representation of the water quality of a small river. At a current meter station, the river should be sampled at several points across the...

## River flow analysis

The ultimate aim of many computational techniques in engineering hydrology is the derivation of river discharges, and it might appear that, once these are obtained, the hydrologist's work is done. However, whether they are gained indirectly from considerations of other hydrological variables (to be described in following chapters) or directly from river discharge measurements, the discharge data are only samples in time of the behaviour of the river. The hydrologist then must assess the utility...

## Depthdurationfrequency DDF curves

The analysis of rainfall frequencies is somewhat more complex than the analysis of river discharge frequencies to be considered in Chapter 11. In the case of rainfall extremes we have to consider not only the frequency of a single variable (e.g. flood peaks) but occurrences for which both magnitude and duration are important. Rainfall frequency clearly has an effect on discharge frequency (for both high and low flows) but the relationship between the two will depend on other variables, in...

## Catchment change

In many parts of the world the landscape is far from a natural environment, but is the product of hundreds or thousands of years of human management. The way in which the land is managed has important effects on hydrological processes, influencing how water moves through the hydrological cycle and the balance between storage, evaporation, runoff, river flows and groundwater. Throughout the evolution of the hydrological sciences, hydrologists have therefore been interested in understanding the...

## The Wallingford procedure

From the 1970s, access to computers meant that simplifications of former methods were no longer necessary and many more relevant factors could be introduced into runoff design calculations using software packages. In 1981 the Wallingford Procedure was published. The Wallingford Procedure describes the hydraulic design and analysis of pipe networks for both new schemes and existing systems. It can accommodate both independent storm water sewers and combined sewers, but the waste water flows must...

## Hydrometric networks

The concept of the hydrological cycle forms the basis for the hydrologist's understanding of the sources of water at or under the Earth's surface and its consequent movement by various pathways back to the principal storage in the oceans. Two of the greatest problems for the hydrologist are quantifying the amount of water in the different phases in the cycle and evaluating the rate of transfer of water from one phase to another within the cycle. Thus measurement of the components of the cycle...

## 1 urbext1990

The ReFH rainfall-runoff model and the design event inputs have been calibrated so as to match, on average, flood frequency curves derived from pooled statistical analysis at 100 gauging stations. The model was calibrated up to return periods of 150 years, relatively long compared with the FEH rainfall-runoff model, which was only calibrated to the 10-year return period. The number of catchments used to calibrate the two methods is similar however, calibration of the ReFH method used more data...