Encyclopedia of Environmental Science and Engineering, Volume I and II

465

HYDROLOGY

THE PURPOSES OF HYDROLOGICAL STUDIES

Hydrology is concerned with all phases of the transport of

water between the atmosphere, the land surface and sub-

surface, and the oceans, and the historical development of

an understanding of the hydrological process is in itself

a fascinating study.^6 As a science, hydrology encompasses

many complex processes, a number of which are only

imperfectly understood. It is perhaps helpful in developing

an understanding of hydrological theory to focus attention

not on the individual physical processes, but on the practi-

cal problems which the hydrologist is seeking to solve. By

studying hydrology from the problem-solving viewpoint,

we shall see the interrelationship of the physical processes

and the approximations which are made to represent pro-

cesses which are either imperfectly understood or too com-

plex for complete physical representation. We shall also

see what data is required to make adequate evaluations of

given problems.

A prime hydrological problem is the forecasting of

stream-fl ow run-off. Such forecasts may be concerned with

daily fl ows, especially peak fl ows for fl ood warning, or a

seasonal forecast may be required, where a knowledge of

the total volume of run-off is of prime interest. More sophis-

ticated forecast procedures are required for the day-to-day

operation of fl ood control reservoirs, hydropower projects,

irrigation and water supply schemes, especially for schemes

which are used to serve several purposes simultaneously

such as hydropower, fl ood control, and irrigation.

Hydrologists are also concerned with studying statistical

patterns of run-off. A special class of problems is the study

of extreme events, such as fl oods or droughts. Such maxi-

mum events provide limiting design data for fl ood spillways,

dyke levels, channel design, etc. Minimum events are impor-

tant, for example, in irrigation studies and fi sheries projects.

A more complex example of statistical studies is concerned

with sequential patterns of run-off, for either monthly or

annual sequences. Such sequences are important when test-

ing the storage capacity of a water resource system, such

as an irrigation or hydropower reservoir, when assessing the

risk of failing to meet the requirements of a given scheme.

A specially challenging example of sequential fl ow studies

concerns the pattern of run-off from several tributary areas

of the same river system. In such studies it is necessary to try

to maintain not only a sequential pattern but also to model

the cross-correlations between the various tributaries.

The question of land use and its infl uences on run-off

occupies a central position in the understanding of hydrolog-

ical processes. Land use has been studied for its infl uence on

fl ood control, erosion control, water yield and agriculture,

with particular application to irrigation. Perhaps the most

marked effect of changed land use and changed run-off char-

acteristics is demonstrated by urbanization of agricultural

and forested lands. The paving of large areas and the infl u-

ence of buildings has a marked effect in increasing run-off

rates and volumes, so that sewer systems must be designed

to handle the increased fl ows. Although not so dramatic, and

certainly not so easy to document, the infl uence of trees and

crops on soil structure and stability may well prove to be

the most far-reaching problem. There is a complex interac-

tion between soil biology, the crop and the hydrological fac-

tors such as soil moisture, percolation, run-off, erosion, and

evapo-transpiration. Adequate hydrological calculations are

a prerequisite for such studies.

A long-term aim of hydrological studies is the clear

defi nition of existing patterns of rainfall and run-off. Such a

defi nition requires the establishment of statistical measures

such as the means, variances and probabilities of rate events.

From these studies come not only the design data for extreme

events but also the determination of any changes in climate

which may be either cyclical or a longterm trend. It is being

suggested in many quarters that air pollution may have a

gradual effect on the Earth’s radiation balance. If this is true

we should expect to see measurable changes in our climatic

patterns. Good hydrological data and its proper analysis will

provide one very important means of evaluating such trends

and also for measuring the effectiveness of our attempts to

correct the balance.

A BRIEF NOTE ON STATISTICAL TECHNIQUES

The hydrologist is constantly handling large quantities

of data which may describe precipitation, streamfl ow,

climate, groundwater, evaporation, and many other factors.

A reasonable grasp of statistical measures and techniques is

invaluable to the hydrologist. Several good basic textbooks

are referenced, 1,2,3,8,9 and Facts from Figures by Moroney, is

particularly recommended for a basic understanding of what

statistics is aiming to achieve.

The most important aspect of the nature of data is the

question of whether data is independent or dependent. Very

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