reports in

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Studies and reports in hydrology 36

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Recent titles in this series:

20. Hydrological maps. Co-edition Unesco-WMO.

21 .* World catalogue of very large floods/Répertoire mondial des très fortes crues.

22. Floodflow computation. Methods compiled from world experience.

23. Water quality surveys.

24. Effects of urbanization and industrialization on the hydrological regimè^àrid on water quality. Proceedings of the Amsterdam Symposium.

October 1977/Effets de l'urbanisation et de l'industrialisation sur le régime hydrologique et sur la qualité de l'eau. Actes du Colloque d'Amsterdam, octobre 1977. Co-edition IAHS-Unesco Coédition AISH-Unesco.

25. World water balance and water resources of the earth. (English edition).

26. Impact of urbanization and industrialization on water resources planning and management.

27. Socio-economic aspects of urban hydrology.

28. Casebook of methods of computation of quantitative changes in the hydrological regime of river basins due to h u m a n activities.

29. Surface water and ground-water interaction.

30. Aquifer contamination and protection.

31. Methods of computation of the water balance of large lakes and reservoirs.

Vol. I Methodology Vol. II Case studies

32. Application of results from representative and experimental basins.

33. Groundwater in hard rocks.

34. Groundwater Models.

Vol. I Concepts, problems and methods of analysis with examples of their application.

35. Sedimentation Problems in River Basins.

36. Methods of computation of low stream flow.

Quadrilingual publication: English—Ftench—Spanish—Russian.

For details of the complete series please see the list printed at the end of this work.

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Methods of computation of low streamflow

Edited by T . A . M c M a h o n and A . Diaz Arenas

A contribution to the International Hydrological Programme

(unesoo

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T h e designations employed and the presentation of material throughout the publication do not imply the expression of any opinion whatsoever o n the part of Unesco concerning the legal status of any country, territory, city or area or of its authorities, or concerning

the delimitation of its frontiers or boundaries.

Published in 1982 by the United Nations Educational, Scientific and Cultural Organization,

7, place de Fontenoy, 7570D Paris Printed by

Imprimerie de la Manutention, M a y e n n e I S B N 92-3-102 013-7

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Preface

Although the total amount of water on earth is generally assumed to have remained virtually constant, the rapid growth of population, together with the extension of irrigated agriculture and industrial development, are stressing the quantity and quality aspects of the natural system. Because of the increasing problems, man has begun to realize that he can no longer follow a

"use and discard" philosophy — either with water resources or any other natural resource. As a result, the need for a consistent policy of rational management of water resources has become evident.

Rational water management, however, should be founded upon a thorough understanding of water availability and movement. Thus, as a contribution to the solution of the world's water problems, Unesco, in 1965, began the first world-wide programme of studies of the hydrological cycle — The International Hydrological Decade (IHD). The research programme was complemented by a major effort in the field of hydrological education and training. The activities undertaken during the Decade proved to be of great interest and value to Member States. By the end of that period a majority of Unesco

1

s Member States had formed IHD National Committees to carry out the relevant national activities and to participate in regional and international co-operation within the IHD programme. The knowledge of the world's water resources had substantially improved. Hydrology became widely recognized as an independent professional option and facilities for the training of hydrologists had been developed.

Conscious of the need to expand upon the efforts initiated during the International Hydrological Decade, and, following the recommendations of Member States, Unesco, in 1975, launched a new long-term-intergovernmental programme, the International Hydrological Programme (IHP), to follow the Decade.

Although the IHP is basically a scientific and educational programme, Unesco has been aware from the beginning of a need to direct its activities toward the practical solutions of the world's very real water resources problems. Accordingly, and in line with the recommendations of the 1977 United Nations Water Conference, the objectives of the International Hydrological Programme have been gradually expanded in order to cover not only hydrological processes considered in interelationship with the environment and human activities, but also the scientific aspects of multi-purpose utilization and conservation of water resources to meet the needs of economic and social development. Thus, while maintaining IHP's scientific concept, the objectives have shifted perceptibly towards a multidisciplinarty approach to the assessment, planning, and rational management of water resources.

As part of Unesco's contribution to the objectives of the IHP, two publication series are issued: "Studies and Reports in Hydrology"and

"Technical Papers in Hydrology". In addition to these publications, and in order

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to expedite exchange of information in the areas in which it is most needed, works of a preliminary nature are issued in the form of Technical Documents.

The purpose of the continuing series "Studies and Reports in

Hydrology" to which this volume belongs,is to present data collected and the main

results of hydrological studies, as well as to provide information on

hydrological research techniques. The proceedings of symposia are also

sometimes included. It is hoped that these volumes will furnish material of

both practical and theoretical interest to water resources scientists and also

to those involved in water resources assessments and the planning for rational

water resources management.

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Foreword

Occurring during long periods of little or no rain and in severe winter conditions, low streamflow constitutes one of the extremes of the hydrological regime. The correct assessment of low flows, appropriately linked with their probability of occurrence and duration, plays an

important role in the design of water supply systems, in the management of water quality and in projects concerned with flow regulation and reservoir operations.

The methodology of low flow computations is much less reflected in the available hydrological literature than the theory of floods. Recognizing this, the IHD Co-ordinating Council decided at its sixth session to broaden the terms of reference of the working group on floods in order to include also aspects of low flow computation. Accordingly, the first session of the Intergovernmental Council of the IHP in April 1975 established a working group to prepare a casebook on methods of computation of low streamflow.

The working group consisted of the following members:

T. A. McMahon (Australia) (Chairman) A. Diaz Arenas (Cuba)

J. 0. Sonuga 'Nigeria) A. M. Vladimirov (USSR).

M. Roche (France) represented the International Association of Hydrological Sciences, and Y.

Bogoyavlensky (UNESCO) provided the Technical Secretariat.

The working group met on three occasions:

Leningrad (USSR) 8-11 June 1976 Paris (UNESCO Headquarters) 12-16 December 1977 Havana (Cuba) 4-9 December 1978.

Individual chapters of the book were prepared by the following members:

The book was edited by T.

March 1980.

Chapter 1 : M.

Chapter 2 : A.

Chapter 3 : J.

Chapter 4 : T.

Chapter 5 : A.

Chapter 6 : A.

A. McMahon and A.

Roche Diaz Arenas 0. Sonuga A. McMahon M. Vladimirov M. Vladimirov Diaz Arenas, i

It should be noted that the technical terms used in the book are consistent with those defined in the International Glossary of Hydrology (World Meteorological Organization - UNESCO, First edition 1974).

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List of tables

2.1 Comparison of summer minimum runoff for river basins composed of different soils.

2.2 Minimum runoffs from drainage basins with lakes of different relative size.

2.3 Thirty days minimum specific discharge in comparison with lake area for four USSR basins.

2.4 Relation between drainage density and specific discharge for three Cuban basins.

2.5 Comparison between present water consumption according to different uses and population.

2.6 Seasonal variations of household and garden water use in three Australian cities.

2.7 Cultivated and irrigated land in Latin America and the Caribbean.

4.1 Average low flows during consecutive periods.

4.2 Examples of twenty-four months running totals of streamflow.

4.3 Typical values of recession constants.

5.1 Minimum 30-day discharges for 97 per cent frequency depending on river embedment'level.

5.2 Minimum 30-day discharges for 80 per cent frequency depending on mean watershed elevation.

5.3 Mean minimum 30-day discharges related to values of coefficient of variation.

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List of figures

2.1 River Niger Basin showing annual hydrographs.

2.2 Discharge fluctuations in a river and associated alluvium.

2.3 Relationship between low flow and annual precipitation.

2.4 Relationship between low flow and annual evaporation.

2.5 Relationship between annual groundwater flow and minimum summer runoft.

2.6 Diagrammatic illustration of relationship between river discharge and associated alluvial groundwater deposits.

2.7 Relationship between minimum runoff and drainage area.

2.8 Relationship between minimum runoff and mean basin elevation.

4.1 Relationship between frequency distribution of flows and flow duration curve.

4.2 Example of annual, monthly and daily flow duration curves.

4.3 Variability of monthly flow duration curves and catchment geology.

4.4 Relationship between normal and extreme value probability scales.

4.5 Example of annual low flow frequency curves.

4.6 Typical shape of some one-day annual low flow frequency curves.

4.7 Example of partial low flow frequency curves.

4.8 Frequency curves based on transition matrix method and independent series.

4.9 Example of hydrologie atlas of low flow characteristics.

4.10 Recession analysis of a hydrograph.

4.11 Derivation of recession constant.

4.12 Relationship between recession constant and surficial geology.

4.13 A classification of reservoir capacity-yield procedures.

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5.1 Diagrammatical illustration of the effect of geographical zones on specific minimum discharge.

5.2 Relationships between minimum 30-day specific discharge and drainage area.

5.3 Relationships between minimum discharge and drainage area for permanent rivers.

5.4 Relationships between minimum discharge and drainage area for intermittent rivers.

5.5 Relationships between minimum specific discharge and river embedment.

5.6 Relationships between minimum 30-day specific discharge and mean basin elevation for rivers of mountain regions.

5.7 Isograms of summer-autumn 80% probability mean minimum monthly runoff.

6.1 Relationship between minimum summer monthly specific flow and sum of winter, spring snowmelt and summer flows.

6.2 Relationship between minimum summer monthly flow and sum of winter flow, losses during spring snowmelt flood and summer rainfall.

6.3 Relationship between minimum summer discharge and mean winter monthly flow.

6.4 Relationships between summer low flow volumes and preceding synoptic meteorological indices.

6.5 Determination of depletion curves.

6.6 Relationship among mean September discharge, mean August discharge and precipitation depth.

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1 Introduction

1.1 BACKGROUND

Whenever a project aims to use run-of-the-river waters, that is, when there is no regulating reservoir, or when flow regulation is to be seasonal, or if, as a result of man's activity, the regime of the stream is to be substantially disturbed, it is of vital importance to have a sound knowledge and understanding of the river's low flows and their characteristics. •

This knowledge must be understood quantitatively. Furthermore, the question of quality of the environment often depends on the availability of low river flows, particularly in areas of urban living, or on problems of public health, such as combating endemic diseases, as well as for thermal or chemical pollution. Thus, the connection between quantitative and qualitative aspects of water resources is especially sensitive during low water periods. For various reasons (health, environmental conditions), it is necessary to maintain a minimum discharge in rivers and, consequently, this water is not available for other water users. Another example of the relationship between quantity and quality concerns water salinity. Where this problem exists, salinity is much greater during low waters than during floods or mean water periods.

For some projects, in addition to discharges, water levels must be considered. However, this is usuallyva high water problem and is rarely considered in low flow studies. Low water levels will not be treated in this book.

1.2 PURPOSE AND SCOPE

A knowledge of low flows is based normally on direct observation of the natural flows of a stream. When measured data are lacking, low flow knowledge depends upon methods of calculation which make it possible to estimate with varying degrees of accuracy the basic information needed

for projects. It is necessary to know how to use these data and to extract from them the characteristics of the regime which, in any given project, will enable the parameters of the scheme to be determined. It is also important to be able to forecast low flow volumes in the short and medium term, since this is an essential factor in the management of some water projects.

This book is written for engineers, water managers and technicians. It is a compilation of methods successfully used in different countries to compute low streamflows and is illustrated with case studies. A chapter is also devoted to theoretical aspects of natural and man- induced factors affecting low flows.

Following this introduction (Chapter 1), Chapter 2 deals with Factors affecting low streamflow. After describing the low flow process, the author explains the influence on low flows of physical factors: climate, geology and morphology. Factors affecting low flows as a result of human activity are also discussed.

Chapter 3 deals with Assessment of data used in low flow analysis. The data necessary for studying low flow characteristics are reviewed and analyses concerned with the determination of trends and cycles are outlined. Errors affecting data, their homogeneity and representativeness of data sets are also considered.

1

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Chapter 4 deals with Computational procédures with adequate hydrometrio data. A great part of this chapter is devoted to methods of statistical analysis with various types of statistical distributions, but other techniques such as recession analysis and stochastic models are also considered. All these procedures require an adequate length of low flow data for the results to be reliable.

Chapter 5 concerns Determination of low flow with inadequate hydrometrio data. Direct use of statistical analysis is no longer applicable because data are not available or the record is too short. If this is so, it is necessary to compare various catchments, some of them having recorded data, and hence deduce, by analogy or by correlation, the low flows in an ungauged catchment. Analogy requires extensive study of catchment physiography and climate. The most objective way is to proceed from a regional point of view, in particular through regional maps of low flow isograms, or else regression equations linking low flow with catchment characteristics.

Chapter 6 deals with Low flow forecasts. This kind of forecast broadly uses the

relationships between rivers and groundwater, taking into account the influence of the preceding hydrometeorological conditions on the soil moisture during the forecast period. For low flow forecasting at a given point on a river (that is, the provision of a local forecast) the recession or depletion curve is widely used. For all but short forecasts it is likely that the low flow forecast will be expressed in statistical terms.

1.3 DEFINITIONS AND CONCEPTS

Before proceeding it is important to define the subject in detail, distinguishing particularly between the notion of low flow and that of drought. Low flow is defined on a seasonal basis and is linked with the annual solar cycle and its regional or even local climatic effects. Low flows may be absolute or relative.

A simple regime, such as the tropical regime, has only one dry season during which there is only one period of low flow. An equatorial regime, on the other hand, is marked by two rainy seasons and two dry seasons, usually of unequal length; there is a main dry season with a corresponding absolute low flow, and a secondary dry season with a secondary or .relative low flow.

In temperate and cold climates low flows also occur. In large regions with extremely cold and long winters, such as in the western part of USSR (Siberia), rivers cease flowing during many months of the year. Whereas in temperate regions, due to the variability of rainfall, one or more low flow periods may occur each year.

Seasonal irregularities -; and hence the severity of low flow - differ considerably

according to a basin's physiography and its climatology, and the low flow may vary from zero to half or more of the largest flow in a year.

On the other hand, drought is defined as a period of abnormally dry weather sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance and carries connotations of a moisture deficiency with respect to man's usage of the water. It may involve different parameters such as annual abundance. We therefore speak of the ten-year recurrence interval dry year when referring to the annual flow which is exceeded with a frequency of 0.9.

It is thus not associated with the idea of low flow, although the statistical study of low flows may lead to a drought characteristic related to a "low flow parameter".

Several concepts relating to the study of low flows are now considered. A period of low flow is usually defined by:

- its duration, which is often equated to that of the dry season. This is defined as the season during which either there is no rain or the rainfall is low having regard to climate;

- the absolute minimum or lowest flow, which is almost always equated to the smallest mean daily flow during the year;

a series of low flow which expresses a correspondence between fixed lengths of time (expressed as a number of days) and flows which have not been exceeded during an

.equivalent number of days, which may be either consecutive or non-consecutive. Examples are:

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- discharge not exceeded for 7 days or 10 days;

- discharge not exceeded for 15 days;

- discharge not exceeded for one month.

It frequently happens that the flow of a stream ceases for one of the following reasons : the water is frozen;

the reserves supplying the streams are exhausted or insufficient to provide a surface flow , (although underflow may continue).

If there is only one major period of zero flow during the year (non-permanent streams), the low flow, which in this case is not defined as the smallest daily flow, may be distinguished by the number of days when no flow is apparent. This definition can be extended without difficulty to the case in which the period of zero flows is interrupted by small and short-lived floods.

Most streams in arid and semi-arid zones, unless they are large rivers which frequently draw their water from less arid regions, are dry most of the time, and their flows occur spora- dically in the form of floods of varying magnitude (intermittent streams). If these flows occur every year, we can still use the analyses relating to non-permanent streams outlined in the following chapters. If there is more than one year's interval between flows, another definition of low flow will have to be sought or the study of low flows as outlined in this book will have to be abandoned and rainfall studies carried out.

The final introductory comment relates to data. We cannot stress too much the importance of low flow measurements. Accuracy in data is especially important. Where possible, appro- priate data measuring equipment should be used and this may differ from that used for medium and high flows.

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2 Factors affecting low streamflow

2.1 DESCRIPTION OF LOW FLOW PROCESS

The period of low flow, which may occur once or several times in a year, is virtually constant for each basin or sub-basin but varies among basins. During these periods, the inflow from the basin to the river system is substantially reduced.

During a period when discharge decreases, there is little or no precipitation contributing to flow and no water is contributed from the basin's surface-water storages; rivers are fed almost entirely by groundwater, except supply from lakes and reservoirs.

In spite of this, in temperate and cold regions it occasionally happens that snowmelt caused by brief thaws or light showers helps to supply the flow during this period. In warm

regions characterised by an incomplete differential pluvial regime, that is, one without a dry season in the strict sense of the term, the low water period is temporarily interrupted in certain years as a result of isolated rainfall. In some extensive basins of Very long continental rivers, the low water period may be different from one cross- section to another along their courses. For example, for the River Niger at Koulikoro, low water extends from March to May, at Dire from April to June and at Niamey from May to July. (Locations and hydrographs are shown on Fig. 2.1.)

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During low flow periods, the groundwater regime is characterized by a gradual depletion of seasonal storage, the capacity of which is impossible to evaluate accurately. Where there is a well defined dry season, the river flow decreases at the same rate as the seasonal groundwater storage decreases and, in many situations, the river attains a relatively stable minimum flow governed by the inflow from deep groundwater.

Depletion or recession curves can be studied to understand the regimes of watercourses and groundwater storages. In a hydrograph, the lower part of the recession limb results from groundwater storage (Fig. 2.2). Castany (1967) has shown that the formulae of the depletion curves of a river are identical to those governing water yield from an aquifer whose regime is not subject to external influences.

Peak

O u ce <

z o

CO Û

Depletion curve

Groundwater runoff

(b)

""*" ""~ — / y fs

V ^ Depletion cur\

TIME

o

ce

<

X o

ço

o

Fig. 2.2. Discharge fluctuations in a river and associated alluvium.

Many factors determine the regime and discharge during a low water period. With present knowledge, the effects of the majority of these factors cannot be differentiated as a rule, since the laws governing them have not been adequately elucidated and their magnitudes are not, in general, known.

From a practical point of view, these factors can be grouped together in two main categories: climatic and azonal. Climatological factors are often more important than basin characteristics. However, the influence of man's activity on the catchment, and hence on low flow, is of enormous importance. This chapter therefore describes in detail not only the natural factors affecting the hydrology of low flows but also factors due to human activity.

2.2 NATURAL FACTORS

According to Vladimirov (1976) natural factors can be grouped into three categories taking into account their primary importance in the genesis of flow. The first category which related to the generation of flow determines directly minimum discharge. The major factor is precipi-

tation. This is the principal source of surface flows and groundwater. Groundwater, of course, depends upon the surface flows and determines the low flow in the absence of precipitation over a prolonged period.

The second group of factors affects the regime and discharge of low flow through temporal and spatial' reduction or distribution of precipitation. These are called indirect factors and include all those that do not directly contribute to the formation of the low flow but affect the variation of its rate. This category includes: evaporation losses (temperature and air humidity deficit, wind velocity) type of soil and plant cover, relief, number of lakes and swamps, hydrogeological characteristics of the basin. With respect to the last factor, Nassar (1973) points out that the storage capacity of an aquifer mainly determines the fluctuations observed in low flow discharges.

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The third category identified by Vladimirov is composed of factors that determine the relationship between river discharges and the subsequent impact of the direct and indirect factors described above. This category includes factors that are most frequently used for practical computation purposes and comprises the azonal characteristics of the basin (area, mean altitude, slope, drainage density, and channel embedment) and the characteristics of flow

(annual runoff, annual groundwater flow to the river, self-regulation of streamflow and other factors).

2.2.1 Climatic factors 2.2.1.1 Precipitation

All water occurring as river flow has at some time been condensed and precipitated from the atmosphere. But, as seen in the preceding paragraphs, rivers are fed during low water essen-- tially from water contained below the ground surface. This storage is repleted by precipitation that occurred prior to the period in which the surface flow has substantially diminished or ceased altogether.

The effect of precipitation on streamflow can be directly observed in the basin's

discharge characteristics. The effect can be modified to a greater or lesser extent by other factors. For example, natural characteristics of the basin (topography, soil^vegetation characteristics, hydrogeology) determine the time it takes for saturated flow to reappear in the form of surface runoff. This may range from a short time in the case of a small karst basin to a month or considerably longer in other types of basins. In order to determine the role of precipitation in the formation of low flow and to explain the nature of its impact, it is possible to prepare graphs showing the relation between precipitation and low flow runoff

(Fig. 2.3). However, to establish this type of relationship, it is necessary to take into

120

1 100 E u.

& 80 Z 3

DZ

g 60 O

_l u.

g 40 O

_l 20

400 500 600 700 800 900

A N N U A L PRECIPITATION ( m m )

Fig. 2.3 Relationship between low flow and annual precipitation.

(East European Rivers).

account a number of features of the basins including the uniformity of natural characteristics, the number of lakes and swamps, and the impact of man's activities (dams, irrigation systems and other hydraulic structures) if these are of appreciable importance.

Furthermore, it is not enough to estimate actual precipitation alone in order to demons- strate its real impact upon groundwater runoff. It is also necessary to take account of losses, through direct evaporation from the ground or from plant cover and through seepage to layers so deep that water contained in them only returns to the river after a long period of time.

The plant cover of the basin, the permeability of soils and the regularity of the slopes are factors that determine the rainfall's penetration to deep-lying horizons or its accumulation in the upper soil layers. Another important factor affecting low flows, in connection with the above mentioned characteristics, is, therefore, the intensity of rainfall.

. • Zones with surplus and sufficient moisture + Zones with water

deficit

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Precipitation as snow contributes directly to the formation of runoff only at the time of thaw. During other times, low flows are supplied by groundwater. This process begins in spring and continues throughout summer and sometimes extends to the following autumn or winter.

According to Komlev (1973), in extremely cold regions such as Siberia, where the period of very low flows during winter is stable, the correlation coefficient between winter discharge and rainfall during the same period is between 0.3 and 0.5.

Low flows in Finland generally occur in winter and at the end of summer although they may occasionally continue over a longer period as a result of low precipitation (Siren, 1960).

Lazarescu (1977) states that in Romania the main periods of low flow occur during summer and autumn as a result of low precipitation during this period combined with high temperatures and hence high evaporation losses. Low flows also occur in winter, when precipitation is low and low temperatures prevent snow from melting.

When a river's regime is mixed, that is, when it is fed both by rainfall and by snowmelt, the occurrence of a rainy season plays an important role in low flow. For example, if rain occurs in winter, minimum discharge will occur at the beginning of spring, which is the time when rainfall is diminishing and the temperature has not yet risen sufficiently for the snow to begin melting. Such is the case in the Chilean central area.

In regions where rivers are fed solely by rainfall, the low water period is governed by the decrease or cessation of rainfall. The amount of precipitation in the preceding rainy season has a marked effect upon the low water flow.

As a result of the substantial difference in evaporation rates the effect of precipitation upon seasonal runoff is different in humid regions compared with temperate regions. But nevertheless, in both these areas rainfall is a principal meteorological element in the formation of low flows.

In small basins, and especially in those characterised by extensive karst formation, a high flow during the low water period will be closely related to heavy precipitation during the prior wet season.

In view of the foregoing, it is fair to say that the characteristics of a basin play an important role in the precipitation-runoff process. This is especially true for small basins.

Rivers possessing extensive basins generally traverse regions characterised by highly dissimilar features, so that it is more difficult to determine their combined effects.

2.2.1.2 Evaporation

Having regard to the practical nature of this book, the term "evaporation" is used in its broadest sense to cover the different processes that constitute an indirect factor significantly affecting the flow during the low water period.

Evaporation implies the process of water emission by a free surface at a temperature below its boiling point and the combined processes whereby snow dissipates from fields or ice disappears from glaciers.

Consequently, evaporation is an extremely important factor in the hydrological

cycle, since it largely determines the river discharge and reduces the flow during low water periods (Fig. 2.4). The effect of evaporation is most significant at the beginning of summer, when a large mass of water returns from the surface soil and from open water bodies to the atmosphere.

7

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120

100

80

60

40

20

n

- . ' v

¹

' ' -

• \ • Zones with surplus '. • \ and sufficient

V » . • \ moisture

— • \ —

\ \ + Zones of water

\ • » \ deficit

\ . • \

\ . • i

' • 1

- > '

r- i

S-u. i - b ^ i i i 300 400 500 600 700

ANNUAL EVAPORATION (mm)

800

Fig. 2.4 Relationship between low flow and annual evaporation. (East European Rivers)

in regions in which the rate of evaporation cannot be compensated by a higher rate of rainfall, an appreciable reduction in river discharge occurs. However, during low water periods, when rivers are fed almost exclusively by groundwater, evaporation is practically insignificant. The amount of evaporation depends mainly on solar radiation, temperature of air and water and of surface soil-water, humidity, vapor pressure, wind velocity and quality of water.

2.2.1.3 Evapotranspiration

Under this heading, it is essential to distinguish between two different processes; one called transpiration which is due to the plant cover, and the other which is related directly to the soil. In both, the depletion of water supplies leads, initially, to a reduction in the dissi- pation of water into the atmosphere and, finally to the cessation of the process.

the intensity and duration of transpiration and evaporation differ. However, on tilled land it is very difficult to measure them individually. The climatic conditions and the availability of water exert a similar effect on evaporation and transpiration, but in the latter case, the type of vegetation and its stage of development take on considerable importance. In regions where water is a limiting factor, not all plants transpire at the same rate. Moreover, crop-farming practices have a significant effect upon moisture consumption.

In this context it is important to consider the influence exerted on low flow by phreatophytes. These are plants found along streams and rivers and in areas characterised by a shallow water table. The consumptive use of these water—loving plants is generally more than twice that of dry crops. It is estimated that in the western region of the United States, the annual loss of non-productive water due to these plants is equivalent to irrigating more than 2 million hectares, a significant figure in comparison with the 235 million hectares of irrigated land in the world (Kharchenko and Maddock, 1981). It has been demonstrated that in Uzbekistan (USSR), phreatophytes consume considerable volumes of groundwater when the water table is close to the

surface.

Basov's (1941) investigations of plantations in the Kamennaya steppes of Kazakhstan (USSR) show the existence of cones of depression of the water table under the plantations during the growing season.

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2.2.1.4 Air and soil temperatures

These indirect factors affect streamflow in two ways. They affect total runoff by influencing other climatic factors, especially evaporation and rainfall. Also, air temperature affects the flow distribution through freezing. Thus it is one of the principal regulatory elements in temperate and cold countries through temporary retention of water within the soil in the form of snow and ice. During the winter season the influence of air temperature upon minimum discharge is largest.

The formation of ice on the surface of rivers, lakes and swamps materially reduces the quantity of water available as discharge. Szilagyi and Muszkalay (1970) explain the formation of this frozen surface with examples taken from Hungary. In the case of major river, ice begins to accumulate upstream of sections where the passage of floating ice is blocked. The frozen surface gradually increases in thickness upstream, and this is accompanied by an

appreciable rise in backwater. For small rivers the discharge greatly diminishes once the period of freezing begins due to the reduced outflow of the basin. Consequently, streams that carry little water tend to become completely frozen over in a short period of time.

In addition to freezing of surface water, enormous quantities of groundwater can also freeze, thereby retarding the groundwater flow and reducing the runoff that reaches the river as base flow. This phenomenon is more evident in years when little snow is recorded. If soil freezing progresses as deep as permafrost, base flow ceases altogether.

In winter there is a relationship betwe e n air temperature and low flow so it is possible to establish a correlation between these two variables. Komlev C1973) has carried out an extensive study of Siberian rivers (USSR) and succeeded in establishing an inverse correlation between the area of the basin and the zonal rates of the minimum mean monthly flow for the winter period.

In some cold regions where air temperature during winter is sufficiently high to produce thaws, the low water discharge may then be higher than it is in summer. If rises in temperature alternate with periods of freezing, the river will flow slowly, and no high flows will occur.

Such fluctuations allow a more effective percolation of water into the soil than occurs through a sudden thaw and high flow. The same temperature conditions that produce a spring flood may also give rise to a low flow regime during summer.

In permafrost regions, an impermeable surface layer results from frozen soil water. This phenomenon has been-studied by Popov (1968) in small and large basins. Generally, minimum

runoff is low. The effect of the permafrost layer is also evident in summer because of the resultant lack of groundwater reserves.

2.2.1.5 Humidity and wind

Humidity and wind affect the total runoff of streams and influence other climatic factors particular precipitation and evaporation. Evaporation is intimately related to air moisture deficit, and any increase therein causes an increase in evaporation, which in turn reduces soil

moisture and possible grounwater recharge. Considering its effect upon flow,air moisture deficit plays an important role only in dry regions.

In some countries, the persistence of particular winds significantly affects the rainfall and hence the low water period. Wind also affects the distribution of the flow of rivers fed by large lakes. The quantity of water flowing into a river from a lake will vary with wind speed and direction.

2.2.2 Hydrogeological factors 2.2.2.1 Geology of basin

The continuous supply of water to rivers during low water periods is an extremely complex process (Roche, 1963). Nevertheless, Waugh (1970) and Riggs (1972) have pointed out that the geology of the catchment area is the main terrestrial influence- on low flows. Areas where surface geology includes unconsolidated sands and gravels produce a sustained flow during periods of drought which contrasts to these streams in which surface formations consist of

9

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unfractured igneous rocks, clays or shales. In crystallized rocks where little fissuring has occurred, there is little groundwater flow. For two adjacent basins with the same meteorological conditions, the basin underlain by the more impervious formation will have lower discharges during low flow periods.

Karstic rocks can have a significant influence on the rate of flow during the low water period. This influence may either increase or decrease the flow, depending on the relationship between the stream and the karst host rock. For example, in cases of large host rock storages and slow water release, the low flow will be large. In those areas where karst is well

developed, rivers may disappear and discharge of neighbouring basins may be affected. It is difficult, in such cases, to define the catchment area, as in Jaruco, Cuba.

The influence of karst on low flow is very significant in small basins. Karst may become submerged in swamps, as in Zapata, Cuba, and the study of its influence becomes very complex.

2.2.2.2 Hydrogeological regime

The hydrogeological conditions of a basin are closely related to its geological structure, since the latter determines the distribution of aquifers. Most of the rainfall that percolates through the soil to groundwater will eventually reach the river as groundwater flow. The type of soil and its composition largely determine the basin absorption capacity. For soils with large effective porosity soil retention is low but water yield and permeability are high. This explains the great dissimilarity in the behaviour of rivers in sandy or loamy areas compared with those that are located in clay regions. Examples of two sets of catchments are compared in Table 2.1.

It is evident that with greater infiltration capacity, the water is able to penetrate further into the sandy soils. Consequently there is a very clear dependence of low flow on infiltration. At times this relation may be adversely affected by other factors that influence infiltration (see Section 2.3.1).

Basins with friable, porous or fissured rock are most favourably placed for groundwater storage which will subsequently contribute baseflow to the river during low water periods. But the composition of the rock does not determine the rate of groundwater flow; this is governed to a large extent by the rock•s structure.

Table 2.1 Comparison of summer minimum runoff for river basins

River

Osuga Tma Vaya Linda

Basin area (km2)

1230 1800 601 1010

composed

Dominating Soils

clay loams sandy soils, sandy loams clay loams sandy soils, sandy loams

of different

Forest (%)

36 34 80 70

soils.

Swamps (%)

0 2 1 0

(Volga basin.

Lakes (%)

1 1 1 1

USSR).

Normal annual minimum daily

discharge (Vs. km2)

0.26 1.64 0.32 1.48

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2.2.2.3 Groundwater

Groundwater, being the main source of surface streamflow during low flow periods, is available in two ways, namely as artesian groundwater, and as phreatic water. The volume of groundwater depends basically on the climate of the region and the geological structure and hydrogeological conditions of the basin.

During the low water season, the groundwater regime is characterised by a gradual reduction of seasonal reserves. As these diminish, the velocity of the flow and hence the groundwater discharge decreases, and, at the end of the period, the flow reaches normally a relatively stable minimum, often determined by the inflow of artesian groundwater. Thus the groundwater regime is governed by the nature of the hydraulic relation between the water-bearing horizons and the river.

Investigations carried out in many countries show that there is a close relation between the low flow of rivers, particularly the minimum flow, and groundwater discharge. An example for the Nemon basin in USSR is given in Fig. 2.5. But it should be pointed out that this relation is significant only in regions with uniform hydrogeological conditions.

In addition to the volume of groundwater storage, the transmissivity of an aquifer also affects groundwater discharge and hence river flow.

,-. ' 3

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JC

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^

£ g so

Li- CC LU 1-

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— 4- Daily runoff

• Monthly runoff

l

0 2.5 5.0

MINIMUM SUMMER RUNOFF (t/s. km²)

Fig. 2.5 Relationship between annual groundwater flow and minimum summer runoff. (Nemon River Basin, USSR).

Studies in the USSR by Koudelin (1959) and Makarenko (1948) explain the different types of groundwater regime and their relationship to low flows. Six types of groundwater are now discussed.

Phreatic water

Phreatic water is found in the active zone of groundwater storage, that is, in the shallower subsoil layers. It seeps to the river system and constitutes the main source of river replenishment during the low water period. This may involve one or more water bearing sediments. The upper aquifer has a close relation with the upper subsoil layers, its

recharge being the result of precipitation that directly seeps into this horizon. The regime

11

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of deep phreatic aquifers is more steady, since they are fed by deep percolation. Where phreatic water is in direct contact with surface water bodies of the basin, such as lakes and reservoirs, it has a marked influence on the discharge and runoff regime during the low water period. Phreatic water reserves are recharged mainly in spring through snowmelt in cold regions, and by rain in temperate and tropical areas. In regions with considerable precipitation and where the groundwater is near the surface, their recharge may take place even

in autumn. This is illustrated diagrammatically in Fig. 2.6.

DISCHARGE LEVEL

+

O

z

I

H

DISCHARGE LEVEL I o +

Fig. 2.6 Diagrammatic illustration of relationship between river discharge and associated alluvial groundwater deposits.

[(a) Volga River; (b) Kazanka River].

Water in unaonsotidated sediments

From the point of view of river flow, alluvial groundwater is very important. Here the stratum water, that is, water occurring in permeable formations, is generally discharged over large areas, although in some places it may take the form of concentrated outflows. This type of groundwater is characteristic of large river valleys.

Cvack or fissure water

Crack or fissure water is formed in massive igneous rocks and is highly metamorphosed sedi- mentary rocks where water accumulates and circulates in fissures. This also occurs in karstic rocks with slight development of stratafied layers. Outflow from cracks or fissures is concentrated. It is of great importance in small and mountain rivers as well as in the middle reaches of valley rivers. In karstic regions concentrated outflows of groundwater predominate.

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Artesian water

Deep groundwater storages depend on the geological structure and hydrogeological characteristics of the river basin. Artesian water is not subject to sudden changes in time and represents an important supply source for base flow. This water, confined under pressure between impervious layers or in fissures in the earth's crust, is found in horizons that are deeper than those where phreatic water is located. In small sectors of a basin, it can rise as a spring yielding a considerable amount of water. Nevertheless, during times of minimum flow of the majority of rivers, its contribution is slight.

Karstic water

Karstic water, along with permafrost groundwater discussed in the next sub-section, constitutes a special category of groundwater. Karstic groundwater varies considerably depending, among other factors, on its relation with the surface, the development of fissures and internal galleries and the storage capacity of the host rock. The importance of karstic water to river baseflow during low water periods is greatest in years of little precipitation. In basins where karst is well developed, it acts as a natural regulator, maintaining a relatively high stable flow during the low water season. In some areas, however, instead of contributing to surface runoff, it may cause the loss of part of the flow, and in small basins the total disappearance of the streamflow through sink holes, caverns or fissures may occur. It should be noted, of course, that the influence of karstic water is greatest in small basins.

Permafrost groundwater

In cold regions streamflow may also be affected by the formation of ice in permafrost zones.

Here part of the undergroundflow is transformed into ice which, on thawing during the warm season, flows into the stream.

2.2.3 Morphological factors 2.2.3.1 Relief

It is logicial that the relief of a basin should have an influence on the low flow of the basin.

Comer and Zimmerman (1969) analysing data from two small basins in Vermont (USA) concluded that as the land use, climate and geology were the same in both catchments, the

difference in low flows was caused by the differences in topography and soils.

Variations in altitude over a basin produce variations in precipitation. Where precipitation increases with altitude., river discharge also increases and, if other factors are favourable for groundwater storage, low flows will also be larger.

2.2.3.2 Lakes

Lakes and other water bodies modify streamflow by reducing the total annual runoff while at the same time they have a stabilizing effect on discharge. In studying the role of lakes and other water bodies in the formation of low flow, other aspects, for example, their size, their location and relation to the stream, in addition to morphological and climatic conditions of the area, must be taken into account (Table 2.2).

In general, it is found thatthe greaterthe number and size of lakes in a basin, the more regular the distribution of annual runoff and the greater the discharge during low water

seasons. In basins with similar physicial and geographical characteristics, those with lakes exhibit low water discharges that are higher than those in basins without lakes. The same effect is observed when water stored in a lake increases. Por this latter case the bank storage in the sediments bordering the lake may represent a considerable part of the lake volume.

The location of lakes and reservoirs relative to the outlet of a basin and to the main river affects the low water period. Lakes that are located close to the outlet yield greater specific basin discharge than those situated further away. When lakes and reservoirs are located along the main streams, the regulating influence is greatest.

Endorheic lakes, those with no discharge into a river network, accumulate surface water and groundwater, which they subsequently lose by evaporation. Consequently, their influence is negative since the catchment area of the basin supplying the river is reduced.

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Table 2.2 Minimum runoffs from drainage basins with lakes of different relative size. (Karelia, USSR).

Drainage Lakes Minimum daily

River area (% Lake runoff (km ) of drainage location (i./s.km^)

area)

1985 7.7 in the upper 2.1 part of the

drainage area

1775 7.7 uniformly over 2.5 the drainage area

1715 11.9 in the lower 3.4 part of the

drainage area

Study of the influence of lakes and reservoirs on low flow demands different approaches, according to whether the lakes are large or small. When they are large, the action of each storage should be considered separately. On the other hand, when the surface of each lake is no more than a few square kilometers, they should be considered as a whole although they may be many in number.

Vladimirov (1976) shows how analysis of the hydrograph facilitates a detailed study of the influence of lakes on the behaviour of a stream regime. He proposes the use of a weighted average of lake area to demonstrate the influence of a lake on the formation of low flow

(Table 2.3).

Table 2.3 Thirty days minimum specific discharge in comparison with lake area for four USSR basins.

Basin Lake area Weighted Swamp area Specific area (% of river value of (% of river discharge River (km ) basin) lake area basin) Winter Summer

U/s.km2)

Pankan-oya 15.3 3 1.6 3 1.30 1.76 Ray-oya 17.1 3 0 0 0.86 0.91 Chernaya 293 9 3.5 2 6.96 4.56 Sestra 390 1 0 6 4.08 3.46 Source: Vladimirov (1976)

2.2.3.3 Swamps

Only the water contained in the active storage zone of swamps makes a significant contribution to the yield of a river draining a swamp. Yield lasts until the water stored in the swamp reaches a specific soil depth called the inert horizon, when for practical purposes, flow ceases.

After the surface water reserve is exhausted, evaporation continues from the soil, so that part of any subsequent precipitation must replenish both reserves and therefore not all the rainfall passes into the river. This illustrates the influence on the flow from lakes and swamps of precipitation falling over them and of evaporation from their surfaces. In regions affected by hurricanes, the path taken by a storm has a marked influence on the lake flow during the subsequent low water period. This effect is more significant when the area that is water- logged is sufficiently extensive to retain much of the resulting precipitation.

Puna

Saari

Yakhtyavan

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In cold zones, freezing of most of the swamp water occurs in winter, thereby diminishing its yield. Vladimirov (1976) points out that, for permafrost zones where the whole swamp freezes, regulation of flow is much less than in other regions because the upper crust of the peat thaws only once a year-

Waterlogging of a basin may increase or decrease the low flow of streams. This will be affected by the location of the swamp within the basin, the thickness of the active layer, and the vegetation.

In wet regions, evaporation from the ground is slightly less than that occurring from swamps. It is considered therefore that during low water periods swamps act as equivalent to a water-bearing horizon and, after the rainy season, they slowly release water to the river.

However, in dry regions, evaporation from swamps is greater than that from the soil. During the low flow period, therefore, the supply of water from these natural sources rapidly dries up, and the flow of rivers fed from these sources may equal, or be even less than, that occurring in similar basins where such sources do not exist.

2.2.3.4 Plant cover

The vegetation of a basin affects river flow mainly through transpiration of water stored in the ground. This effect reduces the runoff.

For the purpose of examining the effect of vegetation on streamflow, plant cover can be divided into three main categories; woods, either virgin or with appreciable secondary growth; perennial or annual bushes, with small secondary growth; and cultivated plants,

natural grazing plants and other native plants with little growth. The influence of these three categories extends to annual runoff, to low water flow and to the flow regime, although the magnitude is not equal in each category.

Vegetation affects streamflow in various ways. It increases soil storage and permeability by its roots breaking up the soil. Further, a surface layer of dead leaves and humus has high infiltration capacity and retards overland flow and promotes infiltration.

Annual crops with shallow rooting systems rapidly exhaust water in the upper soil

layers. Also some plants extract moisture from deeper lying horizons. In both cases, water is transpired that would otherwise go to maintaining river flow, although the deeper rooting plants will have the more significant effect. The effective growing period will also affect transpiration and hence low flows.

The infiltration of rain, or water resulting from the melting of snow, is usually greater in woodland soils than in any other areas. As well, temporary retention of water in the porous layer of humus and in the upper soil zone affects the recharge of aquifers that supply rivers with water during low flow periods. The effect of forests on groundwater storage is less in sandy or unconsolidated soils, which absorb water rapidly under any conditions, than in clayey or compacted soils, where water does not penetrate easily.

In comparison with shrubs and small plants, forests provide greater surface areas for interception and re-evaporation of water, as well as providing a more effective mechanism for absorption and for the exchange of water vapor between the foliage and the atmosphere. Forests, therefore, have an important effect on streamflow patterns.

In basins where a forest covers a large area, its location is also important. If it lies in the higher parts of the basin, because of the area's higher rainfall and, thus, availability of water for transpiration, regulating influence on low streamflow will be more evident.

2.2.4 Morphometrical factors 2.2.4.1 Basin area

Studies have shown that for most rivers there is a direct relation between basin area and minimum discharge during low water periods (Fig. 2.7). This relation is used in many methods for computing low flow.

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E

JÉ (A

ü.

O z ce

i

Z

I I I + Lower basin A Middle basin - • Upper basin 6

5 h

1 I I I I X "

400 800 1200 1600 D R A I N A G E A R E A (km2)

2000

Fig. 2.7 Relationship between minimum runoff and drainage area.

(Severnaya Dvina River Basin, USSR)

The surface of a basin constitutes the catchment area for precipitation. Larger areas usually mean also larger river embedment. In most cases, as the area of the watershed increases, so does the groundwater basin. However, in karst regions or areas with artesian water, the surface water divide may not coincide with the groundwater divide, and the above relation does not hold. This is particularly important in small basins with highly developed karst.

2.2.4.2 Altitude

Generally, rainfall increases with altitude thus creating more favourable conditions for groundwater recharge. The effect of altitude on a catchment is very marked in mountainous regions

(Fig. 2.8). It is observed in some areas that when altitude exceeds a certain limit, precipitation occurs as snow, basin slopes are steeper, rocks are more impervious and therefore low flow is much lower than in basins lying at lower altitudes.

w 3800

<

Ul

cc <

UJ

o <

Z 3000

<

cc Q

u. O

O 2200 z

I-

<

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UJ —I UJ

Z

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s

-

•

— / / •

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1 1

• /

i i

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/•

i i

0 4 8 12 16 20 24 8 10 12

MINIMUM RUNOFF (t/s. k m2)

6 8

Fig. 2.8 Relationship between minimum runoff and mean basin elevation. (Five mountain regions of Middle Asia, USSR)

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2.2.4.3 Slope

The slope of a basin affects mainly the quantity of infiltration that takes place and the rate of overland flow towards river channels. As a result, basins with steeper slopes allow less time for infiltration, and the supply of groundwater is therefore reduced. The importance of this factor is limited to streams in mountainous regions.

2.2.4.4 Orientation

The orientation of a basin (particularly in mountainous areas) can have a considerable influence on the characteristics of the basin's flow. Orientation determines the exposure of the basin to the prevailing water-bearing winds in the region.

Observations carried out in different parts of the world show that where mountain barriers intercept moisture laden winds, river flow is high. However, a sharp reduction in runoff can be observed at lower altitudes. In medium and high mountain regions the difference in precipitation between the windward and leeward slopes can be enormous.

The direction of the slope also has a direct influence on the quantity and intensity of solar energy reaching the soil surface and has, therefore, an indirect influence on water loss through évapotranspiration.

In Cuba, for example, where rivers are fed exclusively by rainfall, the precipitation on the windward sides of the Sierra Maestra, the Baracoa mountains and the Sierra de Cristal is higher than that on the leeward slopes, due mainly to their exposure to the wet winds prevailing in the locality, although as Trusov (1967) points out, air instability, local altitude and the direction of the coast are also influential factors. Thus on the leeward side, the rainfall regime is considerably lower than the rest of the country; this accounts for the low river regime in the area. In South America a similar influence can be seen very clearly along the Pacific slopes of the Peruvian Andean Chain.

Furthermore, Ward (1967) shows that the direction of the slope is an important factor, particularly for high lands, in the accumulation or melting of snow.

2.2.4.5 Drainage density

Drainage density expresses in quantitative terms, the drainage network of a basin. An accepted form uses the relation between the sum of the lengths of all channels and the basin area.

Climate and many physical characteristics of a basin are reflected in the nature of its drainage system. The drainage system is directly related to efficiency of water removal, in two basins with the same depth and distribution of rainfall but with different drainage systems, the basin with the denser system will provide the more efficient removal of runoff.

The greater the basin area and the more highly developed its channel system, the greater will be the probability that surface water derived from rainfall will contribute to flow during the low water period. This occurs because runoff from most distant areas is possible and the regulating capacity is increased (Table 2.4).

Table 2.4 Relation between drainage density and specific discharge for three Cuban basins.

Basin Annual Specific River Station area Drainage rainfall discharge

density (A) (B)

(km2) (mm) (A/s.km2)

Yara A. Sanchez 234 0.81 1940 33.3 15.3 Baconao Trucucu 167 0.64 1600 18.0 9.6 Cauto Pilar 137 0.58 1605 18.2 7.2

(A) Annual specific discharge.

(B) Specific discharge for low rainfall season.

Source: Diaz Arenas (1977).