The Influence of Climate Change and Climatic Variability on the Hydrologie Regime and Water Resources (Proceedings of the Vancouver Symposium, August 1987). IAHSPubl. no. 168, 1987.

Impact of climate change on the morphology of river basins

F.Ho Verhoog Division of Water Science Unesco

Introduction

There is sufficient evidence to show that climate is variable. There are also indications that climate can vary considerably over a relatively short time span. If relatively short means 100 or 200 years, if we are presently undergoing such a change it is important for us to recognise the changes and to adapt our hydrologie means.

Within the framework of the IAHS, we are particularly interested in changes in the morphology of river basins related to the hydrologie regime.

When we want to estimate the impact of climatic change on the morphology of river basins, we first have to estimate the impact of climate change on precipitation and evaporation, secondly on natural ecosystems, thirdly on runoff and lastly on the morphology of river basins.

The following discussion, unreasonably, tries to be quantitative as possible. Due to a lack of data and a lack of verified and tested methodologies the tables given in the text, although based on ideas expressed in scientific literature, can only be regarded as hypothetical.

Much use has been made of the listed consulted literature.This use has been extensive and without restraint. As parts of the consulted literature is used outside its context it does not seem correct to quote it.

Impact of global warming on precipitation and évapotranspiration

The surface of the earth is warm largely due to radiation from the sun in the visible part of the spectrum. This energy is partly absorbed by the surface of the earth which warms up as a result. Like all warm objects, the earth itself radiates energy, but in the infrared part of the spectrum. This re-radiated energy is partly absorbed by water vapor and C0o in the atmosphere and partly radiated back to the earth, the so called greenhouse effect. The mean tempera­ture of the earth is about 15 degrees centigrade. Without water vapour and CO2, the mean global temperature would be -23 degrees.

When the earth was formed it was very warm and the sun was relatively cool. Due to volcanic activity, water vapour and CO2 were put into the atmosphere. The earth cooled and the water vapour precipitated to form oceans. Then about 1 billion years ago, plants began to use the CO2 to produce oxygen. The CO2 content began to fall. But as the warming influence of the green house effect declined

315

316 F.H. Verhoog

the sun got warmer. Hundreds of millions of years ago the atmosphere still contained

much more carbon dioxide than today. The carbon dioxide concentration fell until about 1 million years ago, when series of ice ages started to occur. During the ice ages the carbon dioxide content of the atmosphere was about 200 ppm and during the interglacials about 270 ppm. Over the past 100 years natural variations have ranged from 250 to 310 ppm.

In about 1850 the C0? concentration was around 280 ppm. At the beginning of this century the concentration was about 300 ppm. In 1958 when systematic measurements were started the average concentra­tion was 316, and at present it is around 350 ppm. The increase in CO,. is mainly caused by the burning of fossil fuels. About 58% of the C0,-produced seems to remain in the atmosphere.

In geological terms, an increase in C02 concentration is correlat­ed with an increase of the global mean temperatures. It is thus to be expected that the increase in C02 since the start of the industrial revolution about 1850, will have tiad an impact on temperatures. There was little warming in the nineteenth century, marked warming until 1940, relatively steady conditions until the mid 1970s and a rapid warming until now. Five out of nine of the warmest years since 1860 have occurred since 1978. However, the steady conditions maintained from the late 1930s to the mid 1970s still needs an explanation. Carbon dioxide levels are now expected to reach twice the value of 1850 by the end of the next century.

Future warming brought about an increase in the CCv, concentration is predicted through the use of climatic models — the so-called general circulation models.

The predicted changes are not the same all over the globe. When the world as a whole warms up, the higher latitudes warm up more than the lower ones which reduces the difference in temperature between the equator and the poles creating a profound effect on the circulation of the atmosphere. The equatorial regions may warm up to 0.5 degrees and the higher latitudes up to 6 degrees.

In the first instance, general circulation models (GCMs) try to predict the temperatures of the globe on the basis of a calibration of the present situation and a diminution of outgoing radiation. Some of these models can also give indications of changes in precipitation in the northern hemisphere.

The latest GCMs predict a global warming up of 3.5 to 4.0 degrees celcius brought about by a doubling of C02. One of the features of these GCMs is that they can simulate an exchange of heat between the oceans and the atmosphere creating the formation of sea ice and variations in cloud formation. Warming of tropical surface air ranges from 2 to 4 degrees. The greatest precipitation changes occur between 30 degrees north and 30 degrees south. One of the existing models predicts that the soil in almost all of Europe, Asia and North America will become drier during the summer, while others foresee wetter soil during the summer in most of these continents.

Some of the GCMs predict an increase in temperature and a decrease in precipitation for the areas around the Mediterranean.

Climate model results all point to an increase in the intensity of the hydrologie cycle and an overall small increase (3 to 5%, depending on what models are used) in global mean precipitation. The

Impact of climate change on morphology of river basins 317

effects of precipitation change and vegetation changes are expected to reinforce each other. In some regions very large increases in annual mean runoff can be expected.

Those computer models which predict a temperature rise of 4.5 degrees brought about by a doubling of the C02 concentration also predict an increase in precipitation of 7-11%. Rainfall will increase at the ocean borders of the USA, over most of Canada, at the mouth of the Mississlpi, around the Baltic, Egypt and Sudan, in the Middle East, India and Pakistan and in the middle of China. Precipitation over the rest of the Northern Hemisphere will decline, (source : University of East Anglia, as reported in the Economist of 28 November 1986).

An increase in evaporation is mainly likely to occur in equatorial regions but could also occur at high latitudes.

An approximation of the above information can be found in Table 1.

Table 1 Possible changes in temperatures and precipitation due to a doubling of the C0? concentration. The évapotranspira­tion changes only take into account the increase in temperature. All data are hypothetical

Possible changes in TEMPERATURE PRECIPITATION EVAPOTRANSPI-(°C) (%) RATION(%)

+(2) +(2)

+(8) +(8)

+(12) +(18)

+(74)

Impact of runoff

Static hydrologie changes

Climate change means changes in temperature and precipitation and changes in evaporation caused by changes in temperature, air humidity, windspeed and cloudiness. As hydrologists we are particula­rly interested in the resulting changes in runoff.

The relation of existing regional climate to runoff is not always straightforward as can be seen from the following table of runoff ratios ( the annual volume of discharge divided by the annual volume of precipitation) of large rivers in the world: From

(a) 0.01 to 0.20 Parana, Zambezi, Nile, Niger, Sao Francisco, Murray.

(b) 0.21 to 0.30 Congo, Mississipi, 0b, Danube, Vistula, Volga (c) 0.31 to 0.40 St. Lawerence, Amur, Mckenzie, Indus

TROPICAL

SUB­TROPICAL

TEMPERATE

COLD

Arid Humid

Arid Humid

Warm Cold

+(0.5) +(0.5)

+(2) +(2)

+(3) +(4)

+(6)

+ +/

--

—

+/•

+

318 F.H. Verhoog

(d) 0o41 to 0.50 Amazon, Yang-tse, Ganges, Yenissei, Orinocco, Lena.

(e) 0.51 to 0.60 Columbia, Irrawady (f) 0.61 to 0.70 Brahmaputra

The lower ratios clearly belong to arid and semi-arid climates. But the combination in one group of the Congo, Danube and Ob rivers is not obvious. It means that the usual climate classifications are not directly applicable to hydrology.

Global circulation models are used to predict the impact of a doubling of CO^on climate and likewise hydrologie mathematical models are used to study changes in runoff caused by changes in climate.

Like the GCM models, rainfall runoff mathematical models are calibrated on existing data and situations. The existing data are mostly daily precipitation, temperature and discharges for periods of 20 to 60 years. The most used model technologies are based on the assumption that physical basin boundary conditions do not change, thus vegetation, soil properties and channel morphology are kept constant.

For hydrologie climate impact modelling, the ideal model would be a distributed deterministic model with the following sub-models: an unsaturated zone model, a root zone model, a saturated flow model, a snowmelt model, a canopy interception model, an évapotranspiration model, an overland and channel flow model. An example of such a model is SHE (Systems Hidrologique Européen). With SHE it is possible to model changes in vegetation and land use, but it is not possible to model secondary details such as soil macro-pores and an undergrowth of vegetation below the major vegetation.

For the purpose of rainfall-runoff modelling the modelling of évapotranspiration is crucial. Evapotranspiration, unlike runoff, disappears from the basin system and depends very much on ecological factors. Evapotranspiration depends on net radiation, rate of increase of the saturated vapour pressure of water at air tempera­ture, density of air, specific heat of air at constant pressure, vapour pressure, deficit of air, aerodynamic resistance to water vapour transport, latent heat of vaporization of water, psychrometric constant, canopy resistance to water transport.

Nemec and Schaake used the Sacramento soil moisture accounting model to predict changes in streamflow in three existing river basins by changing both precipitation and évapotranspiration. The évapotran­spiration change was based on changes in temperature.(1 degree Celsius corresponds to a 4 % change in évapotranspiration). The basin as such, including the seasonal distribution of precipitation and évapotranspiration, remained the same.

The runoff ratios for the basins tested were 0.02, 0.13 and 0.31. For the dry basin and decrease of 10% in precipitation and an increase of 1 degree C decreased the runoff by 50%. For the humid one, the decrease was 25 and for the medium basin a decrease in runoff of 40%.

A dry basin is more sensitive to changes in precipitation than to évapotranspiration and is more sensitive to precipitation increases than a more humid basin (humid and dry as indicated by the runoff coefficient). For reductions in precipitation all basins are less sensitive to changes in evaporation than an increase in precipitation. For low runoff ratios, small changes in precipitation may cause large

Impact of climate change on morphology of river basins 319

changes in runoff. An approximation of the above infrastructure is listed in Table 2.

Table 2 Changes in percentage of annual runoff if precipitation and evaporation change as indicated. It is assumed that basin characteristics such as vegetation have not changed. All data are hypothetical

Possible changes in PRECIPITATION EVAPOTRANSPI- RUNOFF (%) RATION (%) (%)

TROPICAL

SUB­TROPICAL

TEMPERATE

Arid Humid

Arid Humid

Warm Humid Cold

+(10) +/-(10)

-(10) -(10)

-(10) +/-(10) +(10)

+(2) +(2)

+8 +8

+(12) (12) +(18)

+20 +40/-30

-50 -30

-50 +10/-35 +5

COLD +(10) +(24) nc

Ecological changes

The importance of vegetation

Runoff ratios for most of the rivers of the world are less than 0.50. The water "lost" is lost through évapotranspiration. The term évapotranspiration in the water balance is therefore important. Taking weather as a boundary condition, évapotranspiration is mainly dependent on the soil-vegetation complex. Vegetation, in addition, influences the base and rise times of the hydrograph.

While precipitation is the input, the soil cover complex plays the role of the discriminating element of the precipitation - runoff relations.

Climate influences the microclimate, the soil and the vegetation. The microclimate influences the vegetation and the soil. The soil influences the microclimate and the vegetation. The vegetation influences the microclimate and the soil. The fauna influences the vegetation.These circular relations are difficult to introduce into mathematical model.

In addition, because the vegetation depends on both climate and soil, it is not always a good indicator of present climate. Vegetation is also dependent on previous climates through the historical diversity in plant species and indirectly through the soils that were formed during previous climatic conditions. Only the climate (the weather over a long period) reacts immediately in temperature and changes in air circulation.

What is the sensitivity of ecosystems to climate change? First of all we should be aware that a change in temperature and/or

320 F.H. Verhoog

precipitation will be accompanied by a change in probabilities of extremes. In certain cases ecosystems may be unexpectedly sensitiveo For example, there is a theory that the reason forests are dying in certain parts of Europe is caused by the combination of increased acid precipitation and the drought of 1976. In some regions in the Alps 40% of the fir trees have died or are dying. This will increase as will the probability of avalanches. Thus a single drought period may have very serious consequences.

In general, when plant and animal generation times are short relative to the timescale of environmental change plant, and animal populations tend to react quickly to environmental processes. Environmentally bad times reduce the population but the return of favourable conditions sees a rapid increase in population growth.

The Northern Hemisphere warmed up from the mid-nineteenth century to about 1940. The predicted warming up and historical warming up are of a comparable characteristic timescale, that is, about a century. The characteristic timescale for animal population growth is from a month to 10 years, for vegetation biomass growth from a year to 10 years, for vegetation biomass growth from a year to 15 years, for soil accumulation from 10 to 800 years and for vegetation range extension from 1000 to 8000 years.

This means that biomass in general will adapt gradually to climate change brought about by global warming up. This may not necessarily be the case for forests. Large scale expanses of trees such as those following the last glaciation operated on a timescale of thousands of years. The reverse will also be true, forests and trees will survive longer if the conditions for tree growth and in particular, reproduc­tion deteriorate.

According to some studies if a doubling of CO2 occurs we may expect the following vegetation range changes :

change

+ +

Ecosystems

tundra 3 - — woodland 58 47 grassland 18 29 ++ desert 21 24 +

According to some studies increasing atmospheric carbon dioxide concentrations will have a direct effect on plants. There stomata will close down, reducing evaporation and increasing water use efficiency. Reduced évapotranspiration would increase runoff and thus could offset the effects of precipitation reduction or enhance the effects of precipitation increases.

Changes in évapotranspiration will in this case occur due to changes in climate, changes in the area of vegetation cover (due to

Forest types

boreal forest cold temperate warm temperate subtropical tropical

present

23% 15% 21% 16% 25%

predicted

1% 20% 25% 14% 40%

Impact of climate change on morphology of river basins 321

either climate change and/or the direct effect if CO? on plant growth) and the direct effect of C0„ on évapotranspiration. If the runoff coefficient is high then runoff is more sensitive to precipi­tation changes than to évapotranspiration changes. Precipitation changes have an amplified effect on runoff, particularly in arid regions where the runoff coefficient is small. Evapotranspiration changes only have an amplified effect on small runoff coefficients.

For a catchment with a present runoff ratio of 0.20, the effect of a 10% reduction in precipitation may range from a 50% reduction in runoff with no direct GO2 effect, to a 70% increase in runoff with a maximum direct CO2 effect. For higher runoff ratios the ranges of possible runoff changes is much less.

The increases in evaporation due to global warming up would be much less than the maximum direct CO2 effects on a global scale. However, there are many unknowns. There will, of course, be large seasonal and regional departures from the global mean.There are also other factors, for example, the seasonal distribution of precipita­tion may be different from that of évapotranspiration. Direct CO2 effects may also be reduced due to increased leaf temperatures, in individual plant leaf areas or in the total vegetated area of a catchment.

When the CO- content is less than 280 ppm it is probably the limiting factor on plant growth. For every increase of 10 ppm above that level plant growth is stimulated by between 0.5% and 2% depending on the species. Also when more CCu is present, plants use water more efficiently in photosynthesis and do not require so much rainfall. More efficient use of water can also cause problems, for example, increased runoff, increased soil erosion, etc.

For rainfall runoff relationships the important geophysical factors are: the nature of geological formation (crystalline or sedimentry); the density of the natural vegetation, the nature of the drainage pattern and the formation of floodplains. Floodplains have high infiltration losses and évapotranspiration losses, which reduce streamflow.

Besides the drainage area and the slopes of the catchment, the influence of the soil vegetation complex, affecting the rainfall runoff transformation, is preponderant. For soils, the clay content seems the most important and as regards the vegetation the percentage of crops in the catchment.

The role played by geophysical factors in determining the runoff coefficient shows that a separate analysis is needed for the arid and semi-arid regions which later are affected by impervious soil surface crusts and hydrographie degradation.

The base and rise times of the hydrographs are much longer in the tropical and forest zones than in the semi and arid zones by:

(a) the attenuating effect of vegetation; (b) in the semi-arid zones floods are produced by surface runoff

alone while in tropical zones floods are, to a considerable extent, generated by sub-surface runoff. In forest zones sub-surface runoff predominates.

The influence of cultivation following the clearing of natural vegetation is as follows :

(1) where mean annual rainfall is less than 650-700 mm, crops reduce mean annual runoff, probably because they consume more water

322 F.H. Verhoog

than the herbaceous stratum. (2) where rainfall exceeds 650-700 mm, the reverse is true, either

because crops consume less water than the tree stratum or because soil exposure promotes runoff.

As variations in climate change the vegetational possibilities in a particular geographic area, changes occur in agricultural methods and extensions or dimunition of cultivated areas. These changes should also be accounted for.

In a forest, any cultivation leads to a considerable increase in runoff. For natural grasslands, subjected to burning every two years and consequently lacking dense plant cover, any modification leads to a decrease in the annual runoff. Finally the installation of anti-erosive structures followed by contour cultivation reduces runoff to a greater extent than traditional cultivation or reafforestation.

Impact of ecological change on hydrology

Table 2 was established assuming that the soil-vegetation complex, the relative distribution of wet and dry seasons, water use and cultivation patterns would not change when the earth warmed up. This, assumption, in the long run, will certainly not be correct.

The following table is an attempt to bring together in a comprehe­nsive form the information described above. The table will certainly be proved wrong as the present knowledge is incomplete and probably partly eroneous. In order to simplify the table, the cold regions and the truly arid regions are left out.

Table 3 Possible relative impacts of vegetation and cultivation changes on the impact figures for runoff of Table 2. The indications in this figure have no real scientific basis

Present s i tuation

Humi d

Sub-humi d

Semi-arid

Possible future si tuation

more humid

more dry

more humid

more dry

more humid

more dry

Changes i n

végétât i on cover

nc

+

+

~

Relative

impact on runoff of

végétât i on

nc

+

.

+

-

+

Relative impact on runoff of

Corrected

runoff changes

cultivat ion i n % changes

nc

+

+

+

+20

less than - 30%

more than + 10%

less than

less than + 10%

more than

-50%

Impact of climate change on morphology of river basins 323

The information in this table, however wrong it may be, is based on the assumption that an increase in C02 will not have a direct effect on the water use efficiency of the vegetation. If this is the case évapotranspiration will be reduced. The direct effect of doubling of CO2 would be most pronounced in semi-arid regions and to a lesser extent in sub-humid regions. The effect would be to increase annual runoff whether the precipitation increases or not. The maximum effect would occur in areas with increased humidity, the runoff may double.

Impacts on river morphology

The variables determining river morphology are: geology, paleoclima-tology, relief, valley dimensions, climate, vegetation, hydrology, channel morphology, water discharge, sediment discharge and flow hydraulics. These variables are not, of course, independent of each other they are listed in this manner to facilitate description.

For engineering purposes, for example for hydrologie engineering, we are interested in the water discharge and the sediment discharge. The dependent variables are the observed water and sediment discharges and the hydraulics of flow. All the other variables are independent variables.

For engineering purposes we look at a timescale of weeks to tens of years and, in this case, we do not usually take into account possible changes in vegetation cover. When looking at the impact of climate change on river morphology we must consider larger timescales, say around 100 to 200 years.

When we want to consider the impact of the climate change on river morphology, the channel morphology becomes the independent variable. The observed water and sediment discharges and the hydraulics of flow become indeterminate variables, and all the others independent variables.

Eventual climate change directly influences the vegetation and the hydrology of the basin, which in turn influences the channel morphology, which influences valley dimensions, which influences relief. Relief again influences the hydrology of the basin, etc.

In a natural stream, over longer periods of time, mean water and sediment discharge are independent variables which determine the morphologic characteristics of the stream and, therefore, the flow characteristics.

A major change in the hydrologie regime would trigger a response that would completely change channel morphology. Channel morphology reflects a complex series of independent variables, but the discharge of water and sediment integrates most of the other independent variables ; it is the nature and quantity of sediment and water moving through the channel that largely determines the morphology of stable alluvial channels.

A decrease in precipitation in the head waters will not only cause a decrease in annual discharge, but through reduction of vegetation density, it will increase peak discharge and greatly increase the amount of sand load with less water, the channel will become wider and shallower. Many of the wide "unstable" rivers of the world could be transformed into the stable channels by reducing flood peaks and

324 F.H. Verhoog

bedload transport. The shape of the channels is closely related to the percentage of

silt and clay in the sediments forming the banks and the bed of the channel» Relatively wide and shallow channels contain only small percentages of silt and clay. Narrow and deep channels contain large percentages of silt and clay.

For semi-arid to sub-humid regions around 90% of the variability of the channel width and 80% of channel depth can be explained by mean annual discharge and type of sediment load. Only about 40% of the variability of channel dimensions can be accounted for by discharge alone.

There does not seem to be much relation between size of sediment and channel dimensions but when an index of the type of sediment load is combined with discharge, good correlations with width and depth were often obtained.

There is a correlation between water and sediment discharge on the one hand and channel width, depth, meander wave length, the width-depth ratio, sinuosity and the slope of the bed on the other hand, Climate change will either change discharge of water and sediment or not. If there are no changes, there will be no changes in morphology. If there are changes, then we may find four cases: the case where both water and sediment discharge increases or decreases and the two cases where one increases and the other one decreases.

Professor Schumm prepared a table giving the consequences of such changes (on a long time scale, a few hundred years);

b

+ -

+ / -+ / -

d

+ / -+ / -

+ -

l a

+ -

+ / -+ / -

S

+ / -+ / -

+ +

b= width of the channel d= depth of the channel la=meander wave length S= gradient P= sinuosity expressed as ratio of channel length to valley length f= width-depth ratio

Earlier, Table 3 took a hypothetical look at possible changes in runoff due to global warming. We did not discuss possible changes in erosion.

Table 4 hereunder gives possible changes in both water and sediment discharges.

When we combine the information in professor Schumm1s table and the information in Table 4 we get Table 5.

The conclusions we can draw from this table are: We have seen from Table 4 that the greatest changes in water and

sediment discharge are likely to occur in the regions between the, at present, humid and semi arid regions. If these regions become more humid, water discharge will increase considerably and sediment discharge will in the long run become less than at present. The result will be that the depth of channels will increase, the gradient

Impact of climate change on morphology of river basins 325

Table 4 Possible hypothetical changes in both water and sediment discharge due to global warming up

Present Possible Possible changes situation future in runoff(Qw)

situation (n%)

Possible changes in sediment discharge

(Qs)

Humid

Sub-humid

Semi-arid

more humid

more dry

more humid

more dry

more humid

more dry

4-

Table 5 Possible hypothetical changes in river basin morphology due to global warming

Possible b future width

situation of channel

d la S P F depth meander gradient sinu- width/ of wave (geol) osity depth channel length ratio

more humid 4- +/-Humid more dry +/-

more humid +/- 4-Sub-humid more dry +/- -

more humid 4- +/-Semi-arid more dry 4-/- -

4-

+/-

+/-

+/-

+

+/-

+/-

4-

-

+

+/-

4-

-

-

+

+

-

-

4-

+

-

+

+

+

will decrease and the sinuosity will increase. On the other hand, if precipitation in the present sub-humid

region decreases, water discharge will decrease and sediment discharge will increase considerably. The result will be that the depth of channels will decrease, the gradient will increase and the sinuosity decrease.

The warming up of the earth due to C0? doubling and the increase in concentration of other man-made gases, will come about gradually and the impacts will also become visible graduaLly. Table 5 gives the directions of the change between the present and the final situation. In between there will be other important changes not reflected in the table. For example, when a semi-arid basin becomes more humid storms may increase in intensity before the vegetation has time to develop.

326 F.H. Verhoog

During this period the erosion and sediment discharge will be greater than at present although as indicated above in the long run it will be less than at present.

References

Abbott, M.B., Bathurst, J.C., O'Conell, P.E., and Rasmussen, J.(1986) an introduction to the European Hydrological System-Système Hidrologique Européen, "SHE", Structure of physically-based, distributed modelling system; Journal of Hydrology, 87:61-77.

Dubreuil, P.; review of relationships between geophysical factors and hydrological characteristics in the tropics; Journal of Hydrology 87 (1986) 201-222.

Hare, F. Kenneth (1985) Climate variations, drought and desertifica­tion WMO publication no. 653.

Manabe, S. and Wetheral, R.T.; Reduction in summer soil wetness induced by an increase in atmospheric carbon dioxide; Science Vol. 232, 1986.

Martin L. Parry (Editor): the sensitivity of natural ecosystems and agriculture change. IIASA and UNEP, 1985, reprinted from climatic Change, Vol. 7.

Nemec, J. and Schaake, J.; Sensitivity of water resources systems to climate variation; Hydrological Sciences Journal, 27, 3, 9/1982.

Schumm, S.A. (1971) Fluvial Geomorphology: Historical Perspective, and Channel Adjustment and River Metamorphosis, in "River Mehanics" edited and published by Hseieh Wen Shen.

Unesco (1978) World Water Balance and Water Resources of the Earth, prepared by the USSR Committee for the Intergovernmental Hydrolo­gical Programme, Unesco series Studies and Reports in Hydrology, no.25.

Wigley, T.M.L. and Jones, P.D.J.; influences of precipitation changes and direct C02 effects on streamflow; Nature, vol. 314 of 14 March 1985.

*The„author is employed by Unesco and active in the division of Water Sciences. Unesco takes no responsibility for the opinions expressed in this discussion.