Hydrology 2000. IAHS Publ. no. 171, 1987. Hydrological sciences in perspective ZBIGNIEW W. KUNDZEWICZ, LARS GOTTSCHALK & BRUCE WEBB IITBODUCTIOI Hydrology is concerned with the study of water although traditionally hydrologists have mainly focussed on the terrestrial and atmospheric rather than the oceanic phases of the hydrological cycle. Hydrology is a basic science belonging to the family of geosciences, and it aims to improve the understanding of the laws which govern the phenomena and processes occurring in the hydrosphere, the spatial and temporal distribution of water over continental surfaces, the transport of material by rivers and the quality of water including its physical, chemical and biological aspects. Hydrology is also involved in the practice of design and management of water resources, where increasingly complex problems are occurring. Hydrology is one of the most applied areas of geoscience which reflects the importance of water and its uneven spatial and temporal distribution. Water is the essence of life; it is the major constituent of living matter and the processes of life are based on the exchange of water between living matter and the environment on a continuous basis. Water is also essential to virtually every human endeavour. Although water covers some three quarters of the Earth's surface, it is fresh water only that is usually suitable for man's use. More than 90 % of the Earth's total water resource is unavailable or unsuitable for direct use, because of its salinity (sea water) or location (polar ice caps, very deep groundwater). Also much of the fresh water available on the Earth is not directly suitable for many uses, either due to natural factors or to a very high degree of pollution. There is enough fresh water on a global scale to fulfil the demands of the present and the anticipated future. However, the ability to meet water needs varies considerably with time and space, and this has caused, causes and will continue to cause (to an even greater degree) serious problems. These problems are often generated by natural factors, especially those of climatological and geological origin, but many have also been caused by mismanagement and abuse of water resources. Because of the limited volume and large fluctuation of fresh water resources of the globe, an understanding of the laws governing water distribution and its circulation in the hydrological cycle, i.e. the domain of interest of hydrological sciences, is now and increasingly in the future will be of paramount importance. 1
2 Zbigniew W. Kundzewicz et al. WATER PROBLEMS 2000 Unavailability of water in sufficient quantity and quality has been and will continue to be an increasing constraint on economic growth. Much of the increased pressure will occur in less developed countries, where high population growth and a desire to improve living standards will rapidly and considerably expand water requirements. In order to meet these demands, even to a moderate degree, the less developed countries would have both to assign a large portion of their national income to investments in water resources development and to implement research programmes to guide this investment. However, the developed countries may also experience serious water problems in the year 2000, the elimination of which would require high capital investment. Such problems include deterioration in water quality and a need to reallocate water to satisfy the changing demands of users. The increase in water demand will encourage exploitation of remaining water resources but these are developable only at higher costs. When considering future water problems at the global scale, the starting point is often the estimation of yearly water supply and demand. The former is usually treated as being nearly constant, whereas an assessment of the latter is more difficult to obtain. There are a number of factors of demographical, technological, agricultural, economic, social and political nature that potentially influence the global withdrawal and consumption of water in the future. An increase in water demand is certain, although there are large discrepancies in the assessment of how this may take place. Global water demand in the year 2000, evaluated by different scientists, ranges from 5000 to 20 000 km year". This is still well below the lower bound estimate of global supply (40 000 km year ), but these figures pertain to the aggregated global scale and give no information about the severity of problems that may arise at a particular place and time. In fact, any significant increase in the rate of water withdrawal will inevitably cause major water supply problems. Several rivers that formerly flowed all the year round now carry no water for most of the time, and there are several instances of serious reductions in the flow of perennial rivers which have been promoted by land-use and other changes in the upstream catchment areas. It is possible to envisage water supply problems as a potential cause of international conflicts in some areas. It is practically and economically unrealistic to imagine that growing drought losses can be eliminated at the global scale with the help of the present costly drought protection measures in a planning horizon up to the year 2000. Water storage, which usually involves impoundments on existing streams, is very costly in terms of investment and causes increasing evaporation losses. The latter could be eliminated by using underground storage. Water transfer, for example via water transmission mains, is costly in terms of both investment and exploitation. Use of potentially inexhaustable sea water resources requires desalination with attendant high costs and energy consumption, and is therefore practical today only for a
Hydrological sciences in perpective 3 small number of locations. Depletion of very deep groundwater, accessible only by deep wells, entails high pumping costs and is an irreversible process. Groundwater reserves are no longer renewable since as fossil resources they are mined when used. Excessive groundwater pumping may cause compaction of aquifers and subsidence of land surfaces and valuable infrastructure. An increase in potable water supply can be obtained in several regions by extending water treatment, although at high cost and with considerable consumption of energy and chemicals. Large potential is still offered by water savings and by more effective water use through new water saving technologies, increased recycling, more efficient irrigation practice and improvements in the use of brackish/polluted water for industrial cooling purposes and for irrigation. It is important in areas without severe shortage that fresh water should not continue to be viewed as a cheap natural resource, or as a principal means of disposing of municipal, rural and industrial wastes. It is also important to rely on "income" from water resources and not to deplete the water capital of the Earth, which has been inherited by our generation. The possibility of an increase in flood losses at the global scale cannot be ignored. In areas subject to urbanization and to deforestation, floods will become more severe with higher peak flow and reduced time to peak. Structural flood protection measures create an illusion that flood danger can be fully eliminated and cause economic developments of areas vulnerable to flooding. Present problems of water quality are not likely to disappear in the future, especially since current pollution of groundwater will ensure continuing problems for many years to come, and it is possible that some water courses may deteriorate very rapidly once a critical threshold level of pollution is transcended. However, treatment and recycling techniques have also greatly advanced, and a progressive deterioration in water quality has been reversed in several major lakes and rivers. Almost every water use has a polluting effect and therefore present water quality problems are likely to be exacerbated in future for every phase of the hydrological cycle. Streams, lakes, soils, sediments, estuaries, seas, groundwater and even rain and snow have been and will continue to be polluted. It is a well established fact, that every change in land use and every manipulation of soil, vegetation, ground surface etc. induces a change in hydrological parameters and affects the local water balance. However, it is rarely possible to anticipate all consequences of human activities for the balances and dynamics of the hydrological cycle. Large and to a certain extent irreversible anthropogenic changes can be identified and include such impacts as interregional water transfer, deforestation, urbanization, pollution, acid rain, stratospheric ozone depletion and the "greenhouse effect" caused by the build-up of C0 and other gases in the atmosphere. The impact of man-induced climatic change is not fully understood but its consequences could produce significant economic and social perturbation in a time horizon of decades.
4 Zbigniew W. Kundzewicz et al. TBEIDS, FASŒEOIS AID BEÂS II HYDROLOGY Scientific development in a subject such as hydrology requires the existence of suitable backing in other sciences from which new ideas can draw support for their growth. Some ideas have proved ahead of their time since the necessary support was not available. Also a new hypothesis may have to await developments in measurement techniques and in computing power for its verification. There are two mechanisms which trigger discoveries of new concepts that push science forwards. One is fascination with theory and with methods the development of which is likely to produce practical results. The other relates to problems that have to be solved in spite of the fact that the available methods do not suffice, this is expressed colloquially as "necessity is the mother of invention". However, both methods-oriented and problems-oriented mechanisms may fail. Fascination with techniques may force a method to an application which is not beneficial. The aim per se is often to show that a method can work but this does not advance hydrology if the approach is based on over-simplified assumptions that cannot be successfully refined. Conversely, the need to solve a problem in a restricted time period may result in a scientific fiasco and the necessity of rehashing doubtful methodology that has been reported as being "successfully used". The classification of mechanisms of progress must reflect the purpose of research and whether it is cognitive, stimulated by curiosity to understand, or whether it is practical and utilitarian and fulfils the human need for information on water resources for their control, development or protecton. The two basic utilitarian aims of hydrology are to aid assessment of water resources, including their spatial and temporal variability, and to improve prediction of future conditons at every possible horizon of interest. Sometimes the term "fashion" is used to describe a problem area which receives considerable attention at a particular time. Since the 1930's there have been several "eras" in the development of hydrological sciences relating to available theory and measurement techniques. It is possible to identify eras when the study of rainfall (IUH), runoff, infiltration, evaporation and water quality predominated in certain countries or more generally. The present period is widely characterized by the use of mathematical modelling, computers and remote sensing and data transmission (RSDT). We can soon expect an era of global hydrology in which the main focus of attention will be on the refining of estimates of world balances of water, heat and materials and on searching for teleconnections, although such activity will probably be restricted by the need for sophisticated and costly computing and RSDT facilities. Much needs to be done in order to understand natural systems at the global scale and the interactions between the hydrosphere and the lithosphère, atmosphere and biosphere including the way these interactions are modified by human impacts. One can also distinguish eras in which particular methods have dominated in solutions to several problems. As far as mathematical
Hydrological sciences in perpective 5 modelling of hydrological systems is concerned, the black box approach (IUH and system integral operator), Kalman filtering, kriging and so on have dominated at particular times. During such epochs, the "fashionable" mathematical tools are applied evermore widely, and a "bandwagon" effect can sometimes be recognized in which particular techniques may be pushed beyond their legitimate areas of application. Both problems studied and methods employed determine a kind of fashion in hydrological research, and it is characteristic that many works on a narrow area appear within a short period of time during which researchers attain considerable progress in understanding. DIVEBSITY II HESEâBCH POTEffCIAL There are countries where twenty-first century research is already being conducted and scientific centres that can afford extremely costly instrumentation. On the other hand, there are large areas of the globe where a lack of hydrological research hampers a proper utilization of water resources, such as in many developing countries. Most activities taking place today in these countries, are aimed to solve acute problems, while the search for basic understanding and for long-term future perspectives are by necessity given low priorities. Although efforts have been made to understand and to describe the problems of hydrology in developing countries, there still exist vital gaps in our knowledge. The sensitive dynamic meteorological, hydrological and ecological balances with complex feedbacks and interactions which are typical of many developing countries, especially semiarid ones, are not fully understood. There is considerable interest in research into new techniques for applications in meteorology, hydrology and ecology from space technology, mathematical modelling and other challenging fields of scientific development which have been taken up in developing countries. Less effort is directed to basic research on physical, chemical and biological processes and to the implementation of the new techniques in operational practice. New techniques do have a great potential in hydrological research in many developing countries, but it must be emphasized that the problems of understanding the processes involved should form the focus of research. The methods, although challenging, should only be tools to enable better research opportunities. Local scientists have limited possibilities to carry out basic process research. At the same time, it is the local scientists who can guarantee a building up of knowledge and awareness in the long term, provided that proper research "facilities exist. A better understanding of basic processes in a long term perspective gives a background for prediction of the future evolution and development of water resources in developing countries. It also provides a background to proposals for land and water resources management policies which eventually can lead to the stabilization of the environmental situation in many countries.
6 Zbigniew W. Kundzewicz et al. MmjHOBTDBOLOGY When trying to forecast the developments of hydrology as a scientific discipline three aspects have to be considered: (a) gaps in knowledge, (b) extrapolation of existing trends, (c) anticipation of further inputs from outside of hydrology. Hydrological research in the future is likely to be directed to areas where the present barriers to development are located. An attempt was therefore made in the present report to identify the critical gaps in our knowledge of particular subareas within the subject. The most important external conditions affecting the development of hydrology relate to fundamental problems whereby water is becoming an increasingly scarce resource in increasingly high demand. In order to facilitate an adequate supply of fresh water of suitable quality, analyses of water resources and likely future demand will continue to be made on many different scales and attention given to the appropriate measures for ensuring supply. Another external factor that will undoubtedly continue to influence hydrological sciences is the development of high technology. This will involve continuing progress in computation technology in the range from personal to supercomputers, further developments in data acquisition can also be expected and ESDT has the potential to revolutionize hydrological monitoring on many scales, including the global level. The new possibilities associated with RSDT will compensate for the decline in monitoring programmes which is currently taking place. This latter tendency and also the decreasing financial support for fundamental research in hydrology, although depressingly widespread at present, need not continue until the year 2000, especially if there is an upturn in the world economy. Anthropogenic effects may also provide external stimuli to the development of hydrological sciences. In 1964, the Committee on "Status and the Needs of Hydrology" of the American Geophysical Union determined a list of 63 challenging areas in hydrology. It is surprising that, in spite of the significant progress achieved, many of the problems listed still remain at the centre of interest in hydrological research. Most problems belonging to older areas are still relevant today since new pressures on the hydrological system require new levels of accuracy and sophistication in problem solving. It is not likely that hydrological research in the near future will be directed by the need to analyse events well beyond our present imagination. There may be unexpected events, such as very large floods or very severe droughts, but these can be classified as rare and not as unthinkable. However, there may be instances when hydrology must participate in analysing the consequences of events beyond our present experience. An example of such an event in the recent past is radioactive contamination of water following an accident in a nuclear power station. Future scenarios whose hydrological and water resources consequences require evaluation in order to encourage preventative measures relate to the effects of man-accelerated climatic change and global nuclear war.
Hydrological sciences in perpective 7 The developnent of hydrological research must provide a trade-off between the possibly conflicting priorities of long-term scientific enquiry and of immediate application to economic and other problems at national and other scales. This requires a balancing of basic research of broad relevance but no immediate application with applied research of immediate relevance to a narrow problem. However, sometimes problems of immediate concern cannot be solved without breaking barriers in fundamental research. A potential increase in the applicability of hydrological sciences calls in general for a better assessment of water stocks. Environmental planning must benefit from progress in understanding and documentation of the distribution, volume and quality of water on the surface of the earth and also underground. Furthermore, forecasting of hydrological variables, most notably in the context of flood forecasting, must be more accurate.