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1. INTRODUCTION

Climate and land cover are major factors that regulate the hydrological cycle, in sense that water, mineral sediments, and biogenic substances are dynamic elements in the river basin landscape.
 
During the last 300 years, most river basins were dramatically modified due to human disturbances such as agriculture, grazing, deforestation, and urbanisation. These disturbances also have been changing the Earth’s albedo and, consequently, its surface energy budget, affecting local and regional climate, and, ultimately, the amount and quality of water in the river basins of the world.
 
The one of the fundamental tenets of the concept of sustainable development is the maintenance of an homeostatic equilibrium within the ecosystem. Over-exploitation, or biotic structure degradation, alters the ecosystem processes to the point where the ecosystem’s ability to produce desired resources is seriously diminished. A decline in water quality and biodiversity, observed at the global scale in both developed and developing countries, has provided sobering evidence that a purely "mechanistic" and fragmented approach to water resources management, based largely on hydrotechnical solutions such as application of sewage treatment technologies and regulation of hydrological processes through flood control and drought mitigation measures, has been less than successful. While elements of this approach remain valid and viable, a technical solution alone is clearly insufficient for the sustainable use of the world’s water resources.

A. The urgent need for a new approach for integrated river basin management

At the beginning of 21^st century, the increasing human population and its aspirations has become a major factor in progressive environmental degradation on the global scale. Degradation of biological structures and ecological processes means a reduction in the ecosystem’s carrying capacity. As a consequence, during the next 30 to 60 years, the human imperatives may clash with the carrying capacity of global environment. Such a clash would be nothing less than catastrophic for humanity.

It is a well-known phenomenon in ecological handbooks that overpopulation, and exhaustion of the resources upon which the population depends, leads to population collapse. This phenomenon appears in many types of populations and across a range of scales, from protozoans in an experimental bottle to deer population introduced to a coastal island. The final effect of such experiments is always the same - over-exploitation of biotic resources lead to sharp decline of population size.

It is worth underlining, however, that the carrying capacity of each ecological system within the mega-ecosystem -the biogeosphere we know as Earth - is not fixed. It can be reduced by pollution and over-exploitation of resources (e.g., through harvesting of biomass, or pollution of water). Likewise, it can be restored and expanded through good husbandry and management (Figure 1.1).

The optimistic aspect of this story is that if, during periods of sharply increased population growth, the carrying capacity of the environment is increased, the population possesses additional time during which homeostatic regulatory mechanisms may be established to achieve a state of dynamic equilibrium between the density of the population and the carrying capacity of ecosystem. One example of such homeostatic feedback regulation is well known. In case of fish populations, when food resources become limiting, females do not produce eggs. This results in a reduction in the population and the recreation of an equilibrium between resource availability, necessary to sustain the population, and the population itself. Thus, the question becomes one of providing an answer to the question of how to achieve and sustain this equilibrium, or, better yet, of how to expand the carrying capacity of global ecosystem to sustain an increasing population?

The answer is through developing an understanding of ecological processes at different spatial and temporal scales. Such an understanding can be achieved by integrating the different sectors or branches of the environmental sciences.

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• International Year of Biodiversity
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