Modifications in water availability and quality over time and space – especially in the context of climate variation and change and growing water needs – call for adaptive management of water resources. The essential requirement for such an approach is detailed knowledge of the availability and usability of water resources in time and space. This implies knowledge of physical systems and their driving processes, and the availability of hydrological data in real time (precipitation, temperature, stream discharge and groundwater levels), together with data on water use and on environmental flows. With this information, it is possible to run water budget models and to know the availability of water for different objectives. These depend on the provision of ecosystem services.
Ecosystem services are the benefits people obtain from ecosystems that maintain the conditions for life on Earth. They include:
- Provisioning services such as food and water
- Regulating services such as flood control
- Cultural services such as spiritual, recreational and cultural benefits
- Supporting services, such as nutrient cycling
Water resources can therefore be considered as “ecosystem products” whose protection and sustainable management will, in turn, also protect and safeguard water resources.
Water bodies – rivers, lakes, groundwater, etc. – provide a wide range of services for ecosystems and human society to thrive on. Fluvial systems – rivers and associated lakes, wetlands, etc. – evidently provide the highest number of such services, for example, for food and agriculture, drinking, natural flood mitigation or energy.
In order to ensure such services, there has to be an appropriate level of functionality of fluvial processes, in terms of flow and sediment regime. This will promote heterogeneous habitats and connectivity and sustain the different biotic communities inside ecosystems. To support the complex task of determining the appropriate level of functionality, WMO is in the process of developing guidance on the estimation of environmental flows.
The establishment and maintenance of such flow and sediment regimes, namely environmental flows, is an essential element in preserving riverine ecosystems and the services they provide. They should be included as a constraint in water resource assessment and in national legislative frameworks.
Environmental flows currently refer to the typical seasonal and interannual variabilities of the natural flow regime, and not only to the minimum amount of water (low flows) to be maintained in a river. In addition to this pure hydrological assessment of natural flow variability, there is also the necessity to link environmental flow definition to related hydromorphological processes and local ecological objectives for the river.
Environmental flows are fundamental to the management of water resources as a tool for ecosystem and biodiversity conservation and adaptation to climate change at a country scale. Additionally, environmental flows should be considered for large uses of water for agriculture and for the development of hydropower projects.
Historically, exploitation of water resources, particularly water, has occurred with no consideration of the sustainability of such management in the long term. For example, rivers have been considered as canals and severely engineered for socioeconomic development. Ecosystems have therefore been disconnected from once-related water bodies and deteriorated, often irreversibly, with a substantial loss of freshwater biodiversity.
River management not taking into account the dynamic nature of rivers has provoked undesired effects. These include incision processes and bank erosion undermining channel stability, sediment starvation, coastal erosion, disconnection with groundwater bodies and, ultimately, loss of habitat and ecosystem services.
A change of method is therefore required to ensure availability and sustainability of water resources. Together with real-time knowledge of water availability and quality, a different approach to river basin management has to be considered. This should be based on a systemic vision of catchments where rivers are seen as dynamic entities providing ecosystems services that have to be managed with these services in mind.
 Millennium Ecosystem Assessment, 2005: Ecosystems and Human Well-being: Synthesis. Washington, D.C., Island Press.