Withdrawal and use of freshwater by various sectors
Throughout history, agriculture has been an important user of water resources, most of which is withdrawn for irrigation purposes and consumption for livestock breeding. Consequently, the impact of agriculture on global water resources is large and often the main originator for the appearance of water stress. Higher levels of irrigation will generally indicate higher levels of water withdrawal, less available water for other sectors, and potential vulnerability to decreases in rainfall as a result of climate change.
Materials
The world’s expanding population is placing increasing demands on water for drinking, food production, sanitation, public services and other social and economic needs.
All natural waters within the hydrological cycle are potential water supply sources: fresh water, surface and underground sources, saline sources, effluents after reclamation, rainwater, icebergs and undersea springs. But water in streams and lakes (natural or artificial) is often the first source that is tapped for water supply.
Variations in water withdrawal and use for municipalities/human consumption depend upon a number of important factors including size of the urban centre, presence of industry, quality of the water, its cost, its pressure, the climate, characteristics of the population, whether supplies are metered, and the efficiency of the waterworks administration.
Materials
Industrial water use depends on the nature and scale of industry in an area. An assessment of the water use by various industries largely depends on available data concerning present and future production in all fields of industry and on the assessment of the present and future water consumption per production unit in all branches of industry. All these parameters can change considerably depending on the investment and economic development dynamics in an area and the implemented technology in the production process, since it can greatly influence the water consumption per unit product.
Materials
The water requirement for the operation of a thermal power plant is dependent on several factors such as type of fuel used (coal, petroleum based, nuclear etc.), power generation capacity of the plant, type of cooling used (once through, recirculation etc.), source of water and treatment to be followed etc. When designing a thermal power plant, one of the key elements to be considered is the amount of water that will be needed for the day-to-day operations and this is one of the first things to be calculated when designing the plant. A water balance diagram provides a good indication of the water requirement across the various streams of a thermal power plant. The design principles can vary based on the standards and guidelines prescribed by the relevant local or national authorities.
Materials
Hydropower is renewable energy, derived originally from the sun, which drives the water cycle, causing rivers to flow over millennia.
Hydroelectric energy is developed by transforming energy in water that falls from a higher level to a lower level into mechanical energy on the turbine and into electrical energy. The power potential of a site (in kWh) is thus a function of the discharge and of the head.
Hydropower uses this energy without consuming water to any great extent. A hydroelectric power station will return the same quantity of water to the natural environment, minus any loss by evaporation from reservoirs, but generally with a somewhat different hydrological regime.
It still has the following impacts on the environment: modification of the river’s flow regime; unnaturally rapid variations in streamflow; a fill of stored water volumes of one part of the year on another and flooding of upstream areas.
Environmental flow (e-flow) refers to a specific flow regime in a river that can sustain a complex set of aquatic habitats and ecosystem processes. It is therefore an essential element in preserving riverine ecosystems and the services they provide. The term e-flow has other names or variants worldwide: instream flow needs, ecological flow, environmental water allocation (or requirement), or minimum flow. The e-flow concept has evolved over time, being its meaning shifted from the traditional view of minimum water amounts to a more comprehensive and holistic understanding of a river system and its dynamics that considers not only the protection of natural ecosystems but also the human livelihoods and well-being that depend on these ecosystems.
An environmental flow assessment is an assessment of how much of the original flow regime of a river should continue to flow down in and onto its floodplains in order to maintain specified, valued features of the ecosystem hydrological regimes for the rivers, the environmental flow requirements, each linked to a predetermined objective in terms of the ecosystem’s future condition. Three basic techniques for assessing e-flows are widely recognized: hydrological methods, hydraulic-habitat methods and holistic methodologies.
Hydrological methods primarily use hydrological data for making e-flow recommendations for maintaining river health at a designated level. They are based on the assessment of the natural flow regime as a key variable in the structure and functioning of aquatic ecosystems. Hydrological-based methods are still the most widely used approach internationally, most probably because of local availability of streamflow time series, low cost and ease of use.
Hydraulic-habitat methods assume that biological communities have evolved to exploit the full range of river habitats. The variability of flows determines when and for how long habitats are available to various species at different locations throughout the stream network.
Hydraulic-habitat approaches are often considered more accurate than hydrological ones. However, these may require a considerable amount of fieldwork and expertise to collect the hydro-morphological and biological data for model calibration.
Holistic methods identify important flow events for all major components of river, model relationships between flow and ecological, geomorphological and social responses, and use in interdisciplinary team approach to establish recommended e-flow regime and the implications of flow scenarios.
Inland water transport is usually a competitive alternative and addition to road and rail transport, offering a sustainable and environment-friendly mode of transport in terms of energy consumption, noise and gas emissions. Of all modes of transport, inland navigation has the least effect on climate change and the lowest environmental impact, which positions it as a key element of sustainable economic development.
The waterways can provide an important means of cheap transport for bulk agricultural and other goods, although they can be affected by hydro-morphological processes and withdrawal of water, which has reduced their navigability in many regions, especially during the dry season. Ensuring sustainable navigability through river improvement and conservation efforts is important in securing not just environmental outcomes but also social and economic benefits.
Inland navigation is highly dependent on natural factors. Some factors that influence navigation remain more or less constant over long periods and can be described by well-defined parameters. Other factors, however, characterize the temporally variable navigation conditions that depend on the streamflow regime of the river, particularly on events such as floods and low-flow periods. Navigation is interrupted during low-flow periods because the reduced water levels cannot accommodate vessels and the water available is insufficient for locks to be operational. The provision of an adequate supply of water is essential to the operation of the inland waterways and the effective management of water resources is fundamental to all navigation authorities.
Without reliable knowledge on the state of the riverbed, the streamflow, the ice regime, and their expected variability over time, and the planning and operation of navigation activities would be seriously hampered. Knowledge of river levels (and derived depths and widths) and discharge is essential for managing shipping schedules and for the optimum use of lock systems. In order to provide this information, it is necessary to continuously collect data on the hydrological regime, predict expected changes and transfer regularly these data and forecasts to potential users.
In the absence of field observations, the depth profile must be estimated using the time series of river flows and hydraulic models. These enable the frequency of interrupted navigation for different vessel sizes to be estimated and proposals can be made for improved channel design, while protecting the natural ecosystem. Navigation is driven by long-term investment and the related infrastructure cannot be easily relocated, redesigned or reconstructed. It is therefore important to forecast low flows so that shipping agencies are warned of navigation restrictions and have an opportunity to provide alternative means of transport in extreme conditions.
Recreational purposes for water include using the water body for swimming, boating, canoeing, water skiing, and fishing. In addition, certain lands around public reservoirs are open for recreational uses such as hiking, hunting, snowmobiling, and snow skiing. These water bodies are protected for both people’s health and aesthetics. Poor water quality can affect recreation in and on the water, posing a health risk for water-contact. Recreation, like some other uses of a reservoir, do not require the release or abstraction of water. However, certain storage limits at specific times of the year, which may impact on the yield characteristics of the reservoir or water resource system, should be observed.
Recreational use of lakes and reservoirs in multi-purpose projects requires, during the design stage, data describing their physiographic characteristics (storage-elevation relationship, shoreline properties, wave possibilities), the climate (rainfall and storms, air temperature and wind distribution), and the water quality. Likewise, during the service life of the reservoir, frequent monitoring of some hydrometeorological variables and water quality parameters may be required, for better usability and safety considerations, either year-round or seasonally, depending on the planned use of the resource for recreational purposes.