A watershed is any surface area, where sources of water, including rainfall and snow melt, collect and drain to a larger water body. Drop by drop, water is channeled into soils, groundwater, creeks, and streams, making its way to larger rivers and eventually the sea. Everyone in the world lives in a watershed, with boundaries defined only by elevation. The watershed at any point is comprised of the upstream area that drains into that point.
Watersheds can be aggregated at different scales – from a small mountainous micro-watershed (such as in Nepal’s Kosi basin), to a large trans-boundary watershed such as the Ganges-Brahmaputra-Meghna or the Indus. The terms watershed, basin, and catchment are often used interchangeably although in some countries, catchment refers to upstream hilly areas where much of the precipitation in a watershed may occur.
Watersheds form the basic building blocks for land and water planning.
Every location on land is part of some watershed. Explore the watersheds around the world by clicking on the map below to outline the upstream land area that drains into that point clicked.
Watersheds are comprised of multiple biophysical and socio-economic elements. Biophysical elements are influenced by climate (rainfall, altitude, solar radiation, winds), drainage, soil, vegetation, topography (slopes, erosion features such as rill, gullies, and landslides) and land use patterns (homesteads, cultivated land, grazing land, forests, degraded areas). Socio-economic elements of a watershed involve population and their economic activities (agriculture, livestock, industry, tourism). Watershed management is multi-sectoral in nature due to the multitude of human activities that coexist within them.
All elements and activities in a watershed are interrelated. Any activity that affects water quality, quantity, or flow rate in one part of the watershed may affect locations downstream. For example, deforestation in upstream reaches could increase soil erosion that could impact the storage of any water storage (e.g. dams) or reduce stream channel capacity through sedimentation that could reduce the availability of water in some cases and increase the hazard of riverine floods in other cases. Understanding this multi-sectoral spatial connectivity within a watershed is essential when planning or managing activities for the future.
For more information on more integrated landscape approaches with examples drawn from around the world, please check out these free resources.
Watersheds around the world have witnessed several challenges in recent decades. Over the last few decades, watershed degradation has become one of the major problems with negative environmental and socioeconomic consequences, particularly in the developing countries. For example, extensive areas of Pakistan in the Thal and Thar have witnessed overgrazing and are now gravely threatened by desertification. Even 1980s-90s studies in Morocco showed that the storage capacity of 34 large reservoirs in the country was threatened by excessive soil erosion and sedimentation. According to an estimation, around 50 million cubic meters of reservoir capacity were being lost each year, 0.5 percent of the total. World Bank 2008
Integrated watershed management is critical to addressing multiple evolving water-related challenges in the South Asia Region. The multiple and increasing uses of land, water, and other bio-physical resources have created opportunities for conflict across internal and international borders that need to be explored along with opportunities for collaboration across the borders in a more integrated manner.
Water scarcity, climate change impacts, erosion, sedimentation, and transboundary water disputes are all landscape-scale challenges effected by linked processes in watersheds that will require integrated, multi-sectoral planning and project implementation across sectors in a spatial context. Coordinated and integrated planning will be required both within watersheds and across them. The COVID-19 pandemic has created additional challenges in this regard and stimulus opportunities for this and other longer-term green investments would also benefit from an exploration of immediate to long-term needs and opportunities in a watershed context.
These challenges can be addressed using evolving opportunities , including leveraging emerging technologies, regional collaboration, and knowledge and lessons learned from regional and global examples. Solutions can be found through information & analytics; institutions & policy; and investment preparation & operations. Together, they can be grouped into integrated approaches to watershed management to solve multiple development challenges and improve social, economic, and environmental development outcomes.
The watershed approach is a coordination framework for environmental management. Following the approach of operating and coordinating programs on a watershed basis makes good sense for environmental, financial, social, and administrative reasons. Besides driving results towards environmental benefits, the watershed approach can result in cost savings by leveraging and building upon the financial resources and people’s willingness to act. It can reduce costly duplication of efforts and conflicting actions through improved communication and coordination. It can also help enhance local and regional economic viability in ways that are environmentally sound and consistent with watershed objectives. Finally, the watershed approach strengthens teamwork between the public and private sectors at the federal, state, tribal and local levels to build a sense of community, increase commitment to the actions necessary to meet societal goals and, ultimately, achieve the greatest environmental improvements with a long-term sustainable impact.
There are several benefits associated with well-managed watershed.
Soil protection and regulation of hydrological flows: Forests provide many important services in watersheds such as the regulation of the hydrological flows. Forests act as runoff buffers, reducing erosion processes, facilitating infiltration of water, groundwater recharge and maintaining base flows in streams and rivers. Additionally, they play an important role in climate change mitigation as carbon sequesters, since it is considered that forests sequester most carbon of any terrestrial ecosystem.
Water filtering and storage: Groundwater aquifers are huge natural stores of freshwater. These deposits stock water that has been naturally filtered when moving from the soil surface to the ground soil. In this process, large particles such as silt, are removed because they can't fit through the small pore spaces. Smaller particles such as clay and microorganisms become adsorbed onto soil particles and some chemicals such as nitrates and pesticides are cleaned up by bacteria. Water can also be further regulated by artificial storage (e.g., ponds, reservoirs, managed aquifer recharge) in addition to the natural storage.
Freshwater supply: One of the main services well-managed watershed can provide is freshwater supply for domestic, municipal, commercial, and industrial uses, as well as to agriculture that accounts for the most water withdrawn in most watersheds around the world. Freshwater in quality and quantity is essential for humankind and highly dependent on the health of the watershed ecosystems. For example, well-managed forested watersheds have been proven to maintain the highest water quality and conserve water supplies through increased groundwater recharge. This is especially important in the context of a progressive augment of human population and an increased variability of the climate.
Streamside (Riparian) forests provide many key services such as erosion control, flood prevention, water quality improvement and biodiversity conservation: The roots of plants stabilize stream banks and prevent erosion of river channels. Soil in these areas slow down the runoff and store water, reducing flooding risks and later releasing water to aquifers and streams. The roots and stems of riparian vegetation are filters that absorb and trap nutrients, diseases and pollutants, improving water quality. Riparian vegetation also provides important habitats for aquatic insects, fish and wildlife.
Food supply: Well-managed watersheds provide appropriated environments such as grasslands, streams and wetlands for food production activities such as livestock, fisheries, crop and fruit production, etc.
Wildlife habitat and biodiversity conservation: Well-managed watershed ecosystems provide a variety of wildlife habitats. Riparian vegetation, rivers and streams offer shelter and feeding opportunities for a wide range of plants and aquatic insects, fish and wildlife animals.
Hydroelectric power: Hydropower is a clean, renewable and reliable energy source which converts kinetic energy from falling water into electricity, without consuming more water than is produced by nature.
Eco-tourism/cultural heritage: Well-managed watersheds ecosystems, such as rivers and lakes, provide many aesthetic and recreational services to population, such as fishing, swimming, boating, hunting, and picnicking or nature appreciation in general. Rivers have a central place in the practices and beliefs of many religions and is part of the cultural heritage and identity of many cultures.
On the contrary, the implications of poorly managed watersheds are as following.
Depletion of water resources: Water is considered a renewable resource, but available freshwater resources are very limited. Water resources can be depleted due to the following reasons. Declining ground water resources, as a result of overgrazing and deforestation, reduce the infiltration of water into the soil. Urbanization replaces natural land cover with impervious surfaces, preventing the natural recharge of ground water levels.
Siltation and pollution of the available water resources decrease the usability of available water sources. Other factors responsible for depletion of water resources include growth in demand, reduction in natural recharge, over exploitation, bad management practices.
Soil erosion and land degradation: Soil erosion is a frequent problem that has both on-site and off-site effects. Onsite effects include- loss of inherent soil resources, collapse of soil structure, a decline in organic matter and nutrients in the soil, and a reduction in available soil moisture. Off-site effects are related to increases in sediment loading and the loss of nutrients adsorbed to the soil particles in the sediment. Subsequent increases in sediment in the streamflow often reduces the capacity of rivers to deliver high-quality water to downstream users, increases the risk of flooding in river basins, reduces or blocks the flow of water through irrigation systems, and shortens the expected operational life of downstream reservoirs.
Impoverishment of the vegetative cover: Impoverishment of the vegetative cover is a reduction of the vegetative cover and biomass caused by climatic factors, over utilization of vegetation, erosion and reduced soil fertility. The linkages are obvious- land degradation is mostly responsible for reduction of the vegetative cover and ultimately depletion of the water resources, which in turn makes the soil, water and vegetation more vulnerable to further aggravation and degradation of the watershed.
Biodiversity loss: Biodiversity is the term given to the variety of life on Earth, within and between all species of plants, animals and micro-organisms and the ecosystems within which they live and interact. Biodiversity loss can occur in several ways- habitat degradation and loss, pollution, introduction of invasive species, and unsustainable use of resources. In particular, land use changes that destroy natural habitats pose the most significant threats to native biodiversity.
Water Pollution: Pollution is the introduction of contaminants into the natural environment that cause adverse change on it. Human pollution is one of the biggest threats to watersheds health. There are two types of pollution- point and non-point pollution. The first comes from a specific source such as a factory, industrial plant or some other facility. The second, non-point pollution is the most serious threat to the watershed water quality and consist of pollutants that are carried off the land by storm water into rivers, lakes, streams or the ocean. The water picks up pollutants left by human actions such as- excess fertilizers, herbicides, and insecticides from agricultural lands and residential areas; oil, grease, and toxic chemicals from urban runoff and energy production; sediment from improperly managed construction sites, crop and forest lands; salt from irrigation practices and acid drainage from abandoned mines; bacteria and nutrients from livestock, pet wastes, and faulty septic systems; and atmospheric deposition.
Climate Change: A warming and changing climate is likely to increase existing watershed stresses and create new ones. The changes on the watershed climatological conditions can have important implications on the landscape, the hydrology and geomorphology, the biotic conditions and the chemical and physical parameters of water flows. They can entail a vast array of impacts, such as- Shifts in streamflow and deficit supplies during peak seasons of water demand; shifts in vegetation conditions and changes in the biomass, production, and composition of terrestrial communities; loss of cold water habitat for aquatic species; loss of organic matter that supplies nutrients; higher concentrations of pollutants; increased algal blooms and associated water quality effects; greater frequency and intensity of flood and fire events; and an increase in landslides during extreme events.