Water on Earth circulates continuously through the water cycle, moving between oceans, atmosphere, land, rivers, lakes, soils, and groundwater. The same overall stock of water is repeatedly redistributed through processes such as evaporation, condensation, precipitation, runoff, infiltration, and transpiration. In water resources, sanitation, agriculture, and environmental management, water is sometimes described using functional categories (such as “blue water,” “green water,” “greywater,” and “blackwater”) to distinguish where water is stored, how it is used, and how it is affected by human activity. These categories are used in different technical communities and do not form a single universal standard. Background Water classifications such as “blue water,” “green water,” “greywater,” and “blackwater” are used as practical labels to describe different parts of the water cycle and different human-influenced water flows. Because these terms come from different fields (hydrology, agriculture, sanitation, and urban water management), they are not a single unified system and should be interpreted in the context of the framework that defines them. Water cycle and human use The water cycle (hydrologic cycle) describes how water is stored and moved on Earth. Human activities—including land-use change, water withdrawals, and pollution—can alter flows and water quality, even though the global water cycle continues. Classification of Water as per Usages and Storage The Water caan be classified based on its storage and usage as follows : Blue water - Blue water commonly refers to freshwater in rivers, lakes, reservoirs, and aquifers, which can be withdrawn for domestic use, irrigation, and industry. In water-accounting frameworks, “blue water” contrasts with soil moisture used by plants (“green water”). Over-abstraction of surface water and groundwater can contribute to reduced streamflow, groundwater depletion, and ecosystem impacts. More specific sources are usually needed to quantify these impacts for a particular region. [citation needed] Green water - Green water generally refers to rainwater stored in the soil and returned to the atmosphere through evapotranspiration, supporting natural vegetation and rain-fed agriculture. The “green water / blue water” distinction is widely used in water-resources planning and agricultural water studies. Rainfed agriculture is a major part of global food production. A major FAO reference notes that rainfed agriculture is practised on most cultivated land globally and supplies more than 60% of the world’s food Classification of Water in sanitation and wastewater management In sanitation and wastewater management, water is often classified by source and contamination level to guide decisions on collection, treatment, reuse, and public-health risk control. Many frameworks distinguish household wastewater streams, such as greywater (non-toilet wastewater) and blackwater (toilet wastewater containing excreta), because they typically differ in pathogen load and treatment requirements. [citation needed] Greywater and Blackwater In many sanitation and wastewater frameworks: Greywater refers to household wastewater from activities such as bathing and kitchens (often excluding toilet waste). Blackwater refers to wastewater containing excreta and related toilet waste streams. A widely cited UN-Water analytical brief defines wastewater as potentially including domestic effluent consisting of blackwater and greywater, and also notes that wastewater can include stormwater and other urban runoff. Guidance linked to the World Health Organisation also discusses the safe use of wastewater and greywater, including health-risk management. Because terminology varies by country and standard, Wikipedia articles typically attribute the definition used to a specific authority (e.g., UN-Water, WHO) rather than presenting one definition as universal. Reclaimed (recycled) water Reclaimed water (also called recycled water or water reuse) generally refers to wastewater that has been treated to a quality suitable for beneficial reuse (for example, irrigation, industrial uses, or groundwater recharge), depending on local standards and treatment levels. The U.S. EPA describes water reuse as treating and repurposing wastewater (and in some contexts stormwater) for alternative uses. Stormwater Stormwater runoff is generated when rain or snowmelt flows over land or impervious surfaces (roads, rooftops, paved areas) and does not infiltrate into the ground. Stormwater can transport pollutants such as sediment, chemicals, and trash into water bodies. Stormwater management approaches may include detention/retention, infiltration systems, and green infrastructure (e.g., bioswales and permeable surfaces). Specific claims about effectiveness generally require sources. [citation needed] Virtual water Virtual water refers to the volume of water used to produce goods and services and thus “embedded” in trade. It is often used to discuss how international trade can shift water demand between regions. A foundational reference is Hoekstra’s early work on virtual water trade and associated methodologies. Fossil water (paleowater) Fossil groundwater (also called paleowater) refers to groundwater that infiltrated long ago—often under different climatic conditions—and is stored in aquifers with little or no modern recharge on human timescales. References U.S. Geological Survey (USGS): Water cycle USGS FAQ: What is the Earth’s “water cycle”? UN-Water (PDF): Wastewater Management: A UN-Water Analytical Brief — https://www.unwater.org/sites/default/files/app/uploads/2017/05/UN-Water_Analytical_Brief_Wastewater_Management.pdf World Health Organisation: Guidelines for the safe use of wastewater, excreta and greywater WHO (IRIS) PDF: Guidelines for the safe use of wastewater, excreta and greywater (2006) U.S. EPA: Urbanisation and Stormwater Runoff EPA. “Basic Information about Water Reuse.” Hoekstra, A.Y. (2003). Virtual Water Trade (PDF). Water Footprint Network. Report (PDF) on virtual water trade quantification. FAO. “Improving irrigated production…” (section noting rainfed agriculture share). UNESCO (UNESDOC). Non-renewable groundwater resources (guidebook page). Taher et al. (2018). “Yellow water…” (definition used in EcoSan context).