Current water availability

Lake in the desert


Strategies to deal with water scarcity must be based on a thorough understanding of the water balance, including water supply and demand and monitoring of water resources in all its dimensions. In such a context, water accounting and auditing is a vital component in the water planning and allocation process for good water resources management. 

Water accounting is defined as the systematic acquisition, analysis and communication of information relating to stocks, flows, and fluxes of water (from source to sinks) in natural, disturbed or heavily engineered environments (FAO, 2016). Its concept is based on the argument that knowledge of the current state of water resources and trends in demand and use is a prerequisite for sustainable water management. 

The objective of water accounting and auditing is to make better use of water-related information in the implementation of adaptation strategies to different biophysical and societal contexts. 

Water auditing goes beyond water accounting by placing trends in water supply, demand, accessibility and use in the broader context of governance, institutions, public and private expenditure, legislation and broader water saving policies in specific sectors. Water accounting and auditing can also be used to set the extraction volumes allowed and managed by codes or water rights.


Water accounting and auditing - A sourcebook (FAO Water reports N°43, 2016)

Water Footprint and Corporate Water Accounting for Resource Efficiency (UNEP, 2011)


River basin inflows and outflows are flows of water into and out of an area having a common outlet for its surface runoff (WMO/UNESCO, Glossary of Hydrology). They are one of the most important sources of information, as it is an essential link of water circulation and represents the foundation of water resources assessment.

Even though the flow fluctuation of a river basin depends on various factors, climate change and anthropogenic activities are widely expected to alter streamflow and potentially disrupt water systems. Increased variability of rainfall, evaporation and groundwater recharge, increased temperature and earlier snowmelt runoff intensify river basin water flow variability in many places around the world. Human activities (reservoirs, deforestation, agriculture, etc.) also have a direct or indirect influence by causing changes in the basin water storage.


UNESCO (1998). Guidelines for conducting water resources assessment. Chapter 5: Evaluating the surface water resources management balance


Snow and ice are important phases of the hydrological cycle over a significant part of each year in mountainous and polar areas, and also in mid-latitude countries. Snow that accumulates in a drainage basin is a natural storage reservoir from which a major part of some basin’s water supply is derived. If harnessed it can be used for water supply, agriculture, industry and energy production. Consequently, the time lag for river flow forecasts and water management derived from snowfall is much greater than from rainfall.

Observations of ice storage on rivers, lakes and reservoirs are of great interest in regions where ice formation affects navigation, results in damage to structures or obstructs streamflow causing serious local flooding. Long-term data on ice conditions in rivers are extremely valuable in designing various structures, in studying processes of ice formation and dissipation, and in developing methods of ice forecasting.


WMO (2020). Guide to Hydrological Practices, Volume I: Hydrology – From Measurement to Hydrological Information – Chapters 3.5 and 5.2. WMO-No. 168. 

WMO (1992). Snow cover measurements and areal assessment of precipitation and soil moisture. WMO-No. 749, OHR 35 

WMO (1994). Applications of remote sensing by satellite, radar and other methods to hydrology: Chap.6 Snow and Chap.7.1 Remote sensing of glaciers. WMO-No. 804, OHR 39. 


Surface water storage is water stored in natural or built systems such as rivers, lakes, glaciers, wetlands, soils, canals, dams and storage reservoirs.
Natural surface water storages are important stores of water that are widely utilised for domestic water supply and agriculture around the world. Natural seasonal and interannual variability results in significant changes in these stores. In addition, human interventions can change their availability. These natural features, in some circumstances, can also play an important role in flood protection systems. Soils also play an important role in the rainfall–runoff response of a catchment: water can be stored in soils in the landscape, even though globally its total volumes are small compared with other natural terrestrial stores.

As human needs have extended beyond that which nature has been able to provide, humans have constructed a broad variety of water retention structures at a variety of scales. Storage reservoirs and dams and have been constructed to increase the reliability of bulk water supply to urban areas, irrigation schemes, and industry, to generate hydropower, manage floodwaters, and to enable navigation of rivers – all often in combination. They have also sometimes provided recreational facilities and opportunities to support fisheries.


WMO (2020). Guide to Hydrological Practices, Volume I, Chapter 4: Evaporation, Evapotranspiration and Soil Moisture; Chapter 5: Surface water quantity and sediment measurement. WMO-No. 168. 

 WMO (2018). Guide to Instruments and Methods of Observation – Vol. I: Measurement of meteorological variables – Chapter 11 - Measurement of soil moisture. WMO-No. 8.

WMO (2009). Guide to Hydrological Practices, Volume II, Chap. 4.2: Estimating reservoir capacity and yield. WMO-No. 168.

WMO (2012). Technical Material for Water Resources Assessment. Chapter 7: Surface water. WMO-No. 1095, Technical Report Series- No. 02. 

GWP (2021). Storing water: A new integrated approach for resilient development. Perspectives Paper

UNESCO (1982). Methods of hydrological computations for water projects

WMO (1992). Snow cover measurements and areal assessment of precipitation and soil moisture. WMO-No. 749, OHR 35

WMO (2010). Manual on Stream Gauging, Vol. I: Fieldwork. WMO-No. 1044.  

WMO (2010). Manual on Stream Gauging, Vol. II: Computation of discharge. WMO-No. 1044. 

WMO (1986). Level and discharge measurements under difficult conditions. WMO-No. 806, Operational hydrology report (OHR)-No. 24.  

WMO (1986). Methods of measurement and estimation of discharges at hydraulic structures. WMO-No. 658, Operational hydrology report (OHR)-No. 26. 

WMO (1994). An overview of selected techniques for analysing surface-water data networks. WMO-No. 1029, Operational hydrology report (OHR)-No. 41.  

WMO (2019). Guidance on environmental flows - Integrating E-flow science with fluvial geomorphology to maintain ecosystem services. WMO-No.1235

WMO (2008). Manual on Low-flow Estimation and Prediction. WMO-No. 1029

WMO (1994). Applications of remote sensing by satellite, radar and other methods to hydrology: Chapter 4: Soil moisture and groundwater; Chapter 8: Surface water. WMO-No. 804, OHR 39. 


Water not only covers three quarters of the Earth’s surface: it is also present almost Everywhere below ground surface, down to considerable depths and in continuous motion. Groundwater – as we can refer to the vast majority of this subsurface water is an invisible component of the hydrosphere, representing a hidden part of the water cycle. Groundwater is widely used in many countries. It is often the primary source of drinking water (supplying half of the world’s population) and contributes significantly to irrigation, hence to food security in arid and semiarid regions. It therefore represents an important component of the water economy. Unlike most mineral resources, groundwater is in most cases a renewable resource, which allows groundwater to be developed sustainably, making use of the dynamics of groundwater in the present-day water cycle. Only a minor part of the enormous groundwater volumes or ‘reserves’ is dynamic, which means: more or less regularly replenished by recharge and sufficiently mobile to play the role of a natural regulator or buffer.

For the environment groundwater plays a very important role in keeping the water level and flow into rivers, lakes and wetlands. Specially during the drier months when there is little direct recharge from rainfall, it provides the environment with groundwater flow through the bottom of these water bodies and becomes essential for the wild life and plants living in these environment. Groundwater also plays a very relevant role in sustain navigation through inland waters in the drier seasons. By discharging groundwater into the rivers it helps keeping the water levels higher.


WMO (2020). Guide to Hydrological Practices, Volume I, Chapter 6: Groundwater. WMO-No. 168. 

WMO (2012). Technical Material for Water Resources Assessment. Chapter 8: Groundwater. WMO-No. 1095, Technical Report Series- No. 02. 

WMO (1994). Applications of remote sensing by satellite, radar and other methods to hydrology: Chapter 4: Soil moisture and groundwater. WMO-No. 804, OHR 39. 

UNESCO (1998). Guidelines for conducting water resources assessment

UNESCO (2012). Groundwater and global change: trends, opportunities and challenges  

Jousma, G., Roelofsen, F.J., IGRAC (2004). World-wide inventory on groundwater monitoring. Report nr. GP 2004-1 

Jousma, G., Roelofsen, F.J., IGRAC (2003). Inventory of existing guidelines and protocols for groundwater assessment and monitoring. Report nr. GP 2003-1 


Imported water means water transported into a watershed from a different watershed via pipeline or aqueduct. Imported water can be used to replenish aquifers and protect them from seawater intrusion, to supplement local supplies to meet increasing demands or during drought season. The quantity of water imported and available for import is affected by drought, legal and several other factors, including regulatory constraints and costs. This practice does affect the natural water flow patterns in both exporting and importing watershed.



Water reuse (also commonly known as water recycling or water reclamation) reclaims water from a variety of sources then treats and reuses it for beneficial purposes such as agriculture and irrigation, potable water supplies, groundwater replenishment, industrial processes, and environmental restoration. Water reuse can provide alternatives to existing water supplies and be used to enhance water security, sustainability, and resilience.


UNESCO (2020). Water reuse within a circular economy context 

Davis, R. and Hirji, R. (Eds.) (2003). Water Resources and Environment Technical Note F.3 Wastewater Reuse. The World Bank.

UNEP, GEC (n.d.). Water and wastewater reuse: an environmentally sound approach for sustainable urban water management


The scarcity of freshwater resources and the need for additional water supplies is already critical in many arid regions of the world and will be increasingly important in the future. Many arid areas simply do not have freshwater resources in the form of surface water such as rivers and lakes. They may have only limited underground water resources, some that are becoming more brackish as extraction of water from the aquifers continues. Therefore, alternate sources of water supply need to be explored, such as desalination which is used widely around the world and involves taking the salt out of water to make it drinkable. Many countries use this technology as a way of creating a more reliable water supply that is not dependent on rain. Some other alternatives include condensate water and stormwater harvesting.


UNESCO (2020). The United Nations world water development report 2020: water and climate change. Chapter 3.2.3 Unconventional water resources 

UNEP-DHI Partnership, UNEP-DTU, CTCN (2017). Climate change adaptation technologies for water: a practitioner’s guide to adaptation technologies for increased water sector resilience

International Water Association (2015). Alternative Water Resources: A Review of Concepts, Solutions and Experiences.

ESCWA (2001). The role of desalinated water in augmentation of the water supply in selected ESCWA member countries 

UNEP (1983). Rain and stormwater harvesting in rural areas : a report 

UNEP (2001). Rainwater Harvesting And Utilisation - An Environmentally Sound Approach for Sustainable Urban Water Management: An Introductory Guide for Decision-Makers 

Top page