4.2 Water supply problems and solutions

As we saw in the previous section, water withdrawals have increased over time. Water is a renewable resource, and the amount of water on the planet is essentially constant. However, we have seen that most of the water on the planet is not freshwater. In addition, many freshwater resources have been polluted to the extent that they cannot safely be used for human purposes without treatment.

In addition, water, even uncontaminated freshwater, can become less available, depending on where it occurs and where it is needed. Confined aquifers can be emptied and do not recharge in a useful timeframe. Much of the water used for irrigation does not return to surface water or groundwater but evaporates; it remains in the hydrologic cycle, but is lost from the water source from which it was withdrawn.

Fresh groundwater and surface freshwater both exist as patchy resources. In addition to being used for direct human purposes, surface water is also the environment for freshwater ecosystems. Soil moisture, the water in soil above the water table is the water source for rainfed agriculture and for naturally occurring vegetation. Atmospheric moisture – humidity – is also important for agriculture and vegetation in general. This section primarily examines issues of water supply for direct human uses. Ecosystem issues are addressed in a later chapter.

Unsustainable withdrawals – the global picture

So far, we have studied the availability of water around the world, and the levels of withdrawal. How do those quantities compare? We know that withdrawals from confined aquifers must be unsustainable, because these do not recharge on a useful timescale. How fast are we drawing down our unconfined groundwater resources? Are withdrawals from surface waters within the limits of these renewable water bodies?

Blue-water scarcity is a measure of water deficit developed by Mekonnen and Hoekstra.^[1] Blue water is a term that combines surface water and groundwater, ignoring precipitation (green water, in this jargon). Blue-water scarcity is calculated as unreturned blue-water withdrawal divided by the net additions to blue-water availability (area runoff input minus upstream withdrawals minus downstream river outflows). A blue-water scarcity value of 1.0 indicates that withdrawals just balance inputs. Lower values (shown in green in Figure 1) indicate that blue-water withdrawals are less than additions. Blue-water scarcity values >1 indicate shortfalls. Not surprisingly, Figure 1 looks very much like a map of aridity – less sustainable water use tends to occur in drier areas.

The groundwater resource is spatially patchier than surface water, because aquifers are limited in their extent, whereas precipitation falls everywhere. A 2015 [click here to read the] study of the 37 largest aquifers on Earth used remote sensing to detect water use and modeling to estimate recharge and discharge of rechargeable aquifers (Fig 2). The aquifers with greatest depletion for the 2003-2013 study period were the Ganges-Brahmaputra system that drains the Himalayas, the North Caucasus (mountains between the Black and Caspian Seas), the Canning Basin in Australia; the Arabian aquifer system, and the aquifer system under the Central Valley of California. Of these, only the Arabian aquifer system has negative recharge due to evaporative losses that exceed precipitation; the other 4 are declining rapidly despite some amount of recharge. Stressed aquifers may occur in arid areas, near high population centers, in areas with high irrigated agriculture, high livestock use, high industrial use, or a combination of these stresses.

Depletion of rechargeable groundwater lowers water tables and reduces spring flow to rivers. Lowered water tables can dewater wetlands and make arable land less suitable for rainfed farming. If irrigation is introduced to allow continued farming, it will further deplete surface and groundwater resources. Reduced spring flow to rivers reduces flow of the rivers themselves and can cause a perennially flowing river to become a seasonal or ephemeral stream. The Phoenix-Tucson area of the southwestern US is notable for large, dry river beds where the Santa Cruz, Gila, Salt, and Agua Fria rivers, among others, once flowed. The rivers were eliminated as surface features by overpumping of groundwater and diversion of upstream flows.

Unsustainable withdrawals – the local picture

As groundwater is pumped from water wells, there usually is a localized drop in the water table around the well called a cone of depression (Fig 3). When a large number of wells have been pumping water for a long time, the regional water table can drop significantly. This is called

groundwater mining, which can force the drilling of deeper, more expensive wells. In addition, on coastlines, the reduced outward pressure of the local water table can allow salt water to intrude into the surface aquifer, leading to salination which may render the water undrinkable and unsuitable for agriculture or industry.

Water in aquifers supports the land above the aquifer. As pumping withdraws that water, the land itself may settle, in localized sinkholes. If larger areas of an aquifer are substantially drained, subsidence of the land surface over a larger area may occur, for example when a sand-and-gravel aquifer compacts as the water is withdrawn. Figure 4 shows the results of years of groundwater pumping of sand and silt aquifers in the San Joaquin Valley of California. Over the period of 1925-1977, the land surface subsided by more than 33 ft (10 m).

Sources of water loss

As we saw earlier, water pollution can render freshwater sources unusable or can increase the cost of making it useable. In addition, water may be lost between the point of extraction from the source and the point of use. Around the world, water is moved in open canals from its source to distant location, even locations in other countries. Canals and raised flumes constructed of metal or wood require maintenance that is not always forthcoming. In Central Asia, much of the water infrastructure was constructed when those nations were part of the Soviet Union. Even in well-maintained systems, leakage occurs, and illegal “poaching” of water from canals also occurs. Remote sensing can often detect leakage; careful monitoring of flow volumes can detect unsanctioned withdrawals but is expensive. Leakage is not only a problem in delivery infrastructure, but also occurs in storage containers, and even to groundwater, from natural streams and from reservoirs.

A Food and Agriculture Organization manual estimates that earthen canals in sand lose 20-40% of their water, depending on length; canals in clay lose 10-20%. Lined canals are estimated to retain 95% of their water, but poor maintenance can reduce all these figures by as much as 50%! ^[2]

In addition to leakage, water delivery systems that are open to the atmosphere are subject to losses due to evaporation. Canals through arid landscapes are particularly at risk for such loss. Researchers estimate that covering California’s almost 4000-mi (6350-km) irrigation canal system could save 63 billion gallons of water annually (>238 million cubic meters).^[3] They suggest covering the canals with solar panels.

Flood irrigation systems, which water entire fields or furrows throughout fields (for example, for rice paddies, but also alfalfa, tomatoes, and other crops) and sprinkler irrigation systems are another significant source of water loss. Flood irrigation loses water to evaporation, run-off, and percolation too deep for crops to reach. Sprinklers can lose water in all these ways, plus drift of water spray off the intended plants. One set of estimates for well-maintained flood and spray irrigation systems estimated that the proportion of water reaching the appropriate root zone varied between 40% and 90%, depending on the method. Drip irrigation methods put >90% of water into the intended root zone.^[4]

Climate change and water availability 

The Intergovernmental Panel on Climate Change (IPCC)

The IPCC is the primary international science body responsible for reporting on climate-change issues. Three working groups underpin the reporting efforts. Working Group I focuses on bringing together the physical science related to climate change, studying aspects of climate, atmosphere, hydrology, oceanography, etc. Working Group II studies impacts of, vulnerability to, and adaptation to climate change, for humans and for the natural world. All aspects of ecology, human health, social sciences, and economics are addressed here, to understand the best changes to minimize harm to humans and the natural world and to increase adaptation to climate-change impacts. Working Group III studies ways to reduce levels of greenhouse gases (GHG) in order to slow and eventually reduce climate change – referred to as mitigating climate change. In the specialized focus of climate change, the term “mitigation” refers only to reductions in greenhouse gases. Any other kinds of mitigation, such as mitigation of harms from various aspects of climate change, require a longer phrase, for clarity – “mitigation of heat impacts,” for example. Because GHGs are produced and absorbed naturally and are also produced by a wide range of human undertakings, WGIII, like WGII, comprises both natural and social scientists. However, the WGIII focus is much tighter, on reducing production and concentration of GHGs.

Every 5-7 years, the working groups produce an assessment report (AR) summarizing new science since the last report. AR6 was produced during 2021-2023. Each working group produces a report, and after those reports are available, a synthesis report is produced that integrates the findings of the three working group reports as well as any special reports that have been produced since the last assessment report. IPCC reports are used throughout the world to understand how climate change will affect the world and what opportunities are available to adapt to it and to reduce it.

Climate-change impacts on water availability

Under climate change, although precipitation will increase as temperatures warm, due to increased evaporation from the oceans and other bodies of water, less precipitation will fall as snow, and evaporation from the land will increase, leading to a loss of groundwater, soil moisture, and streamflow, and decreased water levels in lakes and reservoirs. Carbon dioxide will permit some increase in plant growth and water efficiency, but reductions in soil moisture are expected to offset these potential gains in many places.

Precipitation forecasts are much less certain than temperature forecasts due to the greater complexity of the hydrological cycle – precipitation, evaporation, evapotranspiration, oceans, clouds, rain-vs-snow issues, etc. The IPCC long-term predictions (for the end of the 21st century) suggest reductions in precipitation in winter (December, January, February – DJF – in the Northern Hemisphere; June, July, August – JJA – in the Southern Hemisphere) will be strongest in the American Southwest, around the Mediterranean and in northern Africa, in parts of the Amazon Basin, in southern Africa, and in parts of Oceania (Fig 5). Reductions in summer precipitation will hit hardest in western Europe and the Mediterranean, the Caribbean, along the southeast coasts of Africa and South America. The Mediterranean thus has reductions in both seasons.

The IPCC figure caption is rather technical, but provides numerical information about the level of uncertainty of the results. Here is the full caption from the WGI report of AR 6, 2021.

Figure 4.24 | Long-term change of seasonal mean precipitation. Displayed are projected spatial patterns of multi-model mean change (%) in (top) December –January–February (DJF) and (bottom) June–July–August (JJA) mean precipitation in 2081–2100 relative to 1995–2014, for (left) SSP1‑2.6 and (right) SSP3‑7.0. The number of models used is indicated in the top right of the maps. No map overlay indicates regions where the change is robust and likely emerges from internal variability, that is, where at least 66% of the models show a change greater than the internal-variability threshold (Section 4.2.6) and at least 80% of the models agree on the sign of change. Diagonal lines indicate regions with no change or no robust significant change, where fewer than 66% of the models show change greater than the internal-variability threshold. Crossed lines indicate areas of conflicting signals where at least 66% of the models show change greater than the internal-variability threshold but fewer than 80% of all models agree on the sign of change. Further details on data sources and processing are available in the chapter data table (Table 4.SM.1).

To see the impacts of changing hydrologic regimes on agriculture, we can look at the level of water in the atmosphere and in the soil – the places that control how much dryness plant stems and leaves experience and how much water is available to roots. Water-pressure deficit is a variation of the relative humidity measurements seen in local weather forecasts and is the preferred measure of atmospheric wetness and dryness. Atmospheric wetness affects how much water plants lose through evapotranspiration (think of what your nose feels like when the humidity is low) and how much water is lost from the soil surface due to evaporation.

Soil moisture is more straightforward, but can be measured in the upper levels of the soil, where most evaporation occurs, and where most agricultural crops have their roots, or throughout the entire water column. On average across a variety of temperate-zone crops, 50% of crop roots are in the first 15 cm of soil.^[5] In Figure 6, below, the measure of water in the top 10 cm (4 in) of soil best reflects vulnerability to evaporation of recent precipitation due to dryness, whereas total-column moisture reflects moisture over a longer term, including from previous seasons; it includes the full range of depths from which plant roots might draw moisture but, in deeper soils, also includes depths that most plant roots don’t reach.

Under more severe climate change estimates (SSP5-8.5, the lower line of graphics), all of earth’s land surfaces are projected to experience increasing vapor-pressure deficits – more atmospheric drying (Figure 6). Upper-soil moisture losses will be significant in many places, with significant increases in some presently dry portions of Africa and parts of central China. Soil moisture throughout the soil column shows similar patterns to upper soil moisture, but with reduced strength.

The full IPCC caption for this figure follows.

Figure 8.19 | Projected long-term relative changes in annual mean soil moisture and vapour pressure deficit. Global maps of projected relative changes (%) in annual mean vapor pressure deficit (left), surface soil moisture (top 10cm, middle) and total column soil moisture (right) from available CMIP6 models (number provided at the top right of each panel) for the SSP1.2-6 (a, b, c), SSP2-4.5 (d, e, f) and SSP5-8.5 (g, h, i) scenarios respectively. All changes are estimated for 2081–2100 relative to a 1995–2014 base period. Uncertainty is represented using the simple approach. No overlay indicates regions with high model agreement (‘Robust change’), where ≥80% of models agree on sign of change, diagonal lines indicate regions with low model agreement, where <80% of models agree on sign of change. For more information on the simple approach, please refer to the Cross-Chapter Box Atlas.1. Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Climatologists measure drought of three kinds: meteorological, hydrological and agricultural/ecological. Meteorological drought is measured against regional expectations for precipitation at daily, weekly, monthly, or annual levels. Hydrological drought is measured by the condition of primary water sources – surface and ground water. Because primary water sources get their water in part from runoff that may come from snowmelt or percolation through the soil, hydrological drought may lag behind meteorological drought. Agricultural and ecological drought is measured through conditions for plants, especially soil moisture and evapotranspiration (water loss through plant respiration). Environmental scientists also define environmental drought, which combines not only the water situation, but also the ripple effects from declining water availability – tree mortality, fire, habitat loss, erosion, water quality, etc. In a similar fashion, social scientists look at the socioeconomic impacts of drought including results of crop failure, livestock mortality, and reduced water supply to human undertakings including hydropower.   

The IPCC AR6 projections show at least medium confidence in an increase of agricultural/ecological drought at or above 2°C of warming in the US West and Midwest, across most of Mexico and Central America, through most of northern South America, Chile, and southern Argentina, through Europe, the Mediterranean, and southern Africa, and in southern and southeastern Australia. At that level of warming, drought intensity is predicted to increase by more than 50% (with 90% of estimates ranging from approximately 20-230%) and drought frequency is predicted to increase between 2-fold and 3-fold (with 90% of estimates ranging from approximately 1.5-6-fold) (From Figure 11.18 of the WGI1 AR6 report). The IPCC scenario that best matches the current trend in GHG – SSP5-8.5 – predicts warming of 2°C by around 2040, so these increases in drought may not be far in the future.

Approaches to improving sustainability in water availability

The current and future water shortages described above require multiple approaches to extending the fresh water supply to improve sustainability. A variety of solutions are in use.

Reservoirs that form behind dams in rivers can collect water during wet times and store it for use during dry spells. They also can be used for urban water supplies and to support irrigation. Other benefits of dams and reservoirs are hydroelectricity, flood control, and recreation. However, reservoirs permanently flood land, displacing the original inhabitants, whether these are humans and related agriculture and industry or natural systems. Reservoirs lose water to evaporation, particularly in arid climates; downstream river channels experience increased erosion, and the original river ecosystems are transformed into lake habitats, while the dams interfere with migration and spawning of fish. Fisheries impacts from dams reduce food availability and can also significant economic losses. Cambodia’s Tonle Sap fishery has been sharply reduced by dams on the Mekong by China and downstream countries. Salmon fisheries in the US and Canada have been strongly reduced by dam construction, impacting both commercial and indigenous fisheries.

Dams on rivers that cross international boundaries can create conflict among nations, as upstream nations with dams control the amount and timing of water releases to downstream nations. For example, in 2019, operation of 11 dams on the Mekong within Chinese borders caused the river to run dry for downstream nations during a severe drought.

Aqueducts can move water from where it is plentiful to where it is needed. Like dams, aqueducts can be controversial and politically difficult, especially if the water transfer distances are large. One drawback is that water diversion can cause drought in the area from where the water is drawn. For example, Owens Lake and Mono Lake in central California began disappearing after their river flow was diverted to the Los Angeles aqueduct. Owens Lake remains almost completely dry, but Mono Lake has recovered more significantly due to legal intervention. Water diversion can also move contaminants and invasive species to new areas. Evaporation during diversion in open canals, which occurs in many parts of the world, increases salinity and other contamination of diverted water.

Desalination, which involves removing salts from seawater or saline groundwater, can create additional fresh water. It involves considerable energy and moderate to high expense. Solar desalination has been demonstrated to be feasible, but is used for less than 1% of desalination. Desalination is most common in the Middle East, where aridity, oil, and salt water all occur in the same region. Desalination also creates concentrated salt water as a waste product that is potentially hazardous to local water resources.

Rainwater harvesting involves catching and storing rainwater for reuse before it reaches the ground. It is mostly used for home water use. In the US, rainwater harvesting is illegal in some states, particularly drier western states, because it diminishes the water that reaches streams, and legal water rights are tied to surface water. Fog harvesting is used in a few parts of the world in which fog is somewhat predictable, but like rainwater harvesting, it does not scale up to levels suitable for entire cities or larger regions.

Conservation encompasses using less water and using it more efficiently. Drip irrigation is one of the most widely practiced forms of water conservation. As we saw in water use statistics, use of water by industry and energy production has become more conservative over time, reducing overall water use. Xeriscaping – the use of dry-adapted plants in arid areas – can significantly reduce water used in landscaping. In the home, high-efficiency appliances and an informed approach to water use can also reduce water use significantly.

The majority of the world’s water use is for agriculture, and precision irrigation, drip irrigation, and similar methods are available to reduce water use, but are not widely adopted. Variability in precipitation due to climate change may lead to more irrigation due to potential failure of rainfed agriculture. Increasing irrigation then further stresses area water resources. Such an increase in irrigation is occurring in corn-and-soybean-growing regions of the US.

In developing countries, water infrastructure is often old and inefficient. Much of the water infrastructure of the arid Central Asian nations still dates to Soviet times. Conservation efforts cannot proceed under such conditions.

Given the loss of water from aquifers and the growing uncertainty of water levels in reservoirs and streams, most experts agree that fully sustainable water use will require recycling, or reclaiming, water.

Visit this site from the Colorado School of Mines to understand the process of recycling municipal water. The mobile system shown here is best for emergencies. Permanent versions of this recycling process are in place in a growing number of cities, worldwide, including cities in the US, Namibia, Australia, India, Spain, and Singapore.

Municipal water reclamation is sometimes referred to as toilet-to-tap water use, which does not reduce the “ick” factor but does at least provide transparency. Some industrial water users already recycle at least a part of their water in order to reduce costs and uncertainty.

Not all reused water needs to be treated to the level of drinking water standards. Water professionals distinguish two levels of recyclable water: black water – water from toilets or septic systems, and gray water – domestic water from other sources such as showers, laundry, or sinks. If the water is to be returned to municipal use, it must be cleaned to drinking water standards. But under carefully controlled conditions, gray water can be used for irrigation, watering gardens, parks, and golf courses, or even for “rewatering” short stretches of streams that have been dewatered by overpumping of groundwater and/or diversion of upstream water. In Arizona, over 20 miles (32 km) of the Santa Cruz river, dry since 1940, now flow, through use of treated gray water.

Reclaimed water can also be used for artificial recharge of aquifers, allowing water managers to use the depleted aquifer as a convenient storage area – evaporation is essentially eliminated. Both treated wastewater and transported water (e.g., from an aqueduct) can be used. However, an aquifer is not an inert container. Aquifers have their own chemical properties, depending on the materials (rock, sediment, residual water) in the aquifer. In addition, water-delivery materials have their own chemical properties, and some still include lead pipes or lead-soldered pipes. Interactions between water chemistry, aquifer properties, and water infrastructure can result in changes in water quality and so much be carefully considered and tested. A change in the river used to supply water to Flint, Michigan in the US mobilized lead from the water infrastructure into the drinking water system, raising blood-lead levels in the city’s children, and creating a major scandal.

Progress towards sustainability in water availability

The issue of water availability appears among the UN Sustainable Development Goals as Goal 6: Clean Water and Sanitation. In their description of progress on the goal, the UN reports these numbers:

From 2015 to 2024, the population using safely managed drinking water, safely managed sanitation and basic hygiene services increased from 68 to 74 per cent, from 48 to 58 per cent and from 66 to 80 per cent, respectively. However, in 2024, 2.1 billion people were without safely managed drinking water, 3.4 billion without safely managed sanitation and 1.7 billion without basic hygiene services. In schools around the world in 2023, 447 million children lacked a basic drinking water service, 427 million lacked a basic sanitation service, and 646 million lacked a basic hygiene service.

Estimates based on data for 129 countries covering 89 per cent of the world’s population suggest that the proportion of domestic wastewater that is safely treated was 56 per cent in 2022 (no change since 2020).

From 2015 to 2022, global water use efficiency improved from $17.5/m3 to$21.5/m3, a 23 per cent increase. However, 57 per cent of countries still face challenges, with low efficiency of below $20/m3. Globally, water stress showed little change from 2015 to 2022. Water stress varies significantly across regions, with Northern Africa and Western Asia as well as Southern and Central Asia facing extreme scarcity.^[6]

We have many tools to use to improve sustainability of water availability, but the capacity to employ those tools, both in terms of funding and in terms of knowledge, is not sufficient to achieve sustainability. Progress is being made, but not at the pace anticipated when the Sustainable Development Goals were drafted.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.