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7: Water Availability and Use - Biology

7: Water Availability and Use - Biology



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Learning Outcomes

  • Understand how the water cycle operates
  • Know the causes and effects of depletion in different water reservoirs
  • Understand how we can work toward solving the water supply crisis
  • Understand the major kinds of water pollutants and how they degrade water quality
  • Understand how we can work toward solving the crisis involving water pollution

Thumbnail image - Great Lakes from Space. The Great Lakes hold 21% of the world’s surface fresh water. Lakes are an important surface water resource.


Water: Major Problems and Water Management

Water is life sustaining liquid. It is one of the most important natural resources which is essential for the existence of living organisms.

Water is the most widely distributed key resource to meet the basic needs of a growing population, social and economic ambitions, demanding agriculture, expanding urbanisation, increasing industrialisation and many other causes.

The demands for water is becoming more and more challenging day by day. Hence water in all its forms (solid, liquid and gas) should be harnessed properly.

Drinking water is one of the basic needs for all of us. Unfortunately, about 5.76 lakh villages have been facing scarcity of water. In Rajasthan, people in certain pockets have to wait for hours to collect one bucket of water from water tankers brought by the trains, trucks and tractors. In many hilly areas, the situation is the same. In many tribal and backward areas un-hygienic and unsafe water collected in ponds, tanks etc., during rainy season are the only source of drinking water.

Our country is facing frequent floods and drought often at the same time in different parts because of variable nature and uneven distribution of rains. Besides these, due to large scale deforestation and soil erosion the rivers are silted up causing floods. For sustained and increased agricultural production, industrial development and economic emancipation of the country the most vital resources are soil and water, which need to be conserved, developed and managed efficiently. In India, floods, water-logging, soil erosion, drought salty groundwater, etc. are some of the major problems of water management for agriculture and other needs.

The major problems of water management and the possible strategies of overcoming them are explained here:

Floods:

Floods refer to the inundation of large parts of land by water, which otherwise remain dry for some duration of time. Flood causes heavy loss to agriculture, livestock and property. Deforestation, overgrazing, mining, industrialisation, global warming, etc. have contributed largely in the incidence of floods.

The best solution to overcome such damage is large scale irrigation projects, which will also protect from other environmental hazards. These hazards may be in the form of increase in water logging, soil sedimentation in reservoirs, damage to forest areas, large scale growth of aquatic weed of nuisance value, displacing wildlife and degradation of valuable landscape etc.

Plantations can reduce the impact of water flow on soil erosion. For water management, Land use State Boards were set-up in 1980 in order to protect the soil and water to enhance their productivity through proper land and water use practices.

Water Logging:

A soil is said to be water logged when it is completely saturated with water, which is caused by water stagnation on flat land and low lying areas. It occurs due to excess rainfall, floods, seepage high water table, obstruction to natural drainage, over irrigation, etc.

In most of the low lying areas, wet conditions persist longer that results in delayed sowing or less crop production. Another impact is that when water dries, salts accumulate on the soil surface resulting in salinity. Since water logging is the second biggest threat to the soil, next to erosion, it is therefore, necessary to study water table fluctuations, groundwater recharge, assessment of seepage from canals, tanks, etc.

Salty Groundwater:

Most parts of the arid and semi-arid regions contain high percentage of sodium salts. Such water is dangerous for agriculture. Continuous use of such water results in the accumulation of sodium salts to produce Usar or alkali soil.

Some of the possible solutions to remove salinity are as follows:

ii. Use of molasses, ash and cane sugar extracts

iii. Cultivation of salt resistant varieties

iv. Recharging with good quality of groundwater

Drought:

Drought is a condition of abnormally dry weather within a geographic region.

Some solutions to overcome drought are as follows:

i. Development of additional surface water resources

ii. Direct pumping from streams, rivers and open water bodies

iii. Proper regulation of water use

iv. Increase utilization of ground water resources

v. Efficient distribution of canal water

vi. Irrigation according to requirement of crops

Watershed Management:

Rain is the main source of water on the earth. When the rainwater reaches the earth surface, a part of it is evaporated and a part of it is taken by plants, a part of ground water is retained, a part is infiltrated and the remaining water flows as surface runoff through drainage systems into the sea. Each of the river system has separate drainage units which depend upon the terrain, geology, climate and land use pattern.

In order to tackle the problems of water resources and various forms of degradation and to develop these resources for sustained and increased productivity in the regions it is essential to focus our attention to natural drainage units called the watershed (Fig. 15.2). Watershed is defined as ‘land area that delivers water, sediment and dissolved substances through small streams to a major stream (river)’.

The management of a single unit of land with its water drainage system is called watershed management. This technique has several components which includes soil, water and vegetation cover. The natural drainage pattern of a watershed unit, if managed properly, can bring about a year-round supply of water.

There are two main steps of its conservation:

1. Construction of many long trenches and mounds along the contours of the hill to hold rainwater and allow it to percolate into the ground. This ensures that underground reservoirs are fully recharged. This is followed by growing of grasses and shrubs which hold the soil firmly.

2. The second step is to form nala plugs in the streams, so that water is held in a stream and does not rush down the hillside.

The proper management of watershed is to maintain the water yield confined to the drainage so as to derive maximum benefits and sustained productivity by proper land-use and suitable cropping patterns. The various conservation and crop management practices should be adopted in such a way that loss of soil, as well as moisture, is minimized.

The main objectives of soil and water conservation are as follows:

i. To conserve these resources in the upper catchments by providing suitable vegetation cover to protect soil erosion and to prevent the silting of river channels and the reservoirs, which prevent the flood.

ii. In case of cultivated areas, the conservation measures are different and depend on agricultural practices, economics and other aspects.

Other conservation measures undertaken for watershed management are as follows:

Engineering practices:

These include land leveling, shaping, contour bunding, terracing, grading of land, bank protection, check-dams, etc.

These include protection by vegetation, such as grass cover, pasture development, contour farming, strip-cropping, crop rotation, etc.

Under this, plantation is done, while overgrazing and forest fires are controlled.

Integrated Watershed Management:

In recent years watershed management has received wider acceptance.

It incorporates several diversified programmes, such as:

i. Amendment of alkali and acid soils

ii. Control of shifting agriculture

iii. Creation of State Land use Survey Organisation

Indian Space Research Organisation (ISRO), National Remote Sensing Agency (NRSA), Food and Agriculture Organisation (FAO) and United Nations Development Programmes (UNDP), have taken more initiatives for soil and land use survey and for planning of soil conservation programmes. Awareness was created to develop, conserve and manage the land in the wider perspective in the various sectors. It is, therefore, considered necessary to have some high level body where all matters related to soil and water resources could be discussed under one platform.

The Ministry of Agriculture, Government of India, has set up State Land use Boards under the chairmanship of Chief Minister of States, to take up all matters about the natural resources their utilization and degradation of land. The Government of India has launched an integrated watershed management in the catchments of flood prone rivers with the collaboration of NRSA and ISRO.

Rainwater Harvesting:

Old people said ‘capture water when it rains’. It is also true in the present context of water crisis. Today, the world is facing serious water shortage. Every drop of water is valuable because ‘where there is water there is life’. There has been increased aridity in India over the last few decades. Water scarcity is likely to increase in India due to increasing population. Under extreme conditions in future, human society will be bound to use different means of adaptation due to climatic changes.

Objectives of Rainwater Harvesting:

1. To reduce loss from surface runoff

3. To meet the increasing demands of water

4. To raise the water-table by recharging groundwater

5. To reduce groundwater contamination

Need for Rainwater Harvesting:

Why does rainwater need to be harvested? It matters more today than any other time in the past.

There are following reasons:

1. About 50% fresh water goes waste due to runoff.

2. More than 1 billion people lack clean drinking water globally.

3. Population increase is much faster than the increase in the amount of available fresh water.

4. Per capita availability of fresh water will further decrease in the coming years.

5. During summer and droughts, it will supplement the domestic water requirement.

6. Climatic changes also lead to increase in precipitation, evaporation, transpiration, occurrence of storms and changes in biogeochemical processes affecting water quality.

7. It is essential to reduce groundwater pollution and improve the quality of water.

8. It is a better option for providing clean and safe water particularly for drinking and other domestic uses (Table 15.3).

Rainwater Harvesting Technology:

Rainwater harvesting is the method of storing rainwater and thereby increasing the recharge of groundwater. As India since the very beginning was primarily an agricultural country, the need to harness water was felt. This is also due to fact that rainfall in our country occurs only for two to three months therefore, water needs to be conserved for its use throughout the year. Even the ancient civilizations like Harappa, Mohenjo-Daro, etc. provide excellent examples of water harvesting through a network of tanks and reservoirs. Some of the old forts like Jaigarh Fort near Jaipur and Fatehpur Sikri near Agra, also provide good examples of water storage through rainwater harvesting.

Several techniques are in practice to recharge groundwater. One method is to manage rainwater in such a way that it is used at the source. If as much water as possible is collected and stored, it can be used after the rainy season is over. This method has been traditionally practiced in dry areas. Simple local techniques such as ponds and earthen embankments can help in the harvesting and storage of rainwater. Rural and urban water use, restoration of streams for recreation, fresh water fisheries and natural ecosystems, etc. need rainwater harvesting. Local practices for rainwater harvesting can provide sufficient amount of water.

One hectare of land in an arid region with 100 mm rainfall annually could yield one million litres of water per year through rainwater harvesting.

Deep wells may provide a source of clean water, but it is possible only in the rural areas. Traditional systems could become more efficient if scientific attempts are combined to enhance their productivity. Other methods are refilling of dug-wells, recharging of hand pumps, construction of percolation pits, trenches in the agricultural fields, bunds and check-dams etc. (Fig. 15.4). The above practices have been used since long in India. Now there are advanced techniques of water harvesting systems such as canals, tanks, embankments and wells.

In hilly areas, rainwater harvesting has been practiced in rooftops and springs with the help of bamboo pipes. In arid and semi-and regions, wells and step-wells were constructed to tap groundwater. Construction of tanks has been a very popular method in recent years to conserve rainwater.

Rainwater harvesting treatment is very important in the areas where pollution is alarming. It is now possible to use nano-filtration for the removal of hardness, natural organic materials, pesticides, bacteria, viruses, salinity, nitrates, arsenic and other pollutants. Weather and water policy should be integrated and may be streamlined to promote rainwater harvesting in the water stretched regions of the world. In the urban areas, water resources are fast depleting due to population increase and unrestricted use of water.

Modern Rainwater Harvesting:

Availability of water has become a major problem in the urban areas due to high density of population. In many cities, water is available only for an hour or so and that too in a trickle. The urban areas are mostly dependent on groundwater due to which its level is falling day by day. The problem is further compounded by the fact that most of the urban land is covered by concrete structures making hindrance to groundwater recharge.

To overcome this problem, rooftop rainwater harvesting has proved to be an effective method. In this method, the rainwater falling on rooftops, which otherwise flows down the drain, is diverted to an underground tank for future use. Rainwater can also be diverted to dug-wells or pits for recharging groundwater (Fig. 15.3). In Rajasthan, tremendous work has been done by Sri Rajendra Singh to collect rainwater by constructing check-dams. He was awarded the Magasaysay Award for his commendable work.

Top 10 Water-Rich and Water-Poor Countries:

Iceland, Surinam, Guyana, Papua New Guinea, Gabon, Solomon Islands, Canada, Norway, Panama and Brazil.

Water-Poor Countries:

Kuwait, Egypt, United Arab Emirates, Malta, Jordan, Saudi Arabia, Singapore, Moldavia, Israel and Oman.


Water Reservoirs and Water Cycle

Water is the only common substance that occurs naturally on earth in three forms: solid, liquid and gas. It is distributed in various locations, called water reservoirs. The oceans are by far the largest of the reservoirs with about 97% of all water but that water is too saline for most human uses (Figure 1). Ice caps and glaciers are the largest reservoirs of fresh water but this water is inconveniently located, mostly in Antarctica and Greenland. Shallow groundwater is the largest reservoir of usable fresh water. Although rivers and lakes are the most heavily used water resources, they represent only a tiny amount of the world’s water. If all of world’s water was shrunk to the size of 1 gallon, then the total amount of fresh water would be about 1/3 cup, and the amount of readily usable fresh water would be 2 tablespoons.

The water (or hydrologic) cycle (that was covered in Chapter 3.2) shows the movement of water through different reservoirs, which include oceans, atmosphere, glaciers, groundwater, lakes, rivers, and biosphere. Solar energy and gravity drive the motion of water in the water cycle. Simply put, the water cycle involves water moving from oceans, rivers, and lakes to the atmosphere by evaporation, forming clouds. From clouds, it falls as precipitation (rain and snow) on both water and land. The water on land can either return to the ocean by surface runoff, rivers, glaciers, and subsurface groundwater flow, or return to the atmosphere by evaporation or transpiration (loss of water by plants to the atmosphere).

An important part of the water cycle is how water varies in salinity, which is the abundance of dissolved ions in water. The saltwater in the oceans is highly saline, with about 35,000 mg of dissolved ions per liter of seawater. Evaporation (where water changes from liquid to gas at ambient temperatures) is a distillation process that produces nearly pure water with almost no dissolved ions. As water vaporizes, it leaves the dissolved ions in the original liquid phase. Eventually, condensation (where water changes from gas to liquid) forms clouds and sometimes precipitation (rain and snow). After rainwater falls onto land, it dissolves minerals in rock and soil, which increases its salinity. Most lakes, rivers, and near-surface groundwater have a relatively low salinity and are called freshwater. The next several sections discuss important parts of the water cycle relative to fresh water resources.


Results

Contrasting water‐use patterns identified in wild and cultivated lettuce

When grown under well-watered conditions, the wild (L. serriola) and cultivated (L. sativa) parents of the recombinant inbred lines (RILs) showed significant variation in their diurnal pattern of transpiration (repeated measures ANOVA F1,9 = 24.76, P < 0.001, Fig. 1). For cultivated lettuce, transpiration rose from 05:00 until 13:00 h when it declined until measurements ceased at 23:00 (Fig. 1). Transpiration continued to rise until 15:00 h for wild lettuce, which demonstrated a significantly higher transpiration rate than its cultivated relative consistently throughout the course of the day under well-watered conditions (F1,9 = 24.76, P < 0.001, Fig. 1) until 23:00 h. This effect was observed in several experiments (data not shown).

Diurnal transpiration of cultivated (L. sativa cv. Salinas) and wild (L. serriola) lettuce. Transpiration pattern (mmol m − 2 s − 1 ) (a), with example thermal images of cultivated and wild lettuce (b)

Transpiration rate was also higher in wild lettuce under drought (t10 = -2.35, p < 0.05, Fig. 2a) as was stomatal conductance (t10 = -2.90, p < 0.05, Fig. 2b). Although leaf temperature did not vary significantly between the two parents, there was a trend for lower leaf temperatures in wild lettuce when compared to the cultivated parent under drought (Fig. 2c), confirming the data from stomatal conductance. Leaf temperature was significantly higher in wild lettuce under well-watered conditions (t10 = -3.83, p < 0.01, Fig. 2c). Though differences between wild and cultivated lettuce were observed, the gas exchange response of both genotypes was negligible when the well-watered and drought 1 experiments were compared for each individual (Fig. 2a–c), however leaf temperature was significantly reduced by imposing water stress for wild lettuce (t10 = 3.73, p < 0.01, Fig. 2c). Carbon isotope discrimination (Δ 13 C) was consistently higher for wild lettuce compared to cultivated lettuce (Fig. 2d). Oxygen isotope discrimination was higher in cultivated lettuce (31.31 ± 0.75) than wild (29.08 ± 0.03), although differences were not significant.

Drought response of cultivated (L. sativa cv. Salinas) and wild (L. serriola) lettuce. Transpiration (a), stomatal conductance (b), leaf temperature (c), carbon isotope (d) and oxygen isotope discrimination (e). * indicate significant differences (see text for details)

Phenotypic variation for water‐use traits in the RIL population

Phenotypes for water-use traits segregated under well-watered, mild and moderate drought conditions within the RIL population and bidirectional transgressive segregation was evident for transpiration, stomatal conductance, leaf temperature, fresh and dry weight (Figure S1). The RILs demonstrated transgressive segregation below either parent for carbon isotope discrimination under drought, indicating this population may have an improved water-use efficiency under these conditions.

Infrared thermal measurements of leaf temperature correlated well with porometry measurements under well-watered (r 2 = 0.62, p < 0.001, Fig. 3a) and drought conditions (r 2 = 0.81, p < 0.001, Fig. 3b). Transpiration (E) was strongly positively correlated with stomatal conductance (gs) under well-watered (r 2 = 89, p < 0.001, Fig. 3a) and drought conditions (r 2 = 0.75, p < 0.001, Fig. 3b). Both E and gs were significantly negatively correlated with fresh (r 2 =-0.19 and r 2 =-0.20, respectively, P < 0.01) and dried whole plant biomass (r 2 =-0.35 and r 2 =-0.39, respectively, P < 0.001), but positively correlated with fresh:dry weight ratio (r 2 = 0.36 and r 2 = 0.40, respectively, P < 0.001) in the Dr1 trial, although significant variation was observed in the data. Application of drought led to a reduction in gs measured using a porometer (r 2 =-0.18, p < 0.01, Fig. 3b) and by thermal imagery (r 2 =-0.27, p < 0.001, Fig. 3b), though the opposite effect was seen under well-watered conditions when temperature was measured using the porometer (r 2 = 27, p < 0.01, Fig. 3a). As expected, carbon isotope discrimination was found to be significantly negatively correlated with above ground fresh weight biomass under both drought treatments (r 2 =-0.33, P < 0.05, Fig. 3b and r 2 =-0.69, P < 0.001, Fig. 3c, for Dr1 and Dr2 treatments, respectively). Carbon isotope discrimination was positively correlated with E and gs under drought stress (r 2 = 0.36 and r 2 = 0.35, respectively, P < 0.01, Fig. 3b).

Correlations between water-use traits. Observed under well-watered conditions (a) and under drought 1 (b) and 2 (c) trials. Estimated using Spearman’s correlation, with scatterplot (bottom left) and significant r 2 correlation values (top right) shown. * indicates significance at P > 0.001 (***), P < 0.01 (**) and P < 0.05 (*). Transpiration (e), stomatal conductance (gs), temperature measured by porometry (Temp), temperature measured by thermal imaging (IR), carbon isotope discrimination (Δ 13 C), whole fresh weigh (FW), dry weight (DW) and their ratio (FW:DW)

QTL for water‐use traits in lettuce

A genetic linkage map with 1,099 markers spanning a total of 1,414.7 cM across 10 linkage groups was generated using regression mapping in Joinmap. Collinearity of marker ordering was validated using the physical map. Due to a region of high segregation distortion on chromosome 3, which has been previously noted by Truco et al. [7], this linkage group did not coalesce and was split into two segments labelled as 3a and 3b, which were 62 and 33 cM in length, respectively. Maximum marker interval was 16.9 cM with an average spacing of 1.3 cM (approximately 2.2 Mb) across all LGs. Utilising this molecular marker map, 30 significant QTL were identified for nine of the ten traits investigated, with no QTL identified for whole plant dry weight. These QTL accounted for 4.8–23.6 % of the phenotypic variation (PV), with 22 small effect QTL (< 10 % PV) and eight moderate effect QTL (10–25 % PV) and spanned eight of the ten linkage groups, with no QTL identified on LG5 or LG3b (Table 1 Fig. 4).

QTL identification for water-use-associated traits in the RIL population. Bars represent each LG with position in centiMorgan on the left, LG number at the top of each bar and horizontal lines indicating marker positions. QTL are shown as filled boxes to the right of each LG representing the 1-LOD interval, with error bars showing the 2-LOD interval for QTL detected in the well-watered (blue), Dr1 (red) and Dr2 (black) trials. 13 C, Δ 13 C, FW, fresh weight FW_ Lf56, fresh weight of fifth and sixth true leaves, FW:DW, fresh:dry weight ratio gs, stomatal conductance, E, transpiration, E:DW, transpiration:(dry weight) ratio Temp, leaf temperature measured via porometry, IR, leaf temperature measured via IR thermography

Two QTL for E were identified on LGs 2 and 9, under the Dr1 and well-watered treatments and accounting for 8.4 and 7.5 % of the PV respectively, with L. sativa allele inheritance increasing the trait value. A QTL for leaf temperature measured via steady state porometry mapped to the same position as E on LG 9 accounting to 9.1 % of the PV. Four QTL for gs were identified, two under the well-watered treatment on LGs 7 and 8, cumulatively accounting for 17.2 % of the PV and two under Dr1 treatment, located 53 cM apart and accounting for 10.6 % of the PV. Three moderate and one small-effect QTL for Δ 13 C measured in the Dr2 trial were identified on LGs 6, 8 and 9, together accounting for 63.4 % of the PV. QTL for Δ 13 C co-located to those for whole plant fresh weight (FW) on LGs 6 and 8 in the Dr1 trial and a second QTL for Δ 13 C on LG 8 located to the same position as FW, gs and the ratio between E and whole plant dry weight measured in the Dr1 trial. Two QTL for leaf temperature, measured by porometry and IR thermal imaging in the Dr1 trial co-located on LG 4, accounting for 5.6 and 9.5 % of the PV. Mapping has identified interesting candidate regions for further functional investigations.

Candidate genes for WUE in lettuce

Nine locations with large-effect or multiple overlapping QTL were selected for candidate gene analyses (Table S2). The 2-LOD QTL intervals were mined for genomic features, identifying > 1,400 putative genes from 73.8 Mbp of genome sequence and 87 % of these genes retrieved a BLASTp hit against 15 plant protein databases (Table S3).

Four regions of interest were located on LG8 (Fig. 5). QTL_8–51, comprising two QTL for gs and FW:DW, and these harboured a cluster of six xyloglucan endotransglucosylase/hydrolases (XTH) which have roles in modifying the extensibility of the cell wall and have been linked to drought tolerance through influencing stomatal pore size [42]. Other candidates in this region included a subtilisin-like serine protease, which modulate cell differentiation during stomatal development [43], a glutaredoxin family protein, associated with drought stress tolerance through ROS detoxification [44] WRKY and BHLH transcription factors. Significantly enriched GO terms within QTL_8–51 included those for cell wall (GO:0005618), cellular polysaccharide metabolic process (GO:0044264) and xyloglucan:xyloglucosyl transferase activity (GO:0016762 Table S4). A subtilisin-like protease and glutaredoxin family protein were identified within QTL_8–89, a QTL for Δ 13 C accounting for 9 % of the PV. A QTL for gs accounting for 10 % of the PV on LG 8, QTL_8–65, mapped to the same position as two aquaporin-like proteins involved in water transport and an ABA-responsive element binding protein, involved in ABA-induced stomatal closure following water deficit [45]. Another aquaporin protein was identified within QTL_8-100 a hotspot on LG8 in which QTL for Δ 13 C, gs, FW and the ratio between E and DW co-located. Other notable candidates in this QTL hotspot included a subtilisin-like serine protease, a dehydration-associated protein, three BZIP and one BHLH transcription factors (Fig. 5). Significantly enriched GO terms within QTL_8-100 included defence response (GO:0006952), response to stress (GO:0006950), stimulus (GO:0050896), oxidative stress (GO:0006979) and antioxidant activity (GO:0016209 Table S4).

Candidate gene mining of LG8 QTL. Illustration of LG 8, with the QTL investigated for candidate genes highlighted and gene information provided

Under QTL_6–6, a region in which large-effect QTL for Δ 13 C and FW traits measured from the Dr2 trial co-located, a late embryogenesis abundant (LEA) protein along with several transcription factors reported to influence response to drought were identified, including three MYB-like domain containing proteins, a NAC, APETALA2 (AP2)-like ethylene-responsive factor and WRKY transcription factor (Hadiarto & Tran, [46] Nuruzzaman et al., [47] Table S3). The same region contained ten glutathione S-transferases and a glutathione peroxidase, involved in reactive oxygen species (ROS) detoxification in response to drought [48]. A region in which a QTL for E and leaf temperature co-localised, designated QTL_9–27, contained a LEA protein, an aquaporin-like protein, a glutathione S-transferase and several transcription factors (AP2, MYB, NAC, WRKYand BZIP Table S3).


Freshwater Availability

Water is the fundamental ingredient for life on Earth. You can find it in the atmosphere above us, in the ocean, rivers and lakes around us, and in the rocks below us. Of all of the water on Earth, 97% is saltwater, leaving a mere 3% as freshwater, approximately 1% of which is readily available for our use. The world’s population is becoming more and more reliant on this precious resource for power, irrigation, industrial practices, and daily consumption.

Discover Data

Global Precipitation Measurement (GPM) mission data from Earthdata Search. Earthdata Search is a data discovery and data access application that enables access to the NASA Earth Observing System Data and Applications System (EOSDIS) Earth science data across the Distributed Active Archive Centers (DAACs).

Discover Freshwater Data

Groundwater

Groundwater storage around the world is on the decline, largely as a response to human consumption, primarily from irrigation. Understanding our impacts on this resource provides for better management and sustainable use.

Data Tutorials/Recipes

Rainfall

Measuring rainfall helps advance our understanding of Earth's water cycle, improving forecasts of extreme events such as flooding, landslides, and drought. Knowing when, where, and how much rain will fall improves crop forecasts and can benefit agriculture.

Data Tutorials/Recipes

Rivers and Lakes

The amount of water in our rivers and lakes is important in assessing water availability and in preparing for possible water-related events, such as floods and drought.

Data Tutorials/Recipes

Sensors on a suite of NASA satellites observe and measure freshwater resources, including rainfall, snow and ice, rivers and lakes, groundwater, soil moisture, and water quality. These measurements are important to understanding the availability and distribution of Earth's water, which is both vital to life and vulnerable to the impacts of climate change and a growing world population.

Visualize Data

Visualization of the Advanced Microwave Scanning Radiometer 2 (AMSR2) surface rain rate data in Worldview. The EOSDIS Worldview mapping application provides the capability to interactively browse global, full-resolution satellite imagery layers.

Soil Moisture

Understanding soil moisture aids in improving weather forecasts, monitoring drought, predicting floods, and assessing agricultural needs.

Data Tutorials/Recipes

Snow and Ice

Most freshwater is frozen in glaciers, snow, ice caps, and ice sheets. Approximately one-sixth of Earth’s population is dependent upon freshwater output from this seasonal snowpack and glacial ice melt for daily use. Both seasonal and long-term changes to snow cover and ice can impact the amount of freshwater that is available.

Snow and Snow Cover

Snow Water Equivalent (SWE)

Water Quality

Healthy water is essential to support and sustain life. Water quality can be monitored in a variety of ways, but through satellite remote sensing, the primary way is through ocean color, which is impacted by chlorophyll content, sediment, and dissolved organic matter.

Data Tutorials/Recipes

Getting Started

About the Data

NASA provides data from a variety of sources including satellites, airborne campaigns, field campaigns, in situ instruments and model outputs. The Earth Observing System Data and Information System (EOSDIS) offers a wide variety of freely and openly available data that can be used to evaluate freshwater availability, hydrology, and the movement of Earth’s water between the atmosphere, the ocean, and land.

Dr. Faisal Hossain studies ways to improve water management and accelerate economic development in Asia and Southeast Asia.
Dr. Eric Sproles studies water’s eco-social effects.
Ben Holt studies polar sea ice, coastal oceanography, and marine pollution
Bridget Seegers develops new ways to study water quality and track harmful algal blooms
David Mocko uses land-surface models to study drought
Dr. Anne Nolin studies mountain ecosystems
Dr. Christian Kummerow studies the water budget
Dr. Gina Henderson studies the connections between tropical and Arctic weather and climate
Dr. Joan Ramage studies glaciers and snowmelt
Dr. Mark Anderson studies the way frozen surfaces react to changing atmospheric conditions
John Lehrter studies the water quality of estuarine and coastal environments
Dr. Pierre Kirstetter improves our understanding of precipitation and flooding

Read about how researchers are studying wetlands, drought, precipitation, and more, using NASA satellite data.


Water Availability

Water is constantly moving on the Earth between the atmosphere, ocean, rivers and streams, snowpacks and ice sheets, and underground. Water availability, both as surface water and groundwater, is essential for agriculture, human consumption, industry, and energy generation.

Basics

Fresh water is available as surface water (such as lakes, rivers, reservoirs) and groundwater (found underground in rock or soil layers, and accessed through wells or natural springs). Water is constantly moving on the Earth between the atmosphere, ocean, and different fresh water bodies. Climate, land use, local geology, and water quality all affect the availability of fresh water resources in addition to the direct demands people place on them. Read more


5. How can the growing demand for water be met?


Demand for water is increasing

Meeting a continuous and ever increasing demand for water requires efforts to compensate for natural variability, and to improve the quality and quantity available. More.

5.1 Rainwater has been collected for thousands of years in many parts of the world. Today, this technique is used in Asia to replenish underground supplies. It is relatively inexpensive and has the advantage of allowing local communities to develop and maintain the required structures themselves.

Diverting surface water into the ground can help reduce losses from evaporation, compensate for variations in flow, and improve quality. Middle East and Mediterranean regions are applying this strategy.

Dams and reservoirs have been built to store water for irrigation and drinking. Moreover dams can provide power and help control floods, but they can also bring about undesirable social and environmental impacts.

Transferring water between river basins can also help alleviate shortages. China, for instance, already has major interbasin links, and is planning more. The impact of these projects on people and the environment must be monitored closely. More.

5.2 Wastewater is now reused for different purposes in many countries, especially in the Middle East, and this practice is expected to grow. Worldwide, non-potable water is used for irrigation and industrial cooling. Cities are also turning to water re-use to supplement drinking water supplies, taking advantage of progress in water treatment. More.

5.3 Desalinated water – seawater and other salty water that has been turned into freshwater – is used by cities and by industries, especially in the Middle East. The cost of this technique has dropped sharply, but it relies heavily on energy from fossil fuels and hence raises waste management and climate change issues. More.


Chapter 16 - Water Availability and Potato Crop Performance

Agricultural research can help mankind in several ways to cope judiciously with global problems of producing sufficient food from limiting supplies of water. This chapter: (i) summarizes basic concepts on how the system works, (ii) addresses methods of monitoring the water status, (iii) presents the crop responses to drought, and (iv) addresses the environmental and genetic differences in water-use efficiency and product quality. The importance of ensuring sufficient availability of water is illustrated comparing potential and water-limited potato yields in Africa. Ignoring the very small proportion of water taken up by the plant that is stored, the general equation describing this relation can be repeated to describe the fluxes in the several parts of the SPAC. Several lines of evidence support the idea that abscisic acid (ABA) plays a central role in stress signaling, inducing stomatal reactions. There are two general designs —equilibrium and dynamic—that differ in their control philosophy. This technique is now reaching beyond development and is approaching the stage where it can be used in commercial applications. The relative rate of cell expansion, R, has been described as a linear function of turgor pressure above a threshold value pressure, below which there is no cell growth. Water use, leaf dynamics and potato productivity are also elaborated. Varieties may vary in their tolerance of drought, expressed as the relative yield of crops subjected to drought compared with crops optimally supplied with water.


Discussion

These solid-state NMR data and MD simulations show that the water dynamics, orientation, and hydrogen-bonding in the BM2 channel differs significantly between low- and high-pH states. Low-pH channel activation gives rise to (1) a larger pore size with two-fold more water, (2) faster water reorientation and chemical exchange on the ns–μs timescales, and (3) higher water orientational order and more directional hydrogen bonds. Using 13 C detection of water 1 H polarization, we selectively detected the small population of water inside the BM2 channel. The fact that the low-pH channel contains more water is not surprising, since the four-helix bundle is more loosely packed at low pH due to electrostatic repulsion between the four positively charged H19 residues 27,28 . The excellent agreement between the measured and simulated number ratio of water molecules in the closed and open channels validates both the NMR and simulation methods. Further, the larger amount of water is not an artefact of the low temperature (277 K) at which the experiments were conducted, as shown by control simulations at 297 K (Supplementary Fig. 10).

Interestingly, the number of water molecules in AM2 channels determined by X-ray crystallography was found to be sensitive to the experimental conditions. Initial cryogenic and room-temperature synchrotron crystal structures of AM2 found comparable or more water in the high-pH channel than in the low-pH channel 11 . The number of waters was much higher in the cryogenic structures than the room-temperature structures. These results were subsequently attributed to water ordering at low temperature and radiation damage at room temperature. When crystal structures were obtained at room temperature using an X-ray free-electron laser (XFEL), more water was found in the low-pH AM2 channel than the high-pH channel 10 . Solid-state NMR measurements at moderate temperatures represent a non-perturbing approach for quantifying the amount of water in membrane-bound channels. Interestingly, the XFEL data of AM2 also indicate a doubling of water amount at low pH compared high pH, in good agreement with the current BM2 results. This similarity of acid-activated water increase of AM2 and BM2 channels implies that the conserved histidine dictates the water amount in the pore (vide infra).

At high pH, simulated water densities indicate low water occupancy at L8 and H19 (Fig. 2d) and frequent bottlenecks in the water hydrogen-bonding network (Fig. 7c). This hydrogen-bond disruption is not observed in AM2, which contains unbroken chains of water–water hydrogen bonds from the N-terminus to the histidine tetrad at both high and low pH 11 . We attribute this difference to the different oligomeric structures of AM2 and BM2: AM2 adopts a more open N-terminal vestibule at high pH that is closed after acid activation 24,57 . The interconversion between the two conformations underlies the functional asymmetry of the AM2 transporter, where protons are exclusively conducted inward. In comparison, the BM2 tetramer structure undergoes a symmetric scissor-like motion with respect to the center of the TM domain 27 . This conformational symmetry has been proposed to explain the ability of BM2 to conduct weak outward proton current in addition to strong inward current. The lower water amount at L8 in the closed 0/+1 BM2 channel is thus correlated with a more compact N-terminal pore. The minimum diagonal heavy-atom distance at L8 is 11 Å for the open +4/+4 channel and 5.5 Å for the closed 0/+1 channel (Supplementary Fig. 1b). BM2’s H19 pKa’s have been measured and found to be significantly lower than the H37 pKa of AM2 29 : the average pKa of H19 is 5.1, while the average pKa of H37 in AM2 is 5.9. Although the peripheral histidine, H27, speeds up proton release from H19, even an H27A mutant of BM2 still manifests a low average H19 pKa of 5.6 28 . The current finding of a hydrogen-bond bottleneck at L8 provides a second mechanism of this depressed H19 pKa’s, as the interruption of the water wire at L8 will slowdown proton relay from the N-terminus to H19.

Both NMR relaxation and MD simulations indicate that water in the low-pH channel reorients more rapidly than water in the high-pH channel (Fig. 4, Supplementary Tables 1 and 2, and Supplementary Movie 1). Despite this faster dynamics, water in the low-pH channel has higher anisotropy than water in the high-pH channel (Fig. 5), as shown by larger motionally averaged 1 H CSA. Order parameters computed from simulations are in very good agreeement with the experimental data (Fig. 5c). The unexpected finding of faster water dynamics and higher water anisotropy can be rationalized by the fact that water molecules that reorient quickly break and form hydrogen bonds with adjacent water molecules to permit proton exchange and thus Grotthuss hopping 5 . The higher water anisotropy at low pH is also fully consistent with the increased number and directionality of water–water hydrogen bonds at low pH (Fig. 7). This hydrogen bonding extends in opposite directions on either side of H19, with the C-terminal water favoring the inward hydrogen bonds and N-terminal water favoring outward hydrogen bonds. We attribute this hydrogen-bond polarity switch to the concentrated positive charge on the H19 tetrad.

The pH-dependent dynamics and orientations of BM2 channel water are surprisingly similar to the water properties in AM2 channels, despite the low sequence homology and the distinct oligomeric structures of the two proteins. AM2 also contains more dynamic water at low pH than at high pH, as manifested by picosecond time-dependent lineshapes in 2D IR spectra 58 , longer water T2 ′ relaxation times 25 , and a larger number of half-occupancy water molecules in crystal structures 10 . MD simulations of AM2 also indicated that the water hydrogen-bond direction switches above and below H37 11,13 . The fact that both channels exhibit faster water dynamics and a switch in hydrogen-bond direction at the proton-selective histidine at low pH suggests that the proton-selective histidine, without assistance from other pore-lining residues, controls the water amount, water anisotropy, and water wire directionality in both AM2 and BM2 channels. The ps–ns water reorientation and the μs histidine ring reorientation serve to complete the hydrogen-bonded chain to enable proton transfer.

The fact that the tetrameric conformation of M2 plays a lesser role in channel-water properties than the selectivity-filter residue is also supported by MD simulations that used a coiled-coil structure of BM2 solved in detergent micelles 59,60 . Despite the significant structural differences from the bilayer-bound structure 27 , the simulations reached the same conclusion about increased pore size and pore hydration at low pH. The charge-flipped MD simulations shown here lend further support to the essential role of the H19 charge state for regulating the water-wire properties. When the H19 tetrad is neutral, even if the four-helix bundle is loosened, the number of water molecules and the water orientational order are low. Conversely, when the H19 tetrad is highly charged, even if the four-helix bundle is tightened, the water amount and orientational order increase significantly (Fig. 5f and Supplementary Fig. 9). Therefore, the polarity of the hydrogen-bond network is electrostatically controlled by the charge state of the histidine. For AM2, the protein conformation and dynamics play the roles of limiting the net proton flux and dictating the directionality of the proton flux 24,25 . Whether proton flux in BM2 is rate-limited by protein conformational dynamics has not been shown, but the similar rates of proton conduction and conserved HxxxW motif suggest that this may also be the case for BM2. The millisecond motion of the four-helix bundle accounts for the

1000 s −1 proton flux of the M2 channels, while the symmetry of the helical motion affects the extent of the reverse proton current.

The combined measurements of water amount by polarization transfer NMR, water motional rates by relaxation NMR, and water orientation by recoupling NMR, represent a powerful approach for elucidating water properties in membrane proteins. MD simulations complement these NMR experimental results by providing site-specific information about water dynamics and hydrogen bonding. Our results show that proton conduction in the open state of BM2 is correlated with fast water rotational and translational diffusion, which optimizes the orientation of a hydrogen-bonded water wire 61,62 . The selectivity-filter histidine electrostatically controls water orientation and dynamics, and switches the water hydrogen-bond directions above and below itself. The coexistence of dynamic disorder and orientational order of water may be a general property of channels and pumps that contain more than a single file of water molecules.


Local water availability is permanently reduced after planting forests

A shallow river bed in Buderim Forest Park, Queensland, Australia. Credit: Laura Bentley

River flow is reduced in areas where forests have been planted and does not recover over time, a new study has shown. Rivers in some regions can completely disappear within a decade. This highlights the need to consider the impact on regional water availability, as well as the wider climate benefit, of tree-planting plans.

"Reforestation is an important part of tackling climate change, but we need to carefully consider the best places for it. In some places, changes to water availability will completely change the local cost-benefits of tree-planting programmes," said Laura Bentley, a plant scientist in the University of Cambridge Conservation Research Institute, and first author of the report.

Planting large areas of trees has been suggested as one of the best ways of reducing atmospheric carbon dioxide levels, since trees absorb and store this greenhouse gas as they grow. While it has long been known that planting trees reduces the amount of water flowing into nearby rivers, there has previously been no understanding of how this effect changes as forests age.

The study looked at 43 sites across the world where forests have been established, and used river flow as a measure of water availability in the region. It found that within five years of planting trees, river flow had reduced by an average of 25%. By 25 years, rivers had gone down by an average of 40% and in a few cases had dried up entirely. The biggest percentage reductions in water availability were in regions in Australia and South Africa.

"River flow does not recover after planting trees, even after many years, once disturbances in the catchment and the effects of climate are accounted for," said Professor David Coomes, Director of the University of Cambridge Conservation Research Institute, who led the study.

Published in the journal Global Change Biology, the research showed that the type of land where trees are planted determines the degree of impact they have on local water availability. Trees planted on natural grassland where the soil is healthy decrease river flow significantly. On land previously degraded by agriculture, establishing forest helps to repair the soil so it can hold more water and decreases nearby river flow by a lesser amount.

Counterintuitively, the effect of trees on river flow is smaller in drier years than wetter ones. When trees are drought-stressed they close the pores on their leaves to conserve water, and as a result draw up less water from the soil. In wet weather the trees use more water from the soil, and also catch the rainwater in their leaves.

"Climate change will affect water availability around the world," said Bentley. "By studying how forestation affects water availability, we can work to minimise any local consequences for people and the environment."