Isotopic methods are turning out to be the strategic tools to improving agricultural water management around the world. Isotope hydrology helps scientists and governments get a handle on just how much water is in a particular location, where it’s coming from and where it goes, what it picks up along the way and how it changes from liquid to gas, pristine to polluted. Using its experience in nuclear technology, the International Atomic Energy Agency (IAEA) has been involved in this type of research for more than 40 years. The Agency helps scores of Member States better understand their water resources.
Isotope hydrology is a field of hydrology that uses isotopic dating to estimate the age and origins of water and of movement within the hydrologic cycle. The techniques are used for water-use policy, mapping aquifers, conserving water supplies, and controlling pollution. It replaces or supplements past methods of measuring rain, river levels and other bodies of water over many decades. 
It is no secret that competition among different sectors for scarce water resources and ever increasing public concern on water quality for human, animal, industrial consumption, and recreational activities have encouraged to focus more attention on water management in agriculture. As water resources shrink and competition from other sectors grows, agriculture faces a dual challenge:
- To produce more food with less water; and
- To prevent the deterioration of water quality through contamination with soil runoff, nutrients and agrochemicals.
According to the IAEA Bulletin which was published in September 2011, the world faces acute water shortages. The current African drought is just the latest tragic example. One billion people have no access to adequate drinking water. Five million— mainly children — die each year due to water–borne diseases. Those numbers are expected to rise. For over half a century, the IAEA has been doing everything it can to help, deploying its unique expertise in using nuclear techniques to understand and manage water. In more than 90 countries, the IAEA experts work with national counterparts to find, manage and conserve freshwater supplies and protect the oceans.
Only 2.5 percent of the earth’s water is fresh, not salty. Less than 1 percent of that tiny fraction is available for people to use. The rest is frozen in ice caps and glaciers, or occurs as soil and atmospheric moisture. Almost all of that precious resource, the earth’s accessible fresh water, is located underground – water that is hidden in the earth’s crust and is often hard to access. This vital resource is poorly understood and poorly managed.
It’s 70 to 80 percent of the available freshwater resource in many parts of the world that is consumed for agriculture which makes agriculture the principal user of the water at a global level. The water used for current agricultural practices to grow crops is derived from rain-fed soil moisture, with non-irrigated agriculture accounting for some 60 percent of production in developing countries. Although irrigation provides only 10 percent of agricultural water use and covers just around 20 percent of the cropland, it can vastly increase crop yields, improve food security and contribute 40 percent of total food production since the productivity of irrigated land is three times higher than that of rain-fed land. The Food and Agriculture Organization (FAO) predicts a net expansion of irrigated land of some 45 million hectares in 93 developing countries (for a total of 242 million hectares in 2030) and project that agricultural water withdrawals will increase by approximately 14 percent during 2000-2030 to meet food demand.
Since the 1960s the global nutrition has considerably improved, providing more food per capita at progressively lower prices. This performance was possible through high-yielding seeds, irrigation and plant nutrition. As population keeps increasing more food and livestock feed need to be produced in the future and more water applied to this purpose. Irrigated agriculture will have to claim large quantities of water to produce the food required to feed the world. The main source of food for the population of the world is agriculture: This term also includes livestock husbandry, managed fisheries and forestry.
For vegetative growth and development plants require water in adequate quantity and at the right time. Crops have very specific water requirements, and these vary depending on local climate conditions. The production of meat requires between six and twenty times more water than for cereals. The tables presented under Figure 02, give an overview of the water consumption in food and agriculture, specifying values for the water equivalent of a selection of food products:
Specific values for the water equivalent of a selection of food products are given in the table on the left. The table on the right shows the amount of water needed necessary for a few products per unit of consumption.
Improving water management in agriculture requires an improvement in soil moisture conservation measures and a reduction in wastage of irrigation water. Reduction in water wastage also brings about additional benefits in terms of reducing losses of applied nutrients, water erosion and pollution of surface and ground water. An accurate measurement of soil moisture content and water removal by soil evaporation and plant transpiration processes is therefore essential to establish the optimal soil water balance for crop sowing, fertilizer application and irrigation scheduling under different irrigation technologies, climatic conditions and farm management systems that aim to minimize soil evaporation and increase water accessibility for plant roots. Before the role of soil moisture neutron probe and stable isotopic techniques in contributing such information will be discussed, here is a brief description of these techniques in the context of Water Use Efficiency (WUE).
Nuclear and isotopic techniques can play an important role in improving WUE in agriculture. WUE is a broad concept that can be defined in many ways. For farmers and land managers, WUE is the yield of harvested crop product achieved from the water available to the crop through rainfall, irrigation and the contribution of soil water storage. Here is how isotopic techniques can help by:
- Improving water management through accurate soil moisture monitoring for optimum irrigation scheduling to minimize water losses;
- Optimizing crop water productivity with more crops per amount of water inputs from rainfall or irrigation; and
- Assisting in the selection and evaluation of crop cultivars with tolerance to drought and higher crop water productivity.
Nuclear and isotopic techniques have been shown to be invaluable tools for improving WUE around the world. Here is a brief description according to a document published by IAEA:
1. APPLICATIONS OF STABLE ISOTOPES IN WATER MANAGEMENT:
Significant advances have been made in the development and application of isotopic techniques in water management in agriculture. 
he measurement of natural variations in the abundance of stable isotopes of oxygen, hydrogen, carbon and nitrogen in soil, water and plant components can help to identify the sources of water and nutrients used by plants and to quantify water and nutrient fluxes through and beyond the plant rooting zone as influenced by different irrigation and land management practices. These developments have been possible due to the increased sensitivity of continuous flow isotope-ratio mass spectrometers for analyzing the isotopic composition in soil-plant-water components.
For example, hydrogen and oxygen, as constituents of water can exist as light and heavy isotopes. These isotopes can be used to identify water losses through evaporation from soil surface since the light isotopes (hydrogen-1 and oxygen-16) evaporate more readily than the heavy isotopes (hydrogen-2 {2H} and oxygen-18 {18O}). The natural isotopic ratios of hydrogen (2H/1H) and oxygen (18O/16O), which are often expressed as delta units (G2H and G18O) in soil water, water vapour within a plant canopy and plant leaves can provide estimates of soil evaporation and plant transpiration. Such information will enable irrigation and land management practices to be developed to minimize soil evaporation (the non-productive loss of water) and channel this water for crop production. Some examples of recent applications of stable isotopes in estimating soil evaporation, plant transpiration and sources of water used by plants are reported below:
- Changes in the isotopic composition of hydrogen (G2H) over a ten-day sampling period following surface irrigation of olive trees in Morocco indicated that soil evaporation as a proportion of total water removal (evapotranspiration) from both soil and crops ranged from 0 percent prior to irrigation to 31 percent after surface irrigation. These results highlight that soil evaporation under the environmental condition studied was substantial, indicating that any management factors that minimize soil evaporation such as drip irrigation can be expected to significantly improve WUE;
- Partition of transpiration from over story trees from that of understory grasses in the south west of the USA during the post-rainy period by measuring natural variations of both G2H and G18O indicated that the total water removal (evapotranspiration) from the ecosystem was 3.5 mm/day of which 70 percent was from tree transpiration, 15 percent from the transpiration of grass layer, and 15 percent from soil evaporation. The study highlights that over story trees can minimize soil evaporation, thus improve the overall WUE of the studied ecosystem; and
- Agency sponsored research in a CRP on “Use of Nuclear Techniques for Developing Integrated Nutrient and Water Management Practices for Agroforestry Systems” have demonstrated that natural variation in the abundance of 2H and 18O in the soil, plant and water can be used to quantify the contribution of hydraulically lifted water from the subsoil (4-5 m) by deep rooting trees growing in association with grasses or crops in the dry savannah regions of Africa (e.g., Burkina Faso and Niger). Hydraulic lift is a process of water movement from subsoil to the topsoil through plant roots. This process could be one of the features contributing to the success of tree-crop association growing in parklands in the dry savannahs of the West African Sahel.
The currently developed FAO crop water productivity model AquaCrop which aims to predict yield response to water for most major field and vegetable crops under a range of irrigation and land management practices requires a range of data on transpiration and evaporation. These two components of evapotranspiration can be separated by using stable isotopes as outlined above. The planned activities of the Agency in this area will provide valuable information to FAO’s Aquacrop model, which will be a useful management tool to manage water in both rain-fed and irrigated farming systems around the world.
2. THE USE OF CARBON STABLE ISOTOPES TO SELECT AND EVALUATE DROUGHT-TOLERANT PLANT SPECIES:
Carbon, the major building block of carbohydrate and proteins in plant tissues contains both light and heavy carbon stable isotopes (12C and 13C). The measurement of natural variations in the abundance of 13C and 12C in plant materials is increasingly being used to select and evaluate plant cultivars that can withstand drought. This technique obviates the need for measurements of the water budgets of a large number of plants during a large scale screening for WUE characteristics. 
Under drought, less carbon (in the form of carbon dioxide), particularly 13C from the atmosphere is taken up by plants for growth because of plant stress, thus creating a major variation in the natural isotopic ratios of 13C and 12C in plant materials. A plant cultivar, which is resistant to water scarcity should display less depletion in 13C compared with a susceptible cultivar. Such discrimination against 13C (i.e., difference between 13C and 12C, expressed as delta G13C) in plant tissues (leaves and grains) has been successfully used in the selection of drought-resistant barley, wheat, rice and peanut. Scientists have shown that G13C in plant leaves and grain is negatively related to WUE. Besides acting as a surrogate for WUE, carbon isotope discrimination (often abbreviated as CID) measured in different plant parts at harvest can be used as an historical account on how water availability varied during the cropping season.
The Agency through a CRP “Nutrient and water management practices for increasing crop production in rain-fed arid/semi-arid areas” has shown that the CID technique was successfully used as a diagnostic tool for predicting WUE and wheat grain yield. A current CRP on “Selection for greater agronomic water-use efficiency in wheat and rice using carbon isotope discrimination” also established that CID can be used as a selection criterion for wheat yield under a wide range of environmental conditions, in particular under post-flowering stress that represents the most common drought situation. The breeding lines will be further used to develop WUE crop cultivars matching specific environments prevailing in the participating countries.
IAEA projects apply nuclear technology to evaluate these risks and find ways to make better use of water and soil resources. Many countries have benefited from this programme, including Qatar, Chile, Kenya, Turkey, Vietnam and Bangladesh. Here are some highlights from an article written by Sasha Henriques which was included in the IAEA Bulletin for September 2011:
a) Qatar:
Qatar is one of the 10 most water-scarce countries in the world and all its arable land is irrigated with groundwater. But, more than half of the water being used doesn’t reach the crops, evaporating from the soil to the atmosphere. As more groundwater was used for irrigation, and levels fell, seawater and saline water from deeper aquifers intruded on the supply of fresh groundwater.
Isotopic techniques were used to determine the most efficient way to use saline groundwater and treated sewage water through drip irrigation.
Drip irrigation reduced the amount of water needed by up to 30 percent, compared to sprinkler irrigation. There are now plans to use 100 million m3 of saline groundwater with 60 million m3 of treated sewage water annually, which will effectively increase agricultural acreage eleven fold.
b) Chile:
Nearly 60 percent of arable land in Chile is affected by erosion, and in Central Chile, a shortage of flat land has increasingly compelled wine growers to plant vineyards on hillsides which eventually pollutes water downstream.
Three consecutive IAEA technical cooperation projects were undertaken in Chile to investigate this problem. A fallout radionuclide was used to determine the extent of soil erosion and resulting water pollution. The research showed that current vineyard management practices are untenable.
So there are now plans to investigate the use of permanent ground cover between vines to effectively minimize soil erosion and water runoff on slopes and hence improve downstream water quality. Emilio Sanchez from the La Roblería vineyard in Apalta says, “The vineyard associations have been open to embrace nuclear research techniques as it has been a win-win relationship for the farmers of the region.”
c) Kenya:
In Kenya, agriculture is the second largest contributor to gross domestic product, with 70 percent of the population working in the sector. Yet the majority of farmland is arid or semi-arid, with low and erratic rainfall. And food production is low with frequent crop failures. The IAEA worked with the local scientists to develop small-scale, low-cost drip irrigation technologies for poor farmers.
These technologies, perfected by the Kenya Agricultural Research Institute (KARI), are currently being transferred to smallholder farmers for use with high-value crops like cucumber, tomato, kale and lettuce. An example is a project that provides Maasai farmers at Namanga on the Tanzanian border hands-on training in drip irrigation techniques. KARI now provides technical expertise and know-how on agricultural water management to 23 African countries.
d) Turkey:
Turkey is the world’s 5th largest potato exporter. The main challenge farmers encounter is to use water and fertilizers more efficiently by applying irrigation water mixed with fertilizers, also known as fustigation, to the right place, at the right time and in the appropriate amount. Using drip irrigation significantly reduced the amount of water and fertilizers that were needed. This is having a major impact on Turkey’s potato production, yielding substantial savings for farmers.
e) Vietnam:
Vietnam has also been affected by erosion, loss of soil nutrients, as well as water and fertilizer use efficiency problems. They sought the IAEA’s help. Compound-specific stable isotope techniques were used to identify areas of land degradation. The findings of the project have been used to raise farmers’ awareness and to help them adopt strategies to mitigate the impact of typhoons on agriculture in north-western Vietnam.
f) Bangladesh:
Soil salinization is a major threat to crop production in Bangladesh; such that about 90 percent of potentially arable lands in the coastal area remain unused during the dry season. Improved water management practices through drip irrigation, coupled with the identification of saline-tolerant crop varieties during the project, have enabled farmers to introduce and harvest a second crop, in addition to aman (paddy) rice, on potentially up to 2.6 million hectares of highly fertile coastal lands. This could, for example, possibly add an additional 4 million tonnes of wheat to the national bread basket. 
Finally, IAEA projects support the development of comprehensive national and trans-boundary water resource plans for domestic, livestock, fishery, irrigation and other water uses, and help Member States to develop regulations, procedures, standards, minimum requirements and guidelines for sustainable water management. Regional monitoring networks and databases on isotopes and the chemical constituents of surface water and groundwater can also help to improve water resource management. Additionally, radiation processing technology, in combination with other techniques offers improved environmental safety through effective treatment of wastewater, and supports there use of treated wastewater for urban irrigation and industrial purposes.
Resources:
- IAEA Bulletin – September 2011 – Water Matters;
- Wikipedia ISOTOPE Hydrology;
- Water Use Efficiency in Agriculture – The Role of Nuclear and ISOTOPIC Techniques; and
- Use of Water in Food and Agriculture
- This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on February 23, 2012; and
- This chapter was updated on 23 June 2020.
