When water is “for fighting over”: LIST commentary on crop water use study

In the West there is a saying, often attributed to Mark Twain: “Whiskey is for drinking; water is for fighting over”. We do not need a proverb to gauge the importance of water in our lives, of course. Water is as essential an element to humans as is air. The difference here is that climate change, population pressure and a combination of several other manmade factors have turned water into an acutely scarce resource – a real bone of contention to be fought over.

Agricultural production, in particular, relies significantly on water and stands out as the foremost consumer in its sector. “Improving water management practices in agriculture is thus imperative to ensure long-term sustainability and productivity in the agro-food domain,” says Kanishka Mallick, Lead R&T Associate within the Environmental Sensing and Modelling unit at LIST. Recently, Mallick was invited to write a commentary article, along with Professor Dennis Baldocchi, University of California, Berkley, on a crop water use study published in Nature Water.

Leveraging satellite technology to measure water use of plants

In the study, which focuses on deriving crop water use from satellite data, John Volk from the Desert Research Institute, USA and his colleagues, have presented the OpenET initiative, where "ET" refers to evapotranspiration—a process that represents the total amount of water utilized by plants throughout their life cycle to carry out metabolic activities, conduct photosynthesis, and generate biomass. Kanishka Mallick emphasizes the significance of monitoring and measuring ET in agriculture, as it plays a crucial role in determining the required irrigation water for sustained plant metabolism and productivity throughout their growth cycle.

Traditionally, estimates of evaporative water use are derived using observational methods. These methods encompass various subgroups, with eddy covariance measurement – an atmospheric measurement technique – being a widely used approach.

“This involves tall towers equipped with sophisticated sensors that record water vapor, carbon dioxide, and heat fluxes with a typical data sampling frequency of 10 Hz,” Mallick explains. “These data are then converted into energy fluxes through complex mathematical equations, ultimately providing insights into the volume of water used by different ecosystems.” While there are around 700 to 750 such towers available worldwide, they may not be sufficient to grasp the nuances of the water cycle, plant water use, and ecosystem functioning across millions of land pixels.

Within the OpenET project, the technology employed to measure the amount of water required relies on satellite technology, allowing for the mapping of specific areas. The core of this technology involves various sub-portions, primarily focusing on what satellites can perceive. Unlike our eyes, which cannot differentiate between all wavelengths and spectrums, satellites utilize different filters across red, blue, green, near-infrared, shortwave infrared, thermal infrared, and mid-wave infrared regions to sense signals reflected from the Earth's surface.

Satellites provide a unique advantage by capturing images of large regions that standard instruments cannot achieve. “To illustrate,” Mallick says, “consider a thermometer measuring the temperature of a single plant. Now, consider numerous thermometers in space collectively taking temperature measurements, eliminating dependence on a single source. Satellites essentially act as these thermometers, sensing signals and converting them into temperature terms.”

Besides temperature, the satellites also measure reflectivity or the amount of reflected sunlight from vegetation. Once collected, this temperature and reflectivity data, combined with other weather information, undergoes integration into various mathematical models. These models, which involve complex physics and biology, generate outputs that indicate how much water is being used by crops.

Crucial information to optimize water use in dry regions

The initiative involves the development of a system that incorporates multiple models capable of leveraging this satellite information to compute crop water use. The resulting open data source is anticipated to be immensely beneficial, particularly for farmers in regions like California, characterized by arid and semiarid conditions.

California, with its unique ecosystem, faces challenges such as significant daytime radiation load and perennial water stress due to limited precipitation throughout the year. Efficient water usage by plants becomes crucial in these conditions. But farmers, often compelled by the urgency to safeguard their crops, may resort to practices like over-irrigation.

“The concern lies in the lack of awareness about potential future consequences. Farmers, driven by the need to protect their crops, may not be fully informed about the water use efficiency of the plants they cultivate,” says Mallick. This can result in unnecessary water application, where the actual water needs of the plants may be less than what is provided.

“Considering the broader scale,” he adds, “regions like the Central Valley of California and many other semiarid regions of Europe and Africa face similar challenges. In Africa, for instance, Lake Chad has significantly shrunk over the past 40 years, impacting agricultural sustainability. The reliance on Lake Chad's water has diminished, leading to mass migration, poverty, and threats to agricultural practices dependent on water availability.”

The key to addressing this challenge lies in obtaining information on evapotranspiration. Mapping this process on a large scale, especially through satellite imagery, provides valuable insights for farmers for them to make informed decisions regarding irrigation practices for their fields.

“The satellite data can thus soon become indispensable in the agricultural sector, especially to cater to the specific needs of farmers,” says Mallick, who is currently leading the development of the European ECOSTRESS hub with LIST colleagues, aiming to implement a similar system for Europe and Africa. European ECOSTRESS serves as a precursor for two upcoming thermal remote sensing satellite missions: TRISHNA (Thermal infraRed Imaging Satellite for High-resolution Natural resource Assessment), an Indo-French Space Research Collaboration, and LSTM (Land Surface Temperature Monitoring), led by the European Space Agency. Both missions share a common objective, aiming to map and monitor water stress from space. “Although it's a work in progress, I anticipate that within a few years, we'll have a comprehensive system in place,” he concludes.