Monitoring meadows from space

Immersed in the silence of the cosmos, two Copernicus Sentinel-2 mission satellites make their orbit around the Earth. The radiometer with which they are equipped, scans the Earth’s surface to obtain both physical and chemical data. 700,000 meters below, amidst the constant hum of insects, Abraham Mejia Aguilar, a researcher at Eurac Research’s Center for Sensing Solutions, makes his way through the tall grass of a mountain pasture. He carries a shoulder bag full of sensors. To check the reliability of data, he has to compare satellite results with those obtained through field measurements. In this way, the information provided by satellites can be used to estimate the damage suffered by meadows due to drought.

Alpine mountain meadows provide numerous ecosystem services. In fact, these environments are the main source of forage for livestock and a popular tourist destination, as well as playing a crucial role in regulating climate, preserving landscape and biodiversity, preserving soil quality and integrity, and protecting against erosion. In recent decades, human activities have resulted in a drying out of their climate and though mountain meadows are composed of plant species that are resistant to water scarcity, frequent and prolonged periods of drought, such as those predicted by climate models, can impair forage production, especially when they take place in spring: at this time of the year, plants are just beginning to grow and require greater amounts of water. As if that were not enough, there is less and less snow on our mountains and because of that, less water resulting from melting snow. To make matters worse, extreme weather phenomena such as torrential rains and hailstorms are destined to become ever more frequent. When events such as these occur, the soil, especially if arid, is unable to absorb such a large amount of water, instead forming streams on the surface without recharging the water table. Protecting farmers against drought-related risks also involves taking out ad hoc insurance. And for farmers to be reimbursed, the damage they have suffered must be estimated. There are several methods for estimating drought damage for insurance purposes. One option is for an insurance appraiser to go to the site and estimate the extent of the damage. This approach, however, is encumbered by its subjectivity, as different appraisers may give conflicting opinions. An alternative is to resort to the use of objective data. Until recently, the most widely used data was meteorological. Through which scientists could obtain information about weather phenomena that occurred in a particular geographical area. Nevertheless, even this source of information has its limitations: the geographical distribution of weather stations is not so widespread as to allow monitoring of all meadows. Moreover, weather conditions may not necessarily reflect the actual damage suffered, since these also depend on factors such as area management and soil type. To get around the limitations imposed by traditional methods, there are those who rely on the meticulous gaze of satellites. From the calving of icebergs to poaching, from pollution of the seas to deforestation, there are numerous phenomena that satellites make it possible to monitor. At the Eurac Research Institute for Earth Observation, Sentinel-2 satellite data is being used to assess the productivity of mountain meadows in Trentino-South Tyrol. It all started with the Agricultural Consortium of Bolzano needing to have an insurance policy that was based on objective parameters for the protection of its meadows and is how the DRI2 research project, aimed at developing a drought index derived from satellite data, physical models and meteorological information, got underway. The main parameter derived from satellites employed by the researchers, is the leaf area index which measures the surface area of leaves over a given area of soil. The research team used this parameter to estimate pasture productivity: a high leaf area index being synonymous with a rich harvest. To test the reliability of satellite data, it is necessary to travel to mountain meadows and take a series of measurements and that’s where the Eurac Research Center for Sensing Solutions comes in. Once in the field, researchers from the center use various sensors to take biophysical parameters, including soil moisture, plant chlorophyll content, grass height and leaf area index. To finish, grass is cut from a square meter of lawn, weighed, and brought to the lab to be dried in an oven. Once dried, the grass is weighed again to determine pasture yield. All the data obtained from the field surveys is then compared with satellite-based information to make sure the models driven by satellite data can reproduce the actual conditions on the ground.

The DRI2 project ended in December 2022 with the development of the new drought index. This, however, is based purely on Sentinel-2 optical satellite data which only uses naturally available solar radiation reflected or emitted by objects. The problem: radiation is not detectable on days when clouds prevent the satellite from “seeing” the Earth’s surface. Owing to this, a second project is underway, this time the goal is to refine the index calculation using Sentinel-1 radar satellite data, which is unaffected by cloud cover because it is obtained through microwave. Sentinel-1 satellites, in fact, emit microwaves directed toward the object of investigation and record the radiation that this reflects. The project, which started in January 2023 and will end in 2026, is called ScaleAgData and again involves both the Institute for Earth Observation and the Center for Sensing Solutions. The data collected in the field and the models and indices developed within the study will be made available to the entire scientific community.