Our First 1500 Soil Sensors Are in the Ground — so What Can We Learn From Them?

By Luca Zappa and Drew Hemment

Soil moisture sensors have been deployed by GROW Observatory in locations across Europe called GROW Places. So far the number deployed is 1500, this is already 6 times more than the number of measuring sites in Europe until now. So what use are citizens and scientists making of the GROW data?

We are now entering a new era in which previously inconceivable amounts of data are becoming available . This is all thanks to the efforts of volunteer citizens and scientists, working with far more accessible low-cost sensors.. Luca Zappa and Drew Hemment reflect on initial findings from data collected by GROW participants.

GROW Observatory is mobilising enthusiastic citizen scientists to distribute thousands of soil sensors across Europe. This novel source of data has the potential to unlock some of the still poorly understood connections between soil moisture and other climate variables. As a result, data collected within GROW will provide useful inputs into a number of downstream applications such as flood risk assessment and heat wave monitoring.

In fact, soil moisture is a pivotal variable regulating the exchange of water, energy, and carbon flows between the soil and the atmosphere, thus strongly affecting the climate.

Increasingly, human activities are having more and more of an impact on the world around us. Mainly this is because of the way heat is trapped by carbon dioxide (CO2) and other gases intensively released in the atmosphere as result or byproduct of those human activities — above all, via fossil fuel emissions.

Figure 1: atmospheric concentration of CO2 (credit: NASA)

The net result is a warming trend of unprecedented rate, which threatens the Earth and all its living inhabitants. Some effects of climate change are already observable on the environment: heatwaves are longer and more intense and wild fires more frequent and severe, and glaciers shrink, leading to loss of sea ice and sea level rise.

Because human-induced warming is superimposed on a naturally varying climate, there will be areas with stronger impacts of climate change (such as increase in temperature and changes in precipitation regimes), while some other areas will be only slightly affected.

We can predict effects of climate change over the globe using computer models. This involves piecing together mathematical equations representing the physical laws beyond certain processes. This could mean, for example, the amount of water transpiring from the soil — depending on both the temperature and the soil type, or the ratio between the absorbed and reflected solar radiation reaching the Earth’s surface — due to its properties). However, because the Earth is a hugely complex system no mathematical model can reflect perfectly its intricate processes, and there will always be differences between predictions of the models and reality. This is especially evident when models are used to provide information about more localised areas. In fact, it is possible to predict the overall behavior of the system (that is, large scale effects), but it is very difficult to get the same level of confidence for each point on the Earth (the local scale).

The Solution is on the Ground — in GROW Places

Because of the difficulty in making predictions at the local scale, ground observations are crucial. Ground observations are necessary for providing reliable information at small scales, which can be used to validate and calibrate models, which in turn leads to more accurate results. However, in-situ measurements are costly and require maintenance, which means they are scarce in number and density. Currently, fewer than than 250 stations measure soil moisture (and provide free access to the data) over Europe (see link below)

Thanks to the contribution of thousands of volunteer citizens and scientists aka the GROWers — we work towards filling this gap by monitoring soil moisture and other landscape properties at unprecedented scales.

And here’s the exciting bit — more than 1,500 sensors have already been deployed and are collecting valuable data. Distribution of the sensors is concentrated in ten focus areas, which we call these GROW Places, selected based on different climates and soil types . In each one, community champions are supporting citizens to install sensors in different locations and land cover types (such as cropped fields, pastures, forests, gardens) in order to cover a large range of environmental conditions.

Figure 2: locations of the sensors installed by GROWers

Not only citizens will directly benefit from this data, the scientific community will too. Citizens will be able to monitor soil moisture and the near-surface air temperature in their own fields and gardens, thus improving their growing practices while acquiring a deeper understanding of different processes (such as, the impact of weather on crop production, distribution of water within the field/garden, and so on). The scientific community will handle an unprecedented amount of data necessary to understand, calibrate, and validate models to help climate science. This will also contribute to answer a long-standing challenge for space science; the validation of remotely sensed soil moisture (e.g. Sentinel-1).

What Have we Learned so Far?

Let’s dive in (some of) this crowdsourced data and see what we can learn from it!

First, look at soil moisture measured in the Greek GROW Place, located near Alexandroupoli, over a three and half month period during summer 2018. Each line depicts a single sensor (approx. 30 in total).

Figure 3: soil moisture patterns from the Greek GROW Place over the summer period

We can see that even though the sensors are closely located, there might be significant differences in soil moisture values. Such disparities arise from the high variability of soil moisture even at very small scales (even just some centimeters below ground) which are mainly controlled by soil texture, topography, and vegetation.

For example, higher soil moisture content might be related to finer soils (i.e. higher content small soil particles, such as clay) which hold more water more tightly. However it could also be linked to the position in the landscape (e.g. closer to rivers or other water sources, or being in a valley where water accumulates from adjacent slopes).

Another variable with a strong impact on soil moisture is vegetation. We can observe its effect (also) on the increase of soil moisture after precipitation events: for some locations it is higher than for other locations. Clearly, this might be because of the rainfall itself, but over small areas it is usually assumed to be homogeneous. If this is the case, such differences are mainly due to the impact the vegetation canopy on rainfall.Obviously, the more vegetation is present, the more water is intercepted; therefore, some might never reach the ground resulting in a smaller increase in soil moisture.

Now let’s broaden our view to look at soil moisture and temperature recorded in three GROW Places characterized by different climates: Austria, Greece, and Hungary. In this case the data are an average of all the sensors within each GROW Place, and thus represent the (average) local conditions.

Figure 4: averaged soil moisture and temperature from 3 GROW Places

In the Greek GROW Place (Fig. 4, top) the highest temperature and lowest soil moisture levels are reached, especially up to mid-June. In fact, the typical trend of the dry Mediterranean climate (as of Greece) is clearly visible, with very dry conditions coupled with high temperatures. However, in the second half of this period, a rainfall event led to an increase in soil moisture and a slight decrease in temperature, mitigating the previous condition. Unfortunately, this might have not been the case for other areas in Greece, where the absence of precipitation exacerbated the existing drought and made the outbreak and spread of fires during the summer easier.

Austria and Hungary show similar patterns, with higher moisture levels and precipitation -in particular, the temperature trends are very alike. This highlights the similarity of climatic conditions among these two areas, especially the solar radiation (energy from the sun), which is one of the main controls of temperature.

Furthermore, we can observe that in the Austrian GROW Place the development of soil moisture is marked by continuous increases, corresponding to light rain and/or isolated showers, as expected from the Austrian climate (a warm temperate humid climate) where the precipitation is quite evenly distributed over the year.

Another remarkable fact that we can verify from these data is the relationship between temperature and precipitation (which corresponds to peaks in soil moisture). Indeed, as we all experience, after a big rainfall the temperature tends to drop, and the more the precipitation the higher the decrease.

Thanks to the efforts of volunteer citizens and scientists in projects such as GROW, and greater accessibility of low-cost sensors, we are now entering a new era, in which previously inconceivable amounts of data are now becoming available, and being used to address local and global challenges. Such data will not only benefit citizens, who will discover and quantify the exciting relations in the world around us, but it can be further used by scientists and researchers. In fact, there is an ever-increasing need of ground measurements for the validation and calibration of models and remotely sensed data, which in turn will help us to monitor, understand, and predict the effects of climate change on a global scale.

Do you live in one of the nine current GROW Places? These are Evros & Laconia (Greece), Southeast and Northwest (Ireland), Miskolc (Hungary), Barcelona (Spain), Lisbon (Portugal), Tayside & Central belt (Scotland), Vienna (Austria), ’s-Hertogenbosch (Netherlands), Luxembourg (Luxembourg). For more, visit our get involved page.

Luca Zappa works at the Department of Geodesy and Geoinformaiton at TU Wien (Vienna). Drew Hemment is a Chancellor's Fellow at Edinburgh Futures Institute.