Ground Truthing in GROW Places with the Flower Power sensor

Sentinel-1 photo © ESA

By Victoria J. Burton (Permaculture Association Britain)

Sentinel-1

According to the United Nations Office for Outer Space Affairs (UNOOSA) there are approximately 1500 active satellites orbiting the earth, some of which collect data that can be used to monitor global climate, vegetation cover and even soil conditions. Sentinel-1 is part of a new family of Earth Observation missions (the Sentinels) developed by The European Space Agency (ESA).

One of the environmental conditions Sentinel-1 measures is soil moisture — a key soil variable essential to agriculture and the understanding of extreme events such as flooding and heatwaves. The two satellites which comprise Sentinel-1 pass over the same location every six days (although one single location can be observed from more orbits, leading to an actual revisit time of 1.5–4 days over Europe), and measure an area a little bigger than a parking space (20 x 20 meters).

Ground truthing

However, soil moisture can vary over much smaller distances than this, due to changes in soil type, vegetation cover, slope, and aspect. To be sure satellite data are reliable, they needs to be compared (and evaluated) with soil moisture data collected on the ground directly in specific locations — known as ground truthing.

Ground truthing is what participants in the GROW Changing Climate are doing in nine GROW Places across Europe. They are using Flower Power soil sensors to measure soil moisture, along with temperature and sunlight. Data from these GROW Places can be used to aid interpretation and analysis of satellite data as well as enable communities to share and use new knowledge and data on growing practices and soil management as a response to our changing climate.

Inside out — how accurate are the Flower Power sensors in our GROW Places?

Flower Power sensors were originally designed to monitor indoor plants so one of the first tasks of the GROW Observatory was to test if they provide data accurate enough for scientific use — and whether they would survive being outside in all weather. In the winter of 2016–2017, 191 Flower Power sensors were field tested in different climate conditions across Europe. During this time sensors survived temperatures as low as -20℃. As winter returns we will find out if the sensors can survive the even lower temperatures that some of our Changing Climate participants experience!

Photo credit: Twitter user @Soupathon

Before the Flower Power sensors were chosen for this Mission, GROW scientists also tested how their measurements compared with actual soil moisture content and how this depends on soil type. In a laboratory, Flower Power sensors were placed in containers of four different types of natural soils (clay loam, sandy loam, loam and loamy sand). The containers were filled with soil of known weight and volume, saturated with water and then dried naturally over 31 days. After this time the actual moisture content of the soil was measured by weighing and subtracting this from the original weight. The measured values were then compared against the sensor values for the same time period.

Photo credit © Pavlos Georgiadis

This test found that Flower Power sensors tend to underestimate soil moisture in very wet conditions and overestimate in very dry conditions but are accurate in the 35–40% moisture range which is typical for soil. The distorted readings occur in a predictable way so results can be statistically adjusted to give a more accurate result. Some containers had several sensors to measure variation between them and differences were slight.

Graph comparing Flower Power soil moisture measurements with laboratory measurements

In April 2017, Flower Power sensors were placed next to high quality professional soil moisture sensors incorporated in the International Soil Moisture Network to test how they compare. Results from the Flower Power sensors were similar to the professional sensors although the former had a stronger response to precipitation events than the professional sensors as the latter are placed deeper in the soil. Testing is still ongoing against professional sensors and next to a rain gauge to directly compare sensor readings with precipitation.

Weather is not all Flower Power sensors have to contend with. Some of our Changing Climate Mission participants have had sensors trampled by wildlife or livestock or even swallowed up by cracks in the soil. Additionally there are always some technology failures but this has been accounted for and sufficient sensors are available to replace broken ones during the Mission.

If you live within 50km of the nine GROW Places you could be eligible to join the Changing Climate Mission and receive a free soil sensor, please fill in this form to apply.

GROW Places are in:

Evros & Laconia (Greece)
Southeast and Northwest (Ireland)
Miskolc (Hungary)
Barcelona (Spain)
Lisbon (Portugal)
Tayside & Central belt (Scotland)
Vienna (Austria)
’s-Hertogenbosch (Netherlands)
Luxembourg (Luxembourg)

Not in a GROW Place? You can still be part of the GROW Observatory by taking part in our online courses, downloading the GROW App, join the GROW Forum and contribute your planting and harvesting dates through the Share My Planting Calendars Facebook group or direct to calendar@growobservatory.org.