Drought Busters: Next Generation Soil Moisture Monitoring

Pepperwood’s ecosystems and topography are an excellent representation of the North Bay, making it an ideal testbed for this soil moisture monitoring project. Photo courtesy of Jack Elston

Pepperwood is a test-bed for new and innovative monitoring technologies with the potential to serve as valuable tools for our regional fire and drought resilience under a changing climate. With over 80 projects onsite run by a combination of staff, community scientists, visiting scholars, and university partners, our Sentinel Site has collected data on the ground since 2010 – capturing the effects of climate change as well as our watershed and ecosystem stewardship.

An expanding role for us is to serve as a “calibration and validation” site for remote sensing data resources: these are data collection methods that use sensors on drones, airplanes or satellites to rapidly measure conditions over vast areas. One example is Landsat, a joint NASA-USGS program that is the longest continually running enterprise for obtaining satellite imagery of Earth. Ever hear of Google Earth? Those images are made possible by Landsat.

Developers of these emerging remote sensing technologies need partners who can collect data on the ground to help ensure the sensor is measuring its target (validation) and that its measurement is accurate to a known degree (calibration). Since our Sentinel Site now has long-term records and collects near real-time data on multiple parameters, we are building relationships with the agencies that build and apply these remote sensing technologies, like NASA and the US Geological Survey, to assist them in their missions of serving our nation through science.

Monitoring Soil Moisture With the Super Swift

A recent partnership with USGS and Blackswift Technologies features the use of a drone to map surface soil moisture. This experimental method aims to detect the relative water content in the top layer of soil using a technique called “radiometry.” Radiometry is a set of techniques for measuring electromagnetic radiation. It can help us read patterns of non-visible light generated by heat like infra-red radiation. Thus, radiometry is about measuring the full spectrum of electromagnetic wavelengths, both visible and invisible to the human eye. It turns out that patterns of soil moisture may be detectable using a sensor called an “L-band” radiometer.

Soil moisture is an indicator of many things from drought stress to wildfire risk – both of which are studied in depth by our Terrestrial Biodiversity and Climate Change Collaborative (TBC3). In the quest to build climate and fire resilience, monitoring soil moisture at a landscape scale is a “holy grail” of indicators, relevant to water supply, agricultural sustainability, forest health, wildfire strategies, species survival, and flood hazards.

From left to right: Jack Elston, Michelle Stern, S2 (the drone), Maciej Stachura, Ryan Ferrell. Photo courtesy of Jon Stock – also part of the team.

Michelle Stern is a Hydrologist with USGS, a UC Davis doctoral student, and a TBC3 affiliate who is undertaking the design of a soil moisture monitoring network for the whole State of California as her PhD research topic. Stern’s work will use Pepperwood’s Sentinel Site to integrate and compare two methods for measuring soil moisture: direct “point” measurement techniques using buried soil moisture probes, and emerging remote sensing techniques using spectroscopy readings of electromagnetic radiation. In this case, those remote sensing techniques are enhanced by an L-band (1–2 GHz) radiometer based on a drone.

Currently, there are L-band sensors on satellites generating global soil moisture products – but these are too coarse in resolution for local applications. From that distance it can be particularly difficult to model in mountainous regions where solar insolation (the amount of sun radiating on a given area), geology, ecology, and climate all vary. Meanwhile, Sentinel Site-style soil moisture data is extremely sparse across the state, especially in the hills where most of our water supply originates. What we need for regional applications in the West is something between these two scales that is capable of measuring soil moisture in real-time, but with high resolution, and this is the impetus for Stern’s work.

S2 situated on the pneumatic launcher and ready for takeoff. Photo courtesy of Jack Elston.

The drone, engineered right here in the USA by Black Swift Technologies and in partnership with NASA Goddard, is called the Super Swift (or S2). Black Swift founders, Drs. Jack Elston and Maciej Stachura, are two aerospace engineers who met in graduate school in Colorado. Their S2 is a fixed-wing, electric UAS with a three-meter wingspan, that is propelled into the air with a pneumatic launcher using compressed air as a propellant. Affixed to S2 is a radiometer system composed of a lightweight suite of sensors that include multispectral cameras in the visible, thermal, and near infrared regions of the electromagnetic spectrum.

The team spent two days in October 2021 flying the first of three missions designed to map seasonal and spatial variability in soil moisture across Pepperwood’s landscape. This first flight captured imagery of the dry soils during one of the worst droughts ever recorded in California, just before a three-day atmospheric river brought approximately ten inches of rain to the reserve. Prior to this rainfall, surface soil moisture conditions were the driest on record, averaging about 15 percent volumetric water content across the reserve.

What’s exciting about this kind of high-tech project is that it isn’t just applicable to monitoring Pepperwood: it’s an approach that could be expanded and replicated throughout the globe. Once these technologies are proven, they can generate incredible cost savings compared to traditional data collection techniques, and build our knowledge base regarding changing environmental conditions at very large scales.

Through these emerging technology partnerships, Pepperwood can better advise our community on exactly how this kind of data can serve multiple objectives including water security, wildfire resilience, biodiversity protection, emergency response, and Indigenous food sovereignty in our region, and beyond.

Drone captured aerial image of Pepperwood’s landscape. Photo courtesy of S2.

Want to learn more about this project and related research? Check out the most recent publication on this topic, here.

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