Radar Data Used to Measure Root-Zone Soil Moisture (RZSM)
AirMOSS built and operates a radar. This radar is mounted in a pod underneath a Gulfstream-III business jet. As the aircraft flies, the radar builds up an image of the ground on the port (left) side of where the aircraft is flying. The image is about 8 km wide. The image can be as long as the aircraft flies in a straight line, but usually that turns out to be about 180 km. We are able to measure root-zone soil moisture for every 100 m x 100 m square in the area that the radar imaged.
The radar transmits and receives radio waves with about a 68 cm wavelength. As these long wavelength radio waves hit things like trees and vegetation, some of the energy bounces back to the radar and some keeps going. These long wavelength radio waves penetrate into the dirt and reflect energy back from the soil layers where plants roots are. The amount of energy bounced back to and captured by the radar is called the radar return.
More energy is reflected back to the radar if the soil is wet than if the soil is dry. In other words, when there is less root-zone soil moisture we see a smaller radar return, and when there is more root-zone soil moisture we see a bigger radar return. Unfortunately, measuring the root-zone soil moisture is more complicated than just looking at the size of the radar return to estimate the amount of water in the soil. We have to account for all the other things that contribute to the radar return in addition to the amount of water in the soil such as trees, buildings, the roughness of the earth surface, etc. So we model what all the contributions to the radar return should be, look at the difference between our modeled radar return and what we actually measured with the radar, and calculate what the root-zone soil moisture was when the radar imaged the site.
The AirMOSS radar is imaging 10 sites across North America multiple times a year over a few years. These 10 sites are spread across North America. Each has a different climate, kinds of plants, and types of animals and is representative of its type of ecosystem. The northernmost site in Saskatchewan is in a boreal forest, and the southernmost site in Costa Rica is in a tropical forest. We fly these same sites over and over again so that we can understand how the root-zone soil moisture and the vegetation change one day to the next, with season, and year-to-year.
We make a new root-zone soil moisture map each day that the radar images a site. The stack of maps for each site is an input, along with other information such as rainfall data, for a modeling effort that estimates the root-zone soil moisture over the same area every hour. The hourly root-zone soil moisture is, in turn, an input into a climate model.
Each site has in-situ soil moisture sensors so that we can check how well we are measuring root-zone soil moisture using the radar data. Each site also has a flux tower which measures the exchanges of carbon dioxide (CO2), water vapor, and energy between vegetation and the atmosphere at that location.
How Does This Relate to Climate?
Plants thrive when they have the right amount of water at their roots. The amount of photosynthesis, respiration, and transpiration that a plant does depends on its root-zone soil moisture. Consequently, knowing the root-zone soil moisture is important for calculating atmospheric carbon since photosynthesis pulls carbon dioxide out of the atmosphere and converts it to sugars in the plant while respiration converts the sugars in the plant back into energy releasing CO2. The amount of soil moisture can change whether an area is uptaking CO2 from the atmosphere or is a net source of CO2 to the atmosphere.
Net ecosystem exchange (NEE) of CO2 is the term that scientists use to describe the land-to-atmosphere carbon exchange from photosynthesis and respiration in terrestrial ecosystems. Figure 2 shows a calculation of the NEE of North America averaged over several years. The figure shows that the agricultural regions of the Midwest are a net sink of CO2, absorbing more CO2 than they release. Figure 3 shows the uncertainty in the measurement. The uncertainties are larger than the flux over much of the map meaning that we do not know the NEE very well! One of the sources of that uncertainty is the root-zone soil moisture.