It was a nice warm sunny day when we arrived at Cary Institute, situated in Dutchess County, NY, for our inaugural day briefing on SMAPVEX 2022 IOP2 campaign. I had enjoyed a good night’s sleep before coming down to the headquarters, as the forecast for the next 26 days indicated that I might not get the chance again! The forecast turned out to be true, there was little to no time to sleep for the good part of the next month. But I have seldom been so happy about losing my sleep.
Going back a year, my advisor informed me about an opportunity to volunteer for a fieldwork by NASA related to soil moisture sampling, named Soil Moisture Active Passive Validation Experiment (SMAPVEX). Most of my research is based on computational work, so I did not think twice before accepting the volunteering role. Unfortunately, due to COVID-19 restrictions, the SMAPVEX campaign scheduled for Summer 2021 was postponed to Summer 2022. The campaign was split up into two different Intense Observation Period (IOP) and two different sites in Northeast US. The IOP1 was scheduled for April 2022, and IOP2 for July 2022. The sites were Millbrook, NY, and Harvard Forest, MA. I was selected for the IOP2 at Millbrook, NY.
As the name suggests, SMAPVEX is a validation campaign for NASA’s Soil Moisture Active Passive (SMAP) mission. The campaign is arranged to scrutinize the performance of surface and root zone soil moisture data collected by the SMAP satellite. The scientific mechanism behind SMAP soil moisture observations is reasonably straightforward. It uses a chunk of microwave band named L-band (1.413 GHz) to interact with the moisture content of the soil. It is the same mechanism that interacts with the moisture content of food items inside the microwave oven to warm them up. The soil moisture data product from SMAP is sensitive to a few physical factors, i.e., vegetation, surface roughness, etc. Given the heterogeneity of the earth’s surface, rigorous ground validation efforts are made regularly to improve the accuracy of the remotely sensed soil moisture data.
To render a picture of soil moisture, vegetation, and surface roughness conditions, a group of 9 volunteers from all over the US gathered in Millbrook this summer, led by scientists from NASA JPL, USDA, and Cary Institute. Every day the volunteers set out to sample either soil moisture or vegetation in the lush green forests of Northern Appalachians. During the soil moisture sampling days, the volunteers formed four groups and collected soil moisture data from 3 different sites, consisting of 14 other data collection points per site. The data collection points for each site and the sites themselves were chosen to provide good spatial coverage of the SMAP grid cell. We collected surface soil moisture data using 6 cm soil moisture probes and root zone soil moisture using 12 cm probes. These probes use the dielectric properties of soil to measure moisture content. We also collected gravimetric soil and forest floor litter samples for measurements of moisture content.
SMAP is a sun-synchronous satellite with a descending overpass at 6 am and ascending overpass at 6 pm over our site. Also, it turns out that soil moisture is the most consistent during the start of the day. A combination of these two conditions made us start sampling at 6 am (so much so for science!). Along with beating the clock, we also had to beat the dense understory of the forest, the rough and rocky terrain of the Northern Appalachians, and black-legged ticks carrying Lyme disease. The challenges sometimes turned into adventures with frequent sightings of wildlife such as gazelles, coyotes, snakes, lizards, and numerous insects (and Yeti, of course).
The vegetation sampling days were different. Those days, the volunteers spread out into the woods and identified trees, collected leaf samples for vegetation moisture content and Leaf Area Index (LAI) calculations, measured tree trunk diameter, and measured bulk density of soil. We used hammered Lignometer nails into tree bark to read the water content. We used traditional Surface Roughness Profilers for surface roughness measurements and tried modern techniques like iPad LiDAR scanners to perform the same task. SMAP’s microwave scanners hit the vegetation and land surface at a surface incidence angle of 40 degrees, thus, the vegetation and land cover type play important factors in SMAP product accuracy.
Being able to do my part for science through the SMAPVEX 2022 campaign was a fulfilling experience, both on a professional and personal level. On a personal level, it did not take long to make friends with this amazing bunch. When the schedule was not so busy, we dined in different restaurants, hiked old railroad trails, learned new board games, and even managed to escape an escape room. The effort put in by the SMAP team to foster the delicate equilibrium of the earth system is genuinely worthy of appreciation.
By: Soelem Aafnan, PhD student at George Mason University