Soil Moisture: An Important Parameter in Weather Monitoring
CoCoRaHS and Weather Monitoring
Each time a rain, hail, or snow storm crosses over your area, volunteers are taking precipitation measurements that are then used to analyze situations ranging from water resource availability to severe storm warnings.
CoCoRaHS (Community Collaborative Rain, Hail and Snow Network) is a non-profit community-based network of volunteers of all ages and backgrounds working together to measure and map precipitation (rain, hail, and snow). Their data is used by the National Weather Service, meteorologists, hydrologists, emergency managers, city utilities, USDA, engineers, farmers, and more. The organization will soon add another layer to their weather-monitoring efforts: soil moisture measurement.
Why Soil Moisture?
CoCoRaHS originated as the brain child of Nolan Doesken, the state climatologist of Colorado, in 1997 in response to a localized flooding event in Fort Collins, CO that was not well-warned due to lack of high-density precipitation observations. Ten years ago the Colorado Climate Center began a partnership with the National Integrated Drought Information System to establish the first regional drought early warning system. This particular system would serve the Upper Colorado River Basin and eastern Colorado.
From the beginning, Nolan was thinking about soil moisture. He says, “When we first started this project, we identified one weakness of the current climate monitoring systems as the inability to quantitatively assess soil moisture. Soil moisture is critical as it affects both short-term weather forecasts and long-term seasonal forecasts, which are important for drought early warning and avoiding the agricultural consequences of too much or too little soil moisture.”It wasn’t until years later in the drought of 2012, which developed rapidly in the mid and late spring across the intermountain west and central plains that Nolan began planning to use CoCoRaHS as a vehicle for improving the soil moisture aspect of drought early warning.
How Will Volunteers Measure Soil Moisture?
Historically, CoCoRaHS has had success using low-cost measurement tools, stressing training and education, and using an interactive website to provide the highest quality data, and soil moisture will be no different. The organization intends to measure soil moisture using the gravimetric method, where the user will take samples using a soil ring, dry samples in their own oven, and measure sample weight with an electronic scale. Peter Goble, a research assistant at Colorado State, has developed the measurement protocols that volunteers will follow. He says, “We have installed several different types of soil sensors and tried gravimetric techniques in a field next to the center, and our experience has helped us set up a protocol that gets observers as educated as they can be by the time they take their measurements. The coring device we use is something that came about through trial and error. We were trying to reconcile the fact that we really wanted deeper root zone measurements in order to satisfy drought early-warning-system users, and the need for an inexpensive set of standardized materials that we could send out to observers in a kit.” Volunteers will take soil samples at each point in a grid pattern, both at the surface and at the 7-9 inch level near the root zone.
What will Happen to the Data?
Initially, while the program is in its test phase, the data will be put in a spreadsheet and shared. However, once CoCoRaHS has finished sending this protocol around the nation to a group of alpha testers, they’ll set up a website infrastructure enabling volunteers to enter their VWC data directly into the CoCoRaHS website.
Why the Gravimetric Method?
Nolan says the challenge of water content is that soil is highly variable across space. And if you add issues like sensor performance, improper installation of sensors, problems with soil contact, changes in bulk density, and soil compaction, you end up with inconsistent data. The gravimetric method will avoid inconsistencies in spatial measurements and ensure higher quality data.
An Overwhelming Task
Nolan says the need for soil moisture measurement in weather monitoring will outweigh the volunteers’ ability to measure, but there is a solution. “People who use soil moisture data in atmospheric applications need high resolution, gridded information in every square kilometer across the country, but it will happen through modeling. The measurements we take of precipitation and soil moisture will help in the refinement of the weather modules the atmospheric scientists will use as input to their weather prediction models.”
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