Intern Showcase: Soil Moisture Variation Across a Snow-Dominated Landscape

About the Intern:

A woman and a teenage boy look at a hand seld soil moisture sensor among the trees, — AGCI intern, Fallon (left) and high school Volunteer Sam (right) use a Fieldscout TDR 150 Soil Moisture Meter to collect data at the Sky Mountain iRON site, on August 8th, 2024. Image Credit: Elise Osenga.

Fallon Santander is a student of sustainability at Colorado Mountain College in the Roaring Fork Valley. She plans to graduate in May of 2025 with a B.A. in Sustainability Studies and a minor in Business. Fallon is a dedicated supporter of local agriculture and has spent her previous summers working on farms and ranches. This summer she took full advantage of the opportunity to develop new skills and participate in field research for the Center for Western Weather and Water Extremes (CW3E) and Aspen Global Change Institute (AGCI). During her internship, Fallon also installed a new weather station near Steamboat Springs with CW3E staff and volunteered in field research for the Independence Pass Foundation and the U.S. Geological Survey.

This summer the focal point of Fallon’s research was soil moisture in the Roaring Fork Watershed, a crucial headwaters of the Colorado River. More specifically, Fallon used handheld soil moisture sensors to collect data useful for understanding soil moisture distribution across a landscape with assistance from her AGCI mentors. This is an important area of research because understanding soil moisture’s role in the water cycle might improve water supply predictions and be useful to water managers. The Independence Pass and Sky Mountain Stations were chosen to be the study sites for the 2024 summer research and were visited multiple times. The stations are part of AGCI’s Roaring Fork Observation Network (iRON), a network of ten stations that collect weather and soil moisture data recurrently (More information iRON can be found here).

Collecting Soil Moisture Data for Independence Pass and Sky Mountain

To understand soil moisture distribution across a landscape scale at the Independence Pass and Sky Mountain sites, Fallon collected soil moisture measurements at 4.8 in and 8 in depths using a handheld FieldScout TDR 150 Soil Moisture sensor. Approximately 79 yard-long transects were performed in all four cardinal directions to collect soil moisture data across the landscape. The station tower was the origin point at both locations, and a total of 60 soil moisture measurements were recorded at each depth per site visit. Independence Pass was visited three times throughout the summer and Sky Mountain twice. Fallon visualized the data in box-and-whisker plots to look at average soil moisture across the landscape and compare spread of soil moisture at different depths and on different dates throughout the summer.

Volumetric water content: The unit used to represent soil moisture in the graphs below. Volumetric water content (VWC) is the ratio of the volume of water to the unit volume of soil. A zero reading would indicate baked soil, with no water content. A reading of around 30 to 50 VMC, or would indicate fully saturated soil in which no more water can be absorbed, with the point of saturation varying with soil type.

Fig.1. Soil moisture the Sky Mountain site on July 7th, 2024 at 4.8” (blue) and 8” (yellow) depths and August 8th, 2024 at 4.8” (Teal) and 8” (tan) depths. The y-axis displays soil moisture levels in volumetric water content (VWC). This graph shows a significant drying of soil from July 7th to August 8th, however, the averages between the 4.8” and 8” depths are similar when comparing different depths on the same date.

Fig.2. Independence Pass site soil moisture June 25 (4.8” depth), July 11th (4.8” and 8” depth), and August 1st, 2024 (4.8” and 8” depth). The y-axis displays soil moisture levels in volumetric water content. This graph beautifully illustrates typical soil moisture behavior in the Roaring Fork watershed as it transitions through the summer. The first box begins with high readings, near or at saturation levels in June, during the late spring when soil interacts with snowmelt. The July boxes (brown) show mid-summer drying after the snowmelt has finished. The August boxes (lighter blue) show late summer rain storms having increased soil moisture slightly.

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