Fiber-optic distributed temperature sensing (FO–DTS) instruments — sometimes called "kilometer thermometers" — use the temperature-dependent properties of glass and precise pulses of laser light to measure temperature continuously along an optical fiber. The U.S. Geological Survey has used them for years to study how groundwater and surface water exchange.
Why it works
Hydrologic processes, especially where groundwater meets surface water and in shallow sediments, leave distinct thermal signals. These systems change constantly in time and space, so single-point temperature readings often miss what's happening.
| Feature | FO–DTS |
|---|---|
| Measurement | On site, not remote sensing; each reading averages about 0.25–2 meters of fiber, continuously along kilometers of armored cable |
| Duration | Usually days to weeks, readings every few seconds to hours |
| Precision | Set partly by the user: from about 0.2°C to 0.01°C as each measurement is integrated longer; averaging afterward improves it further |
| Best for | Finding focused zones of groundwater exchange that are unevenly spread across a stream or shoreline |
USGS staff and collaborators laying fiber-optic cable in a southern California tidal zone (A) and a Colorado mountain streambed (B) to map groundwater discharge. Photographs by Martin A. Briggs, U.S. Geological Survey.
From early tests to new tools
Commercial FO–DTS instruments arrived in the mid-2000s, making it easier to measure snowpack–air exchange, groundwater–surface water exchange and lake stratification.
- First USGS tests: on the Shenandoah River in Virginia, cables down the channel and near the bank mapped groundwater discharge in karst; on Fish Creek near Jackson Hole, Wyoming, about 24 hours of data showed that shading and pooling can mimic groundwater's thermal signature, so other data are needed to confirm; at Waquoit Bay, Massachusetts, a grid of cable tracked how tides pump groundwater into the bay.
- Wrapped fiber: winding thin cable around vertical pipes turns meter-scale readings into centimeter-scale profiles. In New York, these tracked a groundwater plume entering the Delaware River at habitat for the dwarf wedgemussel.
- Heated fiber: where strong groundwater flow mutes natural signals, profilers were heated to estimate high discharge rates — as on the Quashnet River on Cape Cod, home to brook trout.
- Software: in 2019, the USGS released a graphical program for processing, visualizing and mapping raw FO–DTS data.
Riverbed temperatures after active heating along a vertical profile at a groundwater discharge site on the Quashnet River, Cape Cod. USGS.
Tracking contamination
- Bremerton Naval Complex, Washington: shoreline sediments on Puget Sound are contaminated with PCBs and mercury. From July 30 to August 7, 2020, cable along about a kilometer of shoreline was read at daytime low tides during hot weather — up to 28°C on August 4 — to spot cold groundwater seeping into warm marine water and pick sites for chemical sampling.
- Cement Creek, Colorado: this high mountain stream north of Silverton is heavily affected by acid mine drainage. A short deployment in summer 2019, complicated by storms, revealed groundwater discharge and inflow from mine shafts; a solar-powered system ran for several months in 2023, calibrated in deep, well-mixed pools rather than streambank baths that need upkeep and can run dry. Combined with temperature profilers and chemical tracers, it helped measure discharge from two groundwater systems.
An FO–DTS system on an acid-mine-drainage stream near Silverton, Colorado (A), and the tangled cables that make the method laborious (B). Photographs by Martin A. Briggs, U.S. Geological Survey.
Testbeds for new monitoring
The USGS Next Generation Water Observing System (NGWOS) tests new monitoring technology in basins from about 1 to hundreds of square kilometers.
- Upper Delaware River Basin (2019): the first testbed used FO–DTS to find groundwater entering from several aquifer zones — shallow hillslopes, deep alluvium and fractured bedrock — and to build a model of how the stream's base flow is generated.
- Illinois River Basin (2024): at the Mason Tazewell Drainage Ditch in Illinois and the Kankakee River in Indiana, FO–DTS mapped groundwater discharge across the streambed alongside wells 8 to 33 feet deep, to study how groundwater carries nitrate pollution into streams.
What's next
FO–DTS is labor-intensive and needs specialized training, which has limited its use. Promising directions include cables lowered into monitoring wells to find preferential flow in aquifers, pairing with fiber-optic acoustic sensing, heated-fiber mapping of soil moisture in vineyards and orchards vulnerable to climate extremes, and long-term stream deployments to track cold-water fish habitat.
Sources
Based on "Fiber-optic distributed temperature sensing of hydrologic processes—Diverse deployments and new applications by the U.S. Geological Survey," U.S. Geological Survey Fact Sheet 2025–3006; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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