Efficient High‐Resolution Mapping of Small Groundwater Seeps Along an Intermittent Stream Using Thermal Infrared and Hydrochemical Techniques

ABSTRACT

Localised groundwater inflow can create spatial heterogeneity in intermittent stream discharge, temperature, and chemistry. However, locations and physicochemical properties of the groundwater inflow often remain poorly investigated at the reach and sub‐reach scale. Here, we combined high‐resolution mapping of groundwater seeps using thermal infrared (TIR) imaging with longitudinal electric conductivity (EC) and discharge measurements to efficiently reveal this heterogeneity with a short, targeted field campaign. At the transition to the dry summer season (early July), we mapped water temperatures and identified any seeps in the TIR images along a 341‐m stream reach of the semi‐arid Gibson Jack Creek, Idaho. We then measured the stream/seep ECs and longitudinal changes in stream discharge along the reach. Along the reach, the one‐day TIR survey effectively mapped 27 previously unidentified small seeps that lacked distinct topographic features. Although the geological map reveals consistent geology along the entire reach—landslide deposit on the left bank and limestone on the right bank—the seep water characteristics abruptly change at 125 m along the reach. On average, the temperature and EC were 1.3°C lower and 1.7 times higher, respectively, downstream of the 125 m breakpoint than upstream of this point. Salt dilution gauging showed that seep water contributions increased the downstream stream discharge by fivefold relative to upstream of the breakpoint (104 m), and this pattern was independently supported by two‐source separation analyses based on temperature and EC. Our findings demonstrate that a seemingly homogeneous stream reach can contain hydrologically distinct groundwater sources that strongly control longitudinal patterns of intermittent stream discharge, temperature, and chemistry. This highlights the importance of integrating point‐scale characterisation of groundwater inputs with reach‐scale assessments of intermittent streams.