Methodology:

We deployed commercially-available, high-precision water-pressure sensors (nominal precision 0.1% full-scale) on the bed of each lake, either in advance of the winter freeze-up or via a hole drilled through the winter ice cover. To maximise gauge sensitivity, we specified a relatively small pressure range of 1-10 bar and positioned our gauges in water depths of 1-5 m. All three lakes froze over completely during the studied periods. The gauges recorded water pressure and water temperature, and were naturally ventilated with breather tubes (open to the air at the logger box) to compensate for atmospheric pressure changes. We also used a thermistor on the ground at each logger-box site to measure snow-base temperatures, allowing us to detect the onset of snowmelt runoff in spring. At Silsersee we deployed one gauge in 2018/19 and a second gauge running concurrently with G1 from late February to early August 2020.

In winter, the time series of water pressure from all three lakes showed two dominant signals: (i) a declining pressure trend of several centimetres to decimetres water-equivalent over weeks to months, punctuated by (ii) abrupt jumps in pressure of millimetres to centimetres water-equivalent on hourly to daily timescales. The declining trends demonstrate that there was net drainage of water out of the lakes through winter, while the abrupt, shorter-duration pressure jumps indicate net mass gains. The timing and magnitude of these pressure jumps corresponded closely to snow precipitation as independently observed by nearby weather stations.

Data collection:

The water-pressure and water-temperature data are from a Keller PR-36XiW, 1-bar sensor. Ground temperature is from a T107C thermistor. Data were logged on Campbell CR300 loggers.

Data quality:

At the start and end of each time series during instrument deployment and retrieval, some pressures and temperatures appear anomalous as they represent atmospheric conditions rather than lake conditions. Water-pressure data have been converted from bar to metres of water equivalent using a constant factor of 10.1974.