Use of an ultra-long-range terrestrial laser scanner to monitor the mass balance of very small glaciers in the Swiss Alps

Due to the relative lack of empirical field data, the response of very small glaciers (<0.5 km) to current atmospheric warming is not fully understood yet. Investigating their mass balance is a prerequisite to fill this knowledge gap. Application of the direct glaciological method is one option. Since most recently, terrestrial laser scanning (TLS) techniques operating in the near infrared range are successfully applied for the creation of repeated high-resolution digital elevation models and consecutive derivation of annual geodetic mass balances of very small glaciers. This method is promising, as laborious and 5 potentially dangerous field measurements as well as the interand extrapolation of point measurements can be circumvented. However, it still owes to be validated. Here, we present TLS-derived annual surface elevation and geodetic mass changes for five very small glaciers in Switzerland (Glacier de Prapio, Glacier du Sex Rouge, St. Annafirn, Schwarzbachfirn, and Pizolgletscher) and two consecutive years (2013/14–2014/15). The scans were acquired with an ultra-long-range Riegl VZ6000 especially designed for surveying snowand ice-covered terrain. Zonally variable conversion factors for firn and bare 10 ice surfaces were applied to convert geodetic volume to mass changes. We compare the geodetic results to direct glaciological mass balance measurements coinciding with the TLS surveys and carefully assess the uncertainties and errors included in both methods. Average glacier-wide mass balances were negative in both years, showing remarkably stronger mass losses in 2014/15 (–1.65 m w.e.) compared to 2013/14 (–0.59 m w.e.). Geodetic mass balances were slightly less negative but in close agreement with the direct glaciological ones (R = 0.91). Due to the very dense in-situ measurements, the uncertainties in the 15 direct glaciological mass balances were small compared to the majority of measured glaciers worldwide (±0.09 m w.e. yr−1 on average), and similar to uncertainties in the TLS-derived geodetic mass balances (±0.13 m w.e. yr−1).