Reanalysis of the longest mass balance series in Himalaya using nonlinear model: Chhota Shigri Glacier (India)

Abstract. In-situ glacier‒wide mass balances (MB) from traditional glaciological method often carry systematic biases. The glacier‒wide MB series on Chhota Shigri Glacier has been reanalysed by combining the traditional MB reanalysis framework and a nonlinear MB model. The nonlinear model is preferred over the traditional glaciological method to compute the glacier‒wide MBs as the former can capture the spatiotemporal variability of point MBs from a heterogeneous in-situ point MB network. Further, nonlinear model is also used to detect the erroneous measurements from the point MB observations over 2002‒2023. ASTER and Pléiades stereo-imagery show limited areal changes but negative mass balances of ‒0.38 ± 0.05 m w.e. a⁻¹ during 2003‒2014 and ‒0.51 ± 0.06 m w.e. a⁻¹ during 2014‒2020. The nonlinear model outperforms the traditional glaciological method and agrees better with these geodetic estimates. The reanalysed mean glacier‒wide MB over 2002‒2023 is ‒0.47 ± 0.19 m w.e. a⁻¹, equivalent to a cumulative loss of ‒9.81 m w.e. Our analysis suggests that the nonlinear model can also be used to complete the MB series if for some years the field observations are poor or unavailable. With this analysis, we revisit the glacier-wide MB series of Chhota Shigri Glacier and provide the most accurate and up-to-date version of this series, the longest continuous ever recorded in the Himalaya. We recommend applying the nonlinear model on all traditional glaciological mass balance series worldwide whenever data is sufficient, especially in the Himalaya where in-situ data are often missing due to access issues.