Geodetic and gravimetric constraints on regional glacier mass changes in High Mountain Asia

Regional glacier assessments often require mass-change estimates over periods that differ from the intervals measured geodetically. We quantify how native cumulative-mass adjustments and within-interval variation contribute to the regional estimates obtained by adding GRACE. A linear joint inversion represents glacier, lake, non-glacier and exterior mass across High Mountain Asia. Geodetic-only, gravity-only and joint estimates share source geometry, auxiliary observations and temporal regularization. Evaluation uses excluded CryoSat contributor records for complete RGI regions 13 to 15 over January 2011 to January 2015 and January 2015 to January 2019. Joint regional-period mass-rate RMSE is 4.37 Gt yr⁻¹, compared with 8.01 for geodesy and 14.80 for gravity. Adding gravity improves interperiod estimates relative to geodesy, while geodetic constraints mainly reduce the gravity-only mean offset. Under a fixed linear-bridge decomposition, improvement is associated with adjustments to cumulative mass at the native geodetic endpoints. Adding only this component to the geodetic history gives a diagnostic RMSE of 3.43 Gt yr⁻¹; adding only within-interval variation gives 8.51. The joint solution also has substantial geodetic residuals and poorer results in missing-period and local glacier evaluations. The regional gain reflects reconciliation of cumulative mass constraints under the specified decomposition. Evaluating external-period rates alongside native cumulative budgets distinguishes this improvement from recovery of temporal variation within the original geodetic intervals.