Abstract : On the Cordillera Real of Bolivia, we have reconstructed the mass balance of 21 representative glaciers, by digital photogrammetry. The DEM's have been constructed from the aerial photographs of the IGM-Bolivia of 1956, 1963, 1975, 1983, 1997 and 2006, with a decimetric or metric accuracy.
The analysis of the results of Zongo glacier shows that one third of the glacier surface (ablation zone) controls 80% of the annual surface mass balance. The Zongo glacier series constitute the longest and the most precise series of surface mass balance of the Tropical Andes.
We have analyzed the cumulated masse balances of 21 glaciers since 1956, excluding the tendencies of these glaciers, the temporary variations of the cumulated mass balances evidence a common sign of climatic origin.
We have shown that 88% of the variance of the cumulated mass balances can be explained by exposition and elevation. This correlation and the photogrammetric restitutions have allowed us to calculate the surface mass balance and the surface shrinkage of 376 glaciers between 1963 and 2006 (43% of loss of their volume and 48% of loss of their surface since 1975).
In summary, our study evaluates the glaciers contribution to the water resources of La Paz city. In average, over the 1975-2006 period the glacier melting has contributed around 18% and 15% during the recent period 1997-2006 (12% in wet season and 27% in dry season). Finally, we have calculated the runoff expected in the case of a total glaciers disappearance (12% less a year, 9% less in wet season and 25% less in dry season).