Water temperature in water bodies, such as rivers, lakes, and reservoirs, is one of the most fundamental aquatic environmental factors that affect water quality and ecosystem. Lakes located in high altitude area are considered to be sensitive to global warming. In this study, we carried out detailed field measurements on water temperature and meteorological condition in the Tuni Reservoir in Bolivia Cordillera. Heat balance in this area is different from that in low altitude areas like Japan because the low atmospheric pressure decreases sensible heat transfer between the water body and atmosphere. By the analysis of the detailed field data, water temperature changes in the reservoir were successfully reproduced. The river water temperature in the Tuni reservoir catchment was also analyzed, and the transition of river water temperature along the river channel was discussed.

Glacial melt water is major water resources in and around La Paz, the capital of Bolivia in the center of South America. However, it is predicted that the glaciers in this area will disappear in 30 or 40 years because of climate change. In this study, water temperature around the end of the century in Tuni Reservoir, which is located at downstream of glaciers and provides domestic water to the cities nearby, were predicted with 1-dimensional hydraulic model using the outputs of a global climate model. The results indicate that surface temperature in the reservoir will basically depend on temperature rise. However, inflow rates will decrease about 60% by glacier disappearance and change more variably each year. So, it is important to consider how to take and convey water in the future.