Future droughts and floods in Lake Titicaca cannot be prevented by release management alone

Lake Titicaca, located in the tropical Andes of South America and shared by Peru and Bolivia, has experienced extreme variations in water levels, leading to droughts and floods over the last 50 years. In the early 1990s, a master plan was developed, proposing the construction of a gate to regulate the lake's outflows and mitigate hydrological risks. While the gate was completed in the early 2000s, regulation was never implemented due to a lack of management agreement between the two countries. The effectiveness of the operating rules for managing hydrological risks under ongoing climate change remains unknown. To address this issue, we used an integrated water balance model to evaluate both natural and regulated release options under observed climate conditions (1982–2016) and future scenarios of precipitation and air temperature. Future climates were generated using the perturbation method based on changes projected by 21 GCMs from CMIP6 for the period 2036–2070. Drought was defined as a drop in water levels (and associated released flows) below a threshold linked to downstream irrigation requirements. Flooding was defined as the condition when water levels (and associated released flows) exceed the flooding threshold in the shore zone of the lake. The risks were evaluated in terms of their intensity, duration, and frequency using appropriate indicators.  Under a projected warming of 3.4 °C (as suggested by an ensemble of GCMs by 2050) and no changes in precipitation, the lake's water levels could drop below the outlet level, disconnecting it from the Desaguadero River. Regulation under observed climate conditions reduces risks in the shore zone and downstream areas. However, under future climate scenarios, regulation is likely to be less effective. In the Warm–Dry, Hot–Dry, ensemble mean scenarios, more intense and prolonged droughts are expected, while in the Warm–Wet and Hot–Wet scenarios, the risk of flooding could increase significantly. The differences in the effectiveness of natural and regulated release options are minimal. This suggests that managing releases alone will be insufficient to mitigate hydrological risks under future climate conditions. These findings provide valuable insights for improving management and guiding the identification of additional intervention measures, such as land-use planning, to ensure Lake Titicaca's resilience to future droughts and floods.