Analysis of water supply and demand in high mountain cities of Bolivia under growing population and changing climate

Water resources in La Paz and El Alto, high mountain capital cities of Bolivia, strongly depend on the surface and subsurface runoff from partially glacierized catchments located in the Cordillera Real, Andes. Due to growing population and changing climate, the balance between water supply from the source catchments and demand for drinking, agriculture, industry and hydropower has become precarious in recent years as evidenced by a serious drought during the 2015-2016 El Nino event. To predict the long-term availability of water resources under changing climate, we developed a semi-distributed glacio-hydrological model that considers various runoff pathways from partially glacierized high-altitude catchments. Two GCM projections (MRI-AGCM and INGV-ECHAM4) were used for the prediction with bias corrected by reanalysis data (ERA-INTERIM) and downscaled to target areas using data monitored at several weather stations. The model was applied to three catchments from which current water resources are supplied and eight additional catchments that will be potentially effective in compensating reduced runoff from the current water resource areas. For predicting the future water demand, a cohort-component method was used for the projection of size and composition of population change, considering natural and social change (birth, death and transfer). As a result, total population is expected to increase from 1.6 million in 2012 to 2.0 million in 2036. The water demand was predicted for given unit water consumption, non-revenue water rate (NWR), and sectorial percentage of water consumption for domestic, industrial and commercial purposes. The results of hydrological simulations and the analysis of water demand indicated that water supply and demand are barely balanced in recent years, while the total runoff from current water resource areas will continue to decrease and unprecedented water shortage is likely to occur since around 2020 toward the middle of 21st century even if NWR is improved. We showed that the runoff from a partially-glacierized catchment located in the vicinity of the current water resource catchments can greatly compensate the projected shortage in water supply. Therefore, consensus building on diverting water from the new catchment will be critical for sustainable development of the region.