Changing climatic conditions in the Colorado River Basin: Implications for water resources management in the Las Vegas Valley

Climate change affects the water available in a region. It also affects the water demand, because of the increase in temperature. A system dynamics model was developed for the Colorado River Basin (CRB), operating at a monthly time scale, to assess the potential impacts of climate change on streamflow in the Colorado River and its subsequent impact on the water resources management in the Las Vegas Valley (LVV). The effect of climate change on streamflow was evaluated using 16 global climate model outputs for 3 emission scenarios, also referenced in the Inter-Governmental Panel on Climate Change Fourth Assessment Report. Risk analysis of the water supply to the basin states dependent on the Colorado River was performed based on streamflow magnitude estimated using GCM outputs. Probabilities of Lake Mead levels to draw down below 327.7 meters (1075 feet) was investigated based on streamflow estimated using GCM outputs, and also on the future streamflow varying from 60 to 120 % of the historical average streamflow. The model was further developed to evaluate the impacts of climate change and population growth on the water resources in the LVV. Effect of climate change on water demand was also simulated using the same ensemble of 16 GCM outputs for the future temperature in the LVV. Demand management was modeled as the long term solution to obtain the water sustainability in the LVV. Water demand and water supply were investigated for different scenarios of population growth rate and policies implemented for water conservation in the LVV. Policies refer to indoor-outdoor conservation and water pricing. The results showed that climate change has significant effect on streamflow in the CRB. The ensemble average of all the GCMs showed about 3% reductions in future streamflow by 2035. This created the possibility of curtailments to the water supply in the basin states. Approximately 14% probability of supply curtailment to the basin states was observed from the ensemble average of the GCMs. Similarly, averaged over the ensemble of all the GCMs, water supply reliability of about 0.86 was observed for the basin states. Population growth resulted in significant impact on water resources in the LVV. With the population growth as predicted and with no additional policies for water conservation, water demand was observed to exceed the supply in near future and the reliability of water supply to the LVV from the Colorado River was estimated to be 0.06. With no further population growth and no climate change, a reliability of 1 was observed. Reliability of water supply decreased with the changing climatic conditions. The ensemble average of all the GCMs predicted a 5% probability of Lake Mead levels to drop below 305 m (1000 ft) for the future simulation, if water supply to the basin states is continued below 305 m (1000 ft) water levels in Lake Mead. The results suggested a need of combined demand management policies and slower population growth to achieve the water sustainability in the LVV. This study has expanded the existing knowledge of the effect of climate change on streamflow in the CRB with the inclusion of the most plausible range of future climatic conditions. This study may help to facilitate the water managers by providing the broad choice of demand management policy options in developing sustainable water management practices to meet the increasing demand in the LVV. Keywords: System dynamics, climate change, streamflow, Colorado River basin, Las Vegas Valley, Global Climate Model