Evaluating the impacts of water conservation policies on water demand, availability and outdoor water use in the Las Vegas Valley.

摘要

The Las Vegas Valley, located in the arid Southern Nevada region, with a growing population, limited water resources, and a prolonged drought, faces a challenge in meeting its future water needs. Southern Nevada Water Authority (SNWA), the main water management agency in the Valley, is focusing on water conservation to reduce water demand. Current water use is 945 lpcd (250 gpcd) which SNWA aims to reduce to 752 lpcd (199 gpcd) by 2035. Presently the indoor outdoor water use proportion is about 40:60 in the Valley. An important component of the Valley's supply are the return flow credits which SNWA gets for the Colorado river water, the main supply source, that they return back to the river. This return flow mainly comprises the flow from the wastewater treatment plants. The credits process allows SNWA to withdraw additional one unit of river water for every unit of treated river water returned. The objectives of this research are (i) evaluating the extent to which the present available water supply can fulfill the water demand in the Valley in the future. This involves assessing the impacts of various water conservation policies and population projections on water supply and demand in the Las Vegas Valley (ii) evaluating the magnitude and interrelationship of the different outdoor water use components, their response to water conservation policies and their potential for water savings. This involves quantifying outdoor water use in response to water conservation, estimating the effect of nitrate loading in reuse water on the quality of shallow groundwater, and evaluating the potential for water savings from turf replacement in the Valley.To accomplish the research objectives, a water balance simulation model for the Valley has been developed, which documents the water cycle of the Valley and can be used to explore several what-if questions. System Dynamics (SD) modeling approach and software tool Stella are used to develop the model that runs the simulations from 1993 to 2035 while keeping track of demographics, water demands, and water supply. The model runs on an annual time step and is calibrated for a period from 1993 to 2008. Five different conservation policies are evaluated for both research objectives. The first policy considers the status quo situation by projecting the 2008 water use levels till 2035. The second policy explores the effect of conserving water only on the outdoor side. The third policy considers equal conservation both on the indoor and outdoor side while the fourth policy considers 67% outdoor and 33% indoor water use conservation. The fifth policy considers conserving water only on the indoor side.The results from the model for the first objective reveal the importance of outdoor water conservation and present it as a key solution in addressing the water problems of the Valley. Water consumption decrease from 945 lpcd (250 gpcd) to 752 lpcd (199 gpcd) if met completely through outdoor conservation, generates the highest return flow credits and can potentially satisfy the Valley's water needs through 2035.For the second objective the all outdoor conservation scenario gives the highest value of return flow credits and the least values for the components of outdoor water use. The impact of wastewater reuse specifically its nitrate loading, on the shallow groundwater aquifer points to a gradual deterioration in the groundwater quality with time. The model assesses the impact of replacing all convertible (non-golf course) turf with desert landscaping in the Las Vegas Valley on water savings, and determines that replacing the turf will result in a 59 lpcd (16 gpcd) decrease in the water demand. The results can be a guide in developing effective outdoor water conservation policies and the water balance model can be potentially used in helping policy makers make informed decisions on various water management issues.