The role of external and emergent drivers of water use change in Las Vegas

Over the past twenty-five years, per capita water use has declined in many US cities. Technological and policy changes partially explain this decline, but variables beyond the control of water managers also influence water use including external (e.g. drought) and emergent (e.g. public attention) changes. Importantly, these variables interact and the relationships between these variables and water use are non-stationary. However, many models assume fixed relationships between water use and its drivers, and limited interaction between variables. Here we present a flexible socio-hydrological approach to model how conservation strategies, and external and emergent changes, interact to influence per capita water use. We apply this approach to Las Vegas and find that marginal water rates, code changes coupled with population growth, and conservation response to water stress are the key drivers of the observed decline. Critically, modeling these strategies in absence of their connections to population growth and water stress cannot fully account for observed changes.

过去二十五年来，美国多数城市的人均用水量（per capita water use）均有所下降。技术革新与政策调整可部分解释这一下降趋势，但水利管理者无法掌控的其他变量同样会对用水量产生影响，其中包括外部扰动（如干旱）以及偶发型变化（如公众关注度波动）。尤为重要的是，这些变量之间存在交互作用，且各变量与用水量之间的关联并非固定不变（non-stationary）。然而，多数现有模型均假定用水量与其驱动因素之间存在固定关联，且变量间的交互作用十分有限。我们在此提出一种灵活的社会水文学（socio-hydrological）建模方法，用以模拟节水策略、外部扰动与偶发型变化如何交互作用，进而影响人均用水量。我们将该方法应用于拉斯维加斯市的案例研究，结果发现边际水价（marginal water rates）、配合人口增长推行的用水法规修订，以及针对用水压力的节水响应，是观测到的用水量下降的核心驱动因素。至关重要的一点是，若在建模时忽略这些策略与人口增长及用水压力之间的关联，则无法完全解释观测到的用水量变化。