  Cities are the drivers of socio-economic innovation, and are also forced to
address the accelerating risk of failure in providing essential services such
as water supply today and in the future. Here, we investigate the resilience of
urban water supply security, which is defined in terms of the services that
citizens receive. The resilience of services is determined by the availability
and robustness of critical system elements, or 'capitals' (water resources,
infrastructure, finances, management efficacy and community adaptation). We
translate quantitative information about this portfolio of capitals from seven
contrasting cities on four continents into parameters of a coupled systems
dynamics model. Water services are disrupted by recurring stochastic shocks,
and we simulate the dynamics of impact and recovery cycles. Resilience emerges
under various constraints, expressed in terms of each city's capital portfolio.
Systematic assessment of the parameter space produces the urban water
resilience landscape, and we determine the position of each city along a
continuous gradient from water insecure and non-resilient to secure and
resilient systems. In several cities stochastic disturbance regimes challenge
steady-state conditions and drive system collapse. While water insecure and
non-resilient cities risk being pushed into a poverty trap, cities which have
developed excess capitals risk being trapped in rigidity and crossing a tipping
point from high to low services and collapse. Where public services are
insufficient, community adaptation improves water security and resilience to
varying degrees. Our results highlight the need for resilience thinking in the
governance of urban water systems under global change pressures.
