UC Irvine

Urbanization adversely impacts stream water quality and ecosystem services. It also disrupts the water and sediment budgets of streams. Novel technologies and out-of-the-box approaches are required to address the impaired stream health and function in human-impacted catchments. In this thesis, I look at the degraded condition of urban streams from both hydrological and water quality perspectives, and propose methods for returning the impaired urban streams condition close to the preurban state. From the hydrological standpoint, I explore the role of low impact development technologies in restoring natural water balance over urbanized catchments. I demonstrate that over annual time scales, the volumes of stormwater that should be infiltrated and harvested can be estimated from a catchment scale water balance given local climate conditions and preurban land cover. I conclude that for all but the wettest regions of the world, a much larger volume of stormwater runoff should be harvested than infiltrated to maintain stream hydrology in a preurban state. From the water quality perspective, I investigate the negative impacts of excess in-stream nitrate concentration on human health and the ecosystem; and develop a catchment scale modeling framework for its management. This modeling framework advances the state-of-the-art by taking into account the natural ability of streams in treatment of nitrate (by biotic assimilation and denitrification). My model builds on a seminal study [Mulholland et al., 2008, Nature, 452, 202-205] that found in-stream treatment of nitrate declines non-linearly with increasing nitrate concentration in a stream. I explore the implications of this result for nitrate management in an urbanizing watershed in southeastern Australia. To this end, I couple the correlation for in-stream processing of nitrate with a stream network model of the Jacksons Creek watershed (Victoria, Australia). By exploring various scenarios for nitrate loading rate in the effluent of a recycled water plant within the catchment, stream network model predicts that as nitrate loading from a sewage treatment plant increases (or decreases), Jacksons Creek responds by reducing (or increasing) in-stream nitrate removal. Thus, the non-linear nature of in-stream treatment may reinforce socio-ecological feedback loops that drive urban streams into healthy or degraded states.