Will it rise or will it fall? Managing the complex effects of urbanization on base flow

Abstract

Sustaining natural levels of base flow is critical to maintaining ecological function as stream catchments are urbanized. Stream base flow responds variably to urbanization. Base flow or water tables rise in some locations, fall in others, or remain constant. This variable response is the result of the array of natural (e.g., physiographic setting and climate) and anthropogenic (e.g., urban development and infrastructure) factors that influence hydrology. Perhaps because of this complexity, few simple tools exist to assist managers to predict baseflow change in their local urban area. We address this management need by presenting a decision-support tool that can be used to predict the likelihood and direction of baseflow change based on the natural vulnerability of the landscape and aspects of urban development. When the tool indicates a likely increase or decrease, managers can use it for guidance toward strategies that can reduce or increase groundwater recharge, respectively. An equivocal result from application of the tool suggests the need for a detailed water balance. The tool is embedded in an adaptivemanagement framework that encourages managers to define their ecological objectives, assess the vulnerability of their ecological objectives to changes in water-table height, and monitor baseflow responses to urbanization. We tested our framework with 2 different case studies: Perth, Western Australia, Australia and Baltimore, Maryland, USA. Together, these studies show how predevelopment water-table height, climate, and geology together with aspects of urban infrastructure (e.g., stormwater practices, leaky pipes) interacted such that urbanization led to rising (Perth) and falling (Baltimore) base flow. Greater consideration of subsurface components of the water cycle will help to protect and restore the ecology of urban fresh waters.

Stormwater management network effectiveness and implications for urban watershed function: A critical review

“…Although we can make general predictions of effects of different SCM network types on baseflow (Figure E), groundwater recharge, and evapotranspiration (not shown), it is currently challenging to predict how these processes will respond to urbanization for a given watershed or region, thus creating considerable uncertainty as to the starting point to measure SCM effects (Bhaskar, Beesley et al, ). If that conundrum can be sidestepped, then we expect detention‐based SCM networks will not change baseflow or groundwater recharge, though they could increase evaporation (as with water yield).…”

“…The direction of some hydrological metrics (e.g., peak flow) responds consistently to urbanization but exhibit large ranges in magnitude of response at similar levels of urbanization due to physiographic differences (Hopkins et al, ). Other hydrological responses, such as baseflow, have shown contrasting responses to urbanization because of differences in how regions are urbanized over time, including the extent of vegetation removal or irrigation, as well as the age of stormwater and other infrastructure (Bhaskar, Beesley et al, ; O'Driscoll, Clinton, Jefferson, Manda, & McMillan, ; Price, ). Response to urbanization and to SCM networks can also be influenced by a watershed's capacitance, where capacitance is defined as how well the environmental setting facilitates stormwater run‐off entering long subsurface flowpaths or evapotranspiring (Miles & Band, ).…”

Hierarchical multi‐taxa models inform riparian vs. hydrologic restoration of urban streams in a permeable landscape

“…The majority of rain falls between May and September, with long-term mean annual rainfall between 800 and 1,100 mm. In areas of the metropolitan region, where the depth to groundwater is naturally shallow (<4 m), urbanization causes the water table to rise, making some naturally intermittent streams permanent (Bhaskar et al 2016). Many small streams in the region, particularly those in peri-urban areas, dry during summer when the groundwater level drops below the stream bed (Galvin et al 2009a).…”

Urban base flow with low impact development

“…These studies make it clear that there is no consistent effect of urban development on base flow – both increases and decreases in base flow magnitude with urban development have been observed (O'Driscoll et al , ; Price, ; Hamel et al , ; Bhaskar et al , ). Unlike urban stormflow, which is consistently made flashier by directly connected impervious area, there is diversity in urban base flow response (Hopkins et al , ) because of the multitude of processes that can affect base flow in addition to imperviousness, such as leakage in and out of piped infrastructure, changes in evapotranspiration, import and export of water and wastewater, and, as is investigated here, evolving modes of stormwater management (Bhaskar et al , ).…”