Urban Streamflow Response to Imported Water and Water Conservation Policies in Los Angeles, California

Abstract

Los Angeles has a long history of importing water; however, drought, climate change, and environmental mitigation have forced the City to focus on developing more local water sources (target of 50% local supply by 2035). This study aims to improve understanding of water cycling in Los Angeles, including the impacts of imported water and water conservation policies. We evaluate the influence of local water restrictions on discharge records for 12 years in the Ballona Creek (urban) and Topanga Creek (natural) watersheds. Results show imported water has significantly altered the timing and volume of streamflow in the urban Ballona watershed, resulting in runoff ratios above one (more streamflow than precipitation). Further analysis comparing pre‐ vs. during‐mandatory water conservation periods shows there is a significant decrease in dry season streamflow during‐conservation in Ballona, indicating that prior to conservation efforts, heavy irrigation and other outdoor water use practices were contributing to streamflow. The difference between summer streamflow pre‐ vs. during‐conservation is enough to serve 160,000 customers in Los Angeles. If Los Angeles returns to more watering days, educating the public on proper irrigation rates is critical for ensuring efficient irrigation and conserving water; however, if water restrictions remain in place, the City must take the new flow volumes into account for complying with water quality standards in the region.

Intra‐annual variability of urban effects on streamflow

“…Urban watersheds in this study have approximately 30% impervious cover, compared to 3% for rural watersheds (Table 1). Therefore, the increased impervious cover of urban watersheds decreases the importance of antecedent soil moisture—which reduces soil drainage—to produce runoff (Manago & Hogue, 2017; Miller & Hess, 2017).…”

Adapting Urban Water Systems to Manage Scarcity in the 21st Century: The Case of Los Angeles

“…Currently, a broad planning process has been examining opportunities for the channelized Los Angeles River to promote economic development and multibenefit uses such as recreation. But water conservation and cuts to imported water reduce treatment plant outflows that constitute a significant percentage of the artificial summer stream flows, would be reduced (Manago and Hogue 2017). In addition, promoting more stormwater infiltration in upstream basins would decrease downstream urban stream flows across the county in most seasons and years (Porse and Pincetl 2018t;Mika et al 2017b).…”

“…In particular, more extreme rainfall events will require infrastructure capable of capturing larger storms to recharge groundwater basins to meet future water supply goals (USBR 2015;. Aquatic habitats and marshlands will be affected by water conservation, imported water losses, and precipitation changes that reduce runoff (Read et al 2018;Thorne et al 2016;Manago and Hogue 2017), themselves artifacts of the current engineered system. Urban trees may suffer in future years without conversion of the tree canopy to low-water species (Pataki et al 2011;Litvak et al 2013Litvak et al , 2017aLitvak et al , 2017bVahmani and Ban-Weiss 2016).…”

Lawn Irrigation Contributions to Semi‐Arid Urban Baseflow Based on Water‐Stable Isotopes

“…The fraction of applied irrigation water that becomes LIRF depends strongly on the irrigation method (e.g., sprinkler vs. drip), timing, and amount (Bijoor et al., 2014; Oad & DiSpigno, 1997), but also can vary with vegetation type, climate conditions, and soil type. Previous work investigating the effects of lawn‐watering restrictions found that streamflow in an urban watershed in Los Angeles, California was seven times larger compared to streamflow in a nearby nonurban watershed before the restrictions were implemented (Manago & Hogue, 2017). Urban streamflow dropped by 70% during the restriction period, while the nonurban watershed experienced no statistically significant changes in streamflow (Manago & Hogue, 2017).…”

“…Previous work investigating the effects of lawn‐watering restrictions found that streamflow in an urban watershed in Los Angeles, California was seven times larger compared to streamflow in a nearby nonurban watershed before the restrictions were implemented (Manago & Hogue, 2017). Urban streamflow dropped by 70% during the restriction period, while the nonurban watershed experienced no statistically significant changes in streamflow (Manago & Hogue, 2017). Lawn irrigation contributions to streamflow particularly are of interest as the next major focus of municipal water conservation in semi‐arid and arid cities is reduction of urban irrigation (Gober et al., 2016).…”