Impacts of Climate Variability and Land Use Alterations on Frequency Distributions of Terrestrial Runoff Loading to Coastal Waters in Southern California¹

Abstract

The transport of water, sediment, dissolved and particulate chemicals, and bacteria from coastal watersheds affects the nearshore marine and estuarine waters. In southern California, coastal watersheds deliver water and associated constituents to the nearshore system in discrete pulses. To better understand the pulsed nature of these watersheds, frequency distributions of simulated runoff events are presented for: (1) three land use conditions (1929, 1998, 2050); (2) three time periods (all water years 1989‐2002), only El Nino years (1992, 1993, 1995, 1998); and only non‐El Nino years; and (3) three regions (watershed, uplands, and lowlands). At the watershed scale, there was a significant increase (>200%) in mean event runoff from 1929 to 2050 (0.4‐1.3 cm) due to localized urbanization, which shifted the dominant sources of runoff from the mountains in 1929 (78% of watershed runoff) to the coastal plane for 2050 conditions (51% of watershed runoff). Inter‐annual climate variability was strong in the rainfall and runoff frequency distributions, with mean event rainfall and runoff 66 and 60% larger in El Nino relative to non‐El Nino years. Combining urbanization and climate variability, 2050 land conditions resulted in El Nino years being five times more likely to produce large (>3.0 cm) runoff events relative to non‐El Nino years. Combining frequency distributions of event runoff with regional nutrient export relationships, we show that in El Nino years, one in five events produced runoff ≥2.5 cm and temporary nearshore nitrate and phosphate concentrations of 12 and 1.4 μM, respectively, or approximately 5‐10 times above ambient conditions.

Life Histories, Salinity Zones, and Sublethal Contributions of Contaminants to Pelagic Fish Declines Illustrated with a Case Study of San Francisco Estuary, California, USA

“…Winter and spring storms can increase nitrogen and contaminant loading to concentrations that can be acutely toxic (Fig. 3, effect A) (Beighley et al 2008;Kuivila and Hladik 2008). Storm runoff can double or triple mercury loads (Domagalski et al 2004) and increase concentrations of pesticides as much as 10-fold (Brady et al 2006;Guo et al 2007).…”

Integrating hydrologic modeling and land use projections for evaluation of hydrologic response and regional water supply impacts in semi-arid environments

“…), conveyance (culverts, channels, pipes) as well as detention systems, and transition from native to non-native vegetation species (grass, palms, woody plants, etc.). These conversions typically result in hydromodification of stream systems, increasing wet weather runoff and shifting the timing and volume of flood peaks (Murdock et al 2004;Wissmar et al 2004;White and Greer 2006;Vicars-Groening and Williams 2007;Beighley et al 2008;Andriani and Walsh 2009;Im et al 2009;Quan et al 2010). Increased flow rates typically lead to extensive channel erosion and instability (Bledsoe and Watson 2001;Miller and Kochel 2010).…”

Socio‐ecohydrology and the urban water challenge

“…Impervious surfaces and irrigation of urban landscapes cause large increases in dry-and wet-weather runoff and alter the timing and volume of flood peaks relative to natural riparian systems (Tong and Chen, 2002;Murdock et al, 2004;Wissmar et al, 2004;White and Greer, 2006;Beighley et al, 2008). This can lead to extensive channel erosion and instability (Bledsoe and Watson, 2001) and alter long-term recharge to regional aquifers (Simmons and Reynolds, 1982;Finkenbine et al, 2000), impacting water availability.…”