Research Areas > Stormwater > Stormwater Runoff Plumes Observed by SeaWiFS Radiometer in Southern California Bight

Project: Stormwater Runoff Plumes Observed by SeaWiFS Radiometer in the Southern California Bight

Background and Objectives

Southern California is characterized by a series of well-developed, steep watersheds that are subject to extremely dynamic episodic runoff events, often resulting from large winter rainstorms. These runoff events may result in an increase of pollutants and pathogens in near-shore waters, which can adversely affect coastal ocean and beach water quality. The quantitative relationship between plume size and persistence, and meteorological and hydrological conditions (primarily rainstorm magnitude) is still poorly understood. In addition, there have not been estimates of what portion of the precipitated water forms the plume and what portion is retained in the watershed. Better knowledge of the dispersal patterns of pollutants and pathogens discharged to the coastal ocean would allow more effective management of coastal water quality.

The goal of this study was to analyze the spatio-temporal dynamics of plumes over the San Pedro Shelf in southern California, and to identify factors influencing the incidence and dispersal patterns of the plumes.

Status

This study was completed in 2005.

Methods

Satellite images of 1.1-km spatial resolution from 1997-2003 by the SeaWiFS optical radiometer were analyzed, with a focus on four regions: Ventura, Santa Monica Bay, San Pedro Shelf, and Orange County/San Diego. The area of each plume was detected by the backscattering characteristics of surface waters, (i.e., normalized water-leaving radiation of green-yellow wavelength 555 nm, (nLw555)). Plume area size was correlated with the rainstorm magnitude, which was estimated from atmospheric precipitation averaged over the total area of the watersheds connected to the seashore. Assessing maximum correlation between precipitated rainwater and the plume size, the nLw555 values that best characterized the plume boundaries in each of the four study regions were estimated. Another quantitative characteristic, the “coefficient of persistence," was derived from maximum correlation between rainwater and plume size. This coefficient was related to the speed of freshwater discharge and the time needed for plume water dissipation.

Findings

• The time lag between a rainstorm and the maximum runoff plume size was about one day in the San Pedro Shelf and about two days in the other three regions.

• The main factors regulating the relationship between rainstorm and plume characteristics were the watershed's size, elevation, and land-use characteristics.

• Over San Pedro Shelf, twenty-five percent of rainwater dissipated daily, gradually decreasing the plume size.

• The size, alongshore and cross-shelf translocation of the plumes were modulated slightly by the local circulation, forced remotely by the equatorward wind a few hundred km to the south (along the coast of Baja California). The influence of tidal circulation on the plume area was slight and statistically insignificant.