Research Areas > Stormwater > Satellite Imagery vs. Ship-based Sampling for Assessment of Stormwater Runoff Plumes

Project: Satellite Imagery vs. Ship-based Sampling for Assessment of Stormwater Runoff Plumes

Background and Objectives

Characterization of potentially polluted stormwater plumes using satellite and aerial imagery is an important step toward managing stormwater effects on the coastal ocean. Information about the size and intensity of urban runoff plumes in the ocean has traditionally been collected through ship-based surveys, but sampling from ships in the nearshore zone is weather-dependent because of the rough sea conditions that often accompany storms. Satellite imagery can be used as a proxy for field-based measurements of stormwater plume characteristics including salinity, suspended sediments, and colored dissolved organic matter (CDOM). Advantages of using satellite imagery (rather than field sampling) include lowered cost and achievement of synoptic coverage, although the availability of satellite imagery can also be weather-dependent due to interference by cloud cover.

The goal of this project was to compare the logistical availability of ship-based sampling data to the availability and quality of satellite imagery for assessing freshwater plumes from rainstorms in coastal southern California.

Status

This study was conducted from 2005 to 2006.

Methods

A total of 123 rainstorms between 1997 and 2004 in four regions (Ventura, Santa Monica, San Pedro, and San Diego) were assessed using both ship-based and satellite data. The availability of ship-based sampling data was assessed by correlating the deployment success of three local ships with wind and wave data, and then extrapolating those relationships to a longer wind and wave data record. The quality of satellite imagery was assessed by correlating cloud cover and expert opinion about the usefulness of four types of data from NASA-operated satellites. These included: (1) Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Level 2 normalized water-leaving radiation of 555-nm wavelength; (2) SeaWiFS Level 3 chlorophyll; (3) Advanced Very High Resolution Radiometer (AVHRR) Level 3 Sea Surface Temperature (SST); and (4) Moderate-Resolution Imaging Spectroradiometer (MODIS) Level 3 SST.

Findings

In the ten days following storm events, ships were found to be capable of deployment for sampling about 70% of the time, while SeaWiFS produced high quality images only about 23% of the time. The days for which satellite imagery and ship-based data were available often differed, yielding complementary, rather than redundant, information. As a result, plume data were available for about 80% of the study period using at least one of the methods. The probability of obtaining usable satellite imagery was lowest on the day of the rainstorm and increased during the next 5 days, whereas the probability of obtaining ship-based data was highest on the day of the storm and typically declined in the days following a storm. MODIS sensors provided better coverage than SeaWiFS or AVHRR due to better spectral, spatial, and particularly temporal resolution (twice a day), thereby significantly improving information about plume dynamics.