Relative availability of satellite imagery and ship-based sampling for assessment of stormwater runoff plumes in coastal southern California

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. High-resolution satellite imagery is an alternative approach for assessing plume properties, but the availability of satellite imagery can also be weather-dependent. Here we compare the logistical availability of ship-based sampling and the quality of satellite imagery for assessing rainstorm-mediated freshwater plumes. The availability of ship-based sampling was assessed by correlating deployment success of three local ships with wind and wave data and then applying 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 Level 2 SeaWiFS imagery, then analyzing those relationships with respect to cloud cover found in Level 3 AVHRR, SeaWiFS, and MODIS imagery. In the 10 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 one of the methods. The probability of obtaining usable satellite imagery was lowest on the day of a 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.