Using digital cameras for river plume and water quality measurements

Abstract

The results from a pilot study in coastal waters off the east coast of Ireland using both an in situ and airborne mounted digital camera are described. In situ digital pictures are significantly affected by sea surface reflection. This can be eliminated with the use of a hollow pipe, attached to the camera lens, which intersects the sea surface such that a picture of light upwelled from beneath the surface is obtained. Used in this way, linear relations between both the ratio of red/green digital output (O/P) values (at a particular camera exposure) and the difference in green–red digital camera O/P were found with mineral suspended solid (MSS) concentration. A good comparison was also found between the ratios of red/green upwelling light measured with the camera and a conventional irradiance sensor. Semi‐empirical or analytical relationships between camera O/P and the inherent optical properties of the water could not be established, however. This was probably due, in part, to a lack of range in values for the water constituent parameters. An airborne mounted digital camera was used to successfully monitor the dynamics of a river plume discharging into the coastal water. The high dissolved organic material (CDOM) levels within the plume caused the plume to be easily visible in the digital imagery, with a significantly increased signal in the camera's red/green O/P values marking the spatial extent of the plume. The plume dynamics were principally controlled by the tidal flows in the coastal waters. During the ebb tide strong fronts, and inferred convergence zones, marked the edge of the river plume and were associated with increased values of the camera‐derived red/green ratio. As slack water commenced, the plume expanded away from the source and became thinner, with a decreased red/green signal. Associated with this time internal fronts were present within the plume's extent, marked by step changes, or spikes, in the camera's red/green values. 1Now at Fugro Geos Ltd, Hargreaves Road, Swindon, UK.