Relative turbidity index

Research shows that infrared and visible spectral reflectance recorded by satellite or aerial sensors can be used to reliably estimate turbidity, particularly as related to suspended sediment loads, with notable accuracy in water bodies of moderate depths (Carpenter and Carpenter, 1983; Ritchie, et al., 1983). By incorporating satellite imagery, performance of uniform suspended sediment monitoring for the 25 million acre LMAV could be realized with significant efficiencies. Such work is critically needed considering the scale of the landscape and the region's remoteness, both of which limit monitoring and leave many waterbodies unevaluated for biological impairment. As a pilot effort, Landsat 5 TM imagery for Path 23 Rows 34-39 and Path 24 Row 36 was acquired for January 17, 2005 (January 24 for Path 24 Row 36). Winter imagery was chosen to minimize the effects of leaf cover limiting visibility of waterbodies and to minimize algal effects on water clarity. The chosen date resulted in predominantly cloud-free coverage of the LMAV. Image processing occurred first by extracting all surface water using a thresholding technique, then the surface water layer was used to mask the original imagery. A turbidity sensitive band ratio, known as the Relative Turbidity Index (RTI), was derived for areas where surface water was observed. The top tercile (amounting to less than .1% of classified pixels) of the RTI values were recoded to facilitate easier discernment of turbidity changes using a color ramp. Sites observed to correspond with the high RTI range can be assumed to carry higher suspended sediment loads than those at lower ranges. Research as cited above suggests accurate turbidity measurements such as Secchi Disk Depth can be estimated from satellite imagery. However, calibrating the satellite-based turbidity estimates to actual on the ground turbidity measurements requires a range of water quality samples. DU was unable to acquire water quality monitoring data for the dates of imagery. Currently, daily turbidity monitoring stations are rare in the LMAV. DU was able to acquire data for only two known stations, and these two had been logging data for less than two years. However, as other monitoring stations come online and additional data streams are made available, the use of satellite-borne sensors to estimate and monitor turbidity seems very promising. The Relative Turbidity Index indicates the potential benefit of these techniques for identifying high-turbidity zones as well as the potential application as a replicable broad-scale monitoring technique. As the RTI currently exists, it can be used within and across waterbodies to identify relative changes in turbidity. Furthermore, by identifying sites or zones of stark RTI value change within a waterbody, planners can isolate likely zones of high sediment loading for further on-the-ground investigation and focusing conservation efforts to sites of greatest benefit.