COMPARISON OF ESTIMATED SEDIMENT LOADS USING CONTINUOUS TURBIDITY MEASUREMENTS AND REGRESSION ANALYSIS By

Suspended-sediment loads commonly are estimated with a streamflow regression model. However, turbidity, which is the reduction in the transparency of water due to suspended and dissolved particles, may be a better surrogate than streamflow in estimating suspended-sediment loads. To test this hypothesis, regression equations that relate suspended-sediment concentrations to discrete turbidity measurements were developed for eight U.S. Geological Survey stream-gaging stations in Kansas. For comparison, estimates also were calculated using simple regression equations with streamflow and multiple regression equations with streamflow and turbidity. Turbidity Measurements and Regression Analysis: Between 1998 and 2001, about 20 discrete water samples were collected at each of the eight stream-gaging stations and analyzed for suspended sediment. Samples were collected throughout a range of streamflow conditions and sediment concentrations. In addition, samples collected for suspended-sediment analysis represented the range of recorded turbidity values, with a nearly equal representation of high and low values at most stations. The eight stations are equipped with water-quality monitors that provide relatively inexpensive, continuous (hourly) measurements of turbidity. The water-quality monitors are serviced at least monthly to check calibration and to verify that the continuous monitor is representing the stream cross section. Site-specific regression equations were developed relating laboratory analyzed suspended-sediment concentrations in the discrete samples to turbidity measurements recorded by the water-quality monitors. Suspendedsediment loads were estimated using the continuous turbidity measurements and were compared to suspendedsediment loads estimated with streamflow measurements. Examples from two of the eight gaging stations are shown in figure 1. Results: Suspended-sediment concentrations in the Kansas River at DeSoto were strongly related to turbidity with a coefficient of determination (R) of 0.987, compared to an R of 0.792 for streamflow. The suspended-sediment concentrations in the Little Arkansas River at Sedgwick were strongly related to both streamflow and turbidity, and the estimated average daily load did not differ substantially between the streamflow and the turbidity equations. The results showed that, in general, suspended-sediment loads at stream-gaging stations where flows are affected by human-related factors (for example, reservoir releases) were more strongly related to turbidity than to streamflow. The Kansas River is affected by a series of reservoirs that act as sediment traps. During large reservoir releases, the streamflow at DeSoto increased, whereas turbidity increases were relatively small. On the other hand, during periods of substantial storm runoff, large increases were seen in both streamflow volume and turbidity values. The difference in sediment loads estimated using streamflow and turbidity regression equations (fig. 1) is about 8 million tons per year for the Kansas River at DeSoto, which demonstrates the need to determine whether streamflow or turbidity is a better surrogate. There are no large reservoirs on the Little Arkansas River to affect the flow, which may be why suspended-sediment concentration is more strongly related to streamflow at this gaging station (compared to the DeSoto station). Turbidity and Other Surrogates Workshop, April 30— May 2, 2002, Reno, NV 100 1 ,000 10 ,000 100 ,000 1 ,000,000 10 ,000,000 Streamflow -estimated load Turbidity-estimated load 1 10 100 1 ,000 10 ,000 100 ,000 1/ 1/ 20 01 2/ 1/ 20 01 3/ 1/ 20 01 4/ 1/ 20 01 5/ 1/ 20 01 6/ 1/ 20 01 7/ 1/ 20 01 8/ 1/ 20 01 9/ 1/ 20 01 10 /1 /2 00 1