USGS - science for a changing world

Scientific Investigations Report 2007–5224

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5224

Back to Table of Contents

Results

Calibration Equations

Empirical calibrations for optical sensors, developed using regression analysis and used to convert sensor output voltage into SSC, are as follows and also are shown in figures 2 and 3 :

Lagunitas SPT first sensor (n = 114): SSC = –1.229 (OBS) 1.204     (3)

Lagunitas SPT second sensor (n = 249): SSC = –2.253 (OBS) 1.507     (4)

Walker Creek first sensor (n = 94):SSC = –1.287 (OBS) 1.287     (5)

Walker Creek second sensor (n=429): SSC = –2.282 (OBS) 1.60     (6)

Significant scatter in the calibration datasets resulted in moderate-to-poorly fit calibration equations such that the calibrations may not be reliable for actual suspended-sediment predictions.

The total variation in predicted suspended-sediment concentrations using the calibration equations is given by r 2 . The strength of the regression relation can be measured using the r 2 values, which vary from 0.583 to 0.863 ( figs. 2 and 3 ). There is a lower likelihood for predicting suspended-sediment concentrations closest to the true values using calibration equations with lower r 2 values. Although the r 2 values indicate a moderate fit for Walker Creek and the first probe at Lagunitas SPT, the assumed log-linear relation does not hold for the second probe at Lagunitas SPT.

Monitoring Site Data

Suspended-sediment data collected in the Tomales Bay watershed were used to estimate mean daily and annual seasonal suspended-sediment discharge for Lagunitas Creek SPT and Walker Creek, which were published in USGS Annual Water-Data Reports (Webster and others, 2005; U.S. Geological Survey, 2006; 2007). Annual seasonal suspended-sediment yield at Tomales Bay watershed stream-discharge gaging stations is shown in table 1 . Although the drainage area at the Lagunitas Creek SPT and Walker Creek gaging stations are similar, 34.3 and 31.1 mi 2 , respectively, the annual seasonal suspended-sediment yield at Walker Creek is greater. Throughout the 3-year project period, Walker Creek transported 61–68 percent of the combined measured suspended-sediment discharge at the two gaging stations.

The maximum number of OBS data points collected per year, when collected seasonally from October 1 to May 31, is 23,424 data points (96 data points per day for 244 days). The actual number of usable data points in this study was considerably less due to invalid data. The usable percentage of valid optical backscatter data collected at Tomales Bay watershed stream gaging stations ranged from 20 to 87 percent during the 3-year project period ( table 2 ). Interruptions in the OBS data record at all three gaging stations were caused by OBS measurements that exceeded the upper measurement limit, malfunctioning of the sensor and (or) recording equipment, aggradation and scour of the channel bed, and fouling of the sensor. The upper limit of measurable concentrations was violated periodically at all three gaging stations throughout the 3-year project period primarily during peak runoff events. At Lagunitas SPT, the sensor and (or) the data recorder malfunctioned during water year 2005, resulting in data loss from October 1, 2004, through January 21, 2005. Replacement sensors capable of measuring higher concentrations (the sensor gain was turned up) at Lagunitas Creek SPT and Walker Creek were deployed on January 21, 2005. Aggradation of the channel bed periodically buried the sensors at Lagunitas Creek SPT during water years 2004 and 2005 and at Walker Creek during water year 2005, resulting in anomalously high sensor output for associated low measured SSC values. This especially is evident in the calibration data for the second sensor at Lagunitas Creek SPT ( fig. 2 ).

Invalid data were identified and deleted resulting in edited datasets for each gaging station. Streamflow, estimated SSC time-series data, and measured SSC data from samples collected at Lagunitas SPT during water years 2004–06 are pre­sented in figure 4A–C . Streamflow and edited (invalid data removed) but uncalibrated sensor output data collected at Lagunitas PRS during WY 2004–06 are pre­sented in figure 5A–C . Streamflow, estimated SSC time-series data, and measured SSC data from samples collected at Walker Creek during water years 2004–06 are pre­sented in figure 6A–C .

Grain size analysis of suspended-sediment samples collected at Lagunitas Creek SPT and Walker Creek (Appendix B) shows that fine-grained sediment (percent less than 0.063 mm) dominates the suspended load. Fine-grained sediment represented approximately 70 percent of the suspended material collected at Lagunitas Creek SPT and Walker Creek; whereas, only 22 percent of the samples collected at Lagunitas Creek SPT and 28 percent of the samples collected at Walker Creek contained sand-sized particles.

Calibration of the optical-backscatter data requires a relatively stable particle-size distribution. This condition generally was met at Lagunitas Creek SPT, but not at Walker Creek, and may have contributed to scatter in the calibration datasets, thereby resulting in moderate-to-poorly fit calibration equations. Plots of the ratio of suspended-sediment concentration to sensor output voltage, as a function of suspended particles finer than 0.063 mm ( figs. 7 and 8 ) show that at higher concentrations Walker Creek has greater percentages of sand in suspension.

Storm Events

The OBS continuous time-series data captured the rapid temporal variations in sediment concentration during storms throughout water years 2004–06. Peak streamflow, storm duration, estimated suspended-sediment yield, and duration of elevated suspended-sediment concentrations for individual storm events are presented in table 3 . Although regression analyses indicate that OBS calibrations may not be reliable for actual suspended-sediment predictions, the OBS storm data provide a means with which to characterize the relative magnitude and duration of storms that may result in deleterious effects on at-risk aquatic species in the Tomales Bay watershed.

Back to Table of Contents

AccessibilityFOIAPrivacyPolicies and Notices

Take Pride in America logoUSA.gov logoU.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5224
Page Contact Information: Publications Team
Page Last Modified: Thursday, 01-Dec-2016 19:57:33 EST