Evaluation of Pressure Transducers under Turbid Natural Waters*

Abstract

Pressure measurements made in two turbid natural waters have led to the inference that the effective depth-mean in situ density values, ?eff, of these waters are less than (≈2.70%?6.5%) their bulk densities (i.e., densities of water?sediment mixture), and also less than (≈0.4%?4.5%) that of the density of the same water after removal of suspended sediment. The values of ?eff in a given site differed from one tidal cycle to another (≈1.9%). These values varied slightly (<0.8%) from midtide to slack water period of the same tidal cycle, with ?eff being lower at midtide. It was found that the use of bulk density to estimate tidal elevation yielded an underestimation of tidal range (up to 7%). The underestimation has been corrected (to within ±1.5%) with the use of ?eff parameter. For clear waters there was no measurable underestimation in tidal range. The observations indicate an apparent in situ density reduction for turbid natural waters. With the use of two pressure transducers at a known vertical separation, the value of ?eff over this vertical column of water may be determined during each sampling of pressure values. The present studies indicate that when pressure transducers are used for water level measurements in turbid natural waters, the use of ?eff, in contrast to the bulk density, significantly improves the measurement accuracy. For clear waters, precision density measurements made on discrete water samples agreed with ?eff values derived from pressure measurements to better than ±0.4%. Thus, use of ?eff is expected to improve the accuracy of water level measurements also from clear water estuaries where depth-mean water density undergoes marginal changes with differing phases of tide and significant changes with seasons.