Theoretical Comparison of Grab and Composite Sampling Programs

Abstract

AbstractA mathematical formula for determining the sampling frequency on the basis of the coefficients of variation of the various parameters of interest of water quality is developed.The uncertainty or the variability of each sampling program is expressed as a function of the sampling design variable in order to construct a performance index which will enable one to select the best sampling program. The notion of information content is used as a basis for the performance index.Under certain circumstances, the information content of a single composite is more than that of a single grab sample. However, the average of a series of grab samples provides more information than the average of a series of composite samples. This happens because the individual information content (entropy) of discrete portions of the composite sample are confounded in the average.It is shown that if the variability of the proportions is greater than the upper limit (provided in Table 2), the information content of the composite, where the volume of each discrete portion is collected in proportion to the rate of flow (or discharge) at the time it is collected, is much smaller than that of the same number of grab samples or composite samples collected at constant volume‐constant time intervals.Time constant, volume proportional‐to‐flow since last sample composites have the same disadvantages.