Flow-variation-paced sampling: a method for automatic sampling of streamflow during peak runoff periods

Automated sampling of streamflow water in small watersheds is usually controlled by a flow-paced algorithm (i.e. sampling rate is proportional to discharge, l s−1). However, this sampling method typically takes too few samples during runoff peaks (the critical periods for assessment of suspended solids losses) and too many samples during periods of basal or near-basal discharge (increasing the likelihood that there will be no sample bottles left when an important event occurs). We describe a sampling algorithm designed to overcome these problems: specifically, sampling rate is made proportional to the absolute rate of change in discharge (l s−2). We call this approach “flow-variation-paced (FVP) sampling”, and have developed a simple device for its implementation. This device has proved reliable in field trials. Furthermore, simulated sampling studies using data from a small agricultural watershed in northwest Spain indicate that estimation of suspended solids losses on the basis of FVP sampling is considerably more accurate than estimation on the basis of conventional flow-paced sampling.