Calibrating storage tanks for soil erosion measurement from plots

Many plots for soil loss measurements are equipped with a sequence of tanks for measuring runoff volume and sediment concentration. The stored water v olume is easily determined by a water depth measurement while the sediment concentration is often measured by collecting samples of the mixed suspensi on. In this paper, using the IT-theorem of dimensional analysis, the functi onal relationship describing the mixing of the suspension in the tank is ex pressed in a dimensionless form. The recognized dimensionless groups allow the establishment, for given soil and water depth in the tank, of the relat ionship (calibration curve) between the actual and the measured concentrati on. The calibration curve, using measured concentration values obtained fro m sampling taps located at different heights on the vertical of a tank wall , is shown to be linear. For a given soil, the slope of the calibration cur ve is related to the water depth. We also show that the same dimensionless groups allow the deduction of a scale-up procedure and the possibility of u sing a small tank, similar to the field one, for investigating additional e ffects neglected in the theoretical analysis. The effects of the following factors on the calibration curve are examined: (1) sampling direction along the vertical, i.e. starting the sampling from the upper tap to the lower o ne or vice versa; (2) mathematical shape of the concentration profile; (3) representativeness of the calibration curve; (4) sampling volume; (5) water depth into the tank (i.e. filling level); (6) suspension mixing time; (7) grain size distribution of the eroded soil particles for given soil type; ( 8) time between the end of the erosive event and the starting of the sampli ng procedure (delay time). Finally, an attempt is made to define a simpler sampling procedure based on a single measurement with a dipped sampler. (C) 1998 John Wiley & Sons, Ltd.