FAGNA-TYPE HYDROLOGICAL UNIT FOR RUNOFF MEASUREMENT AND SAMPLING IN EXPERIMENTAL PLOT TRIALS

The Fagna-type unit is a new device for the measurement and sampling of runoff in plot experiments. Its name derives from the Experimental Centre of Fagna where the Istituto Sperimentale per lo Studio e la Difesa del Suolo of Florence set up an experiment on 12 plots for monitoring soil erosion and chemical losses. The working principle is based on a revolving pot which concentrates the runoff and, when filled, discharges it into a structure for conveying the water to a water meter. At every pot discharge the same quantity of runoff is sampled so that, at the end of the runoff event, an integrated sample is collected. Coarse materials that creep and hop at the bottom of the conveying flume are forced to settle in a siltation hopper before reaching the pot. With respect to instruments which use multislot dividers, calibrated weirs and water level recorders , the new equipment directly and totally measures the volume of runoff , avoiding the risks linked to the indirect estimate which sometimes presents errors; especially when runoff is very feeble or in the presence of high peaks. The Fagna-type unit has been successfully tested over four years. On the basis of the experimental results we consider the new equipment suitable for clayey soils and probably applicable in a wider range of conditions.