Imhoff cone determination of sediment in irrigation runoff

There is a need to rapidly quantify erosion from irrigated farmland. The prevailing method consists of collecting runoff samples, then filtering, drying and weighing them to determine sediment concentration. Labor cost and slow data availability prompted development of a faster, less expensive technique. Sediment settling volume in a graduated vessel was expected to correlate well with total mass of suspended sediment. Eight soils varying in texture, mineralogy, and organic-matter content were sampled, fragmented, and air dried. A series of 1-L suspensions was prepared with sediment concentrations from 1 to 30 g L-1. Samples were either hand shaken for 30 s or mechanically blended for 60 s. Suspensions were decanted into graduated Imhoff cones and allowed to settle for 0.5 h (1800 s). The series was repented three times for each soil. Settling volume was regressed against sediment concentration (total sediment, g L-1). Field calibrations for two soils were developed from furrow runoff samples. Laboratory regressions had a mean r2 of 0.99. Field regressions of two soils had r2 of 0.94 or higher. Cone design did not permit accurate volume estimates o(the first 1 mL, causing slopes and intercepts to very among field regressions for sediment concentrations < 1.0 g L-1. These samples, however, represent negligible erosion, and therefore have little value. Slope and intercept of field regressions corresponded closely to 30-s-shaken laboratory regressions but different statistically at P less than or equal to 0.05. The technique provided a rapid, inexpensive, and accurate. rate suspended-sediment determination in the field for concentrations > 1.0 g L-1. Several settling-volume predictions based on textural components and organic-matter content had r2 > 0.60. Laboratory 30-s hand-shaken calibrations may be adequate for diagnostic purposes, but individual field calibrations should be performed for research purposes.