A SIMPLE METHOD FOR IMPROVING SOIL LOSS MEASUREMENT IN USLE TYPE EXPERIMENTS

Soil erosion experimentation around the world commonly uses field runoff plots from which runoff and soil loss is collected. Volume of the collected water-sediment mixture is commonly so great that sub-sampling techniques are required to estimate total soil loss in any erosion event. It has been shown that the commonly used method involving a collection tank, agitation of the water-sediment mixture and then sub-sampling can lead to serious under-estimation of total soil loss. This paper outlines and illustrates a practical, simple method for reducing the error by timing any lag that occurs between completion of stirring and sample collection, and measuring the settling velocity characteristics of the sediment involved. Additional key words: USLE, error in USLE, soil loss measurement, settling velocity, Oden theory. Introduction Soil erosion experimentation around the world commonly uses hydrologically defined runoff plots with runoff and soil loss measured at the exit of the plot by collection in a tank system. Following an erosion event, sediment collected in the “silt collection box” is thoroughly stirred, seeking to ensure a uniform distribution of sediment throughout the depth of water in the tank. As soon as feasible after stirring,i.e. after a short time, a sub-sample of the water-sediment mixture in the tank is taken by immersing a collector to a substantial depth beneath the surface of the water in the tank. The (oven dry) mass of this sample of known volume is then factored up to the complete volume of water collected to give the total soil loss from the plot during the erosion event. Soil loss data obtained in this way provides the major body of data collected world wide on soil loss under different soil, climate, and management conditions. In particular it is the basis of the large body of data collected in the USA and summarised in the Universal Soil Loss Equation (USLE) (Wischmeier and Smith 1965, 1978). Experimentation carried out at Scone, NSW, showed that this commonly used method of measuring soil loss could lead to serious error, with actual soil loss being underestimated by a factor of approximately 3 (Lang 1990,1992). This work thus cast doubt on the correctness of the USLE database on soil loss that has been relied upon to the present day to validate process-based erosion prediction technologies (Zhang et al., 1986; Tiwari et al. 2000). Other authors have also noted that there was substantial potential for error in the sampling techniques (Silburn and Hunter, 2002, Herweg and Ostrowski , 1997). The magnitude of error in this methodology is expected to depend upon at least the following four factors: a) Whether stirring of sediment in the collection was sufficiently vigorous to justify the assumption made that it has achieved a uniform distribution throughout the collection tank. b) The lag time between completion of stirring of collection tank contents and sub-sampling. c) The settling velocity distribution of the sediment collected in the tank. d) The actual detailed technique of sediment sampling. ie the method and utensil used to dip the sample. The aim of this paper is to employ the existing theory of sediment settling in water to yield a practical method of making a correction to the estimate of soil loss for errors incurred due to issues (b) and (c) listed earlier. Thus a correction method will be developed and experimentally tested to allow a correction for: (i) any slight delay between stirring and sampling, and to (ii) tailor this correction to the soil type involved, as characterised by its settling velocity characteristics. Further experimentation was also carried out which has implications for the errors listed as (a) and (d). ISCO 2004 13 International Soil Conservation Organisation Conference – Brisbane, July 2004 Conserving Soil and Water for Society: Sharing Solutions Paper No. 603 page 2 Materials and Methods A measured mass of soil was added to some 250mm depth of water in a circular container of diameter 565mm, to yield a mixture of sediment concentration of 20 kg m. The mixture was stirred vigorously with a spade, and samples of the water-sediment mixture taken simultaneously at three depths. The time period between cessation of stirring and sampling was measured with a stopwatch (around 4 seconds). Samples were taken from just below the water surface, just above a hand placed on the container bottom to avoid collecting deposited sediment, with the third sample taken mid-way between the upper and lower samples. Sample containers were 73mm internal diameter and 105mm in length. Containers were inserted in a downward facing direction, and inverted on reaching the required depth. This procedure was replicated four times for each of the two soil types: a vertosol and a krasnozem. Sample data were converted to sediment concentrations which were then plotted against sample depths, yielding an average sediment concentration for the entire depth. The result was then used to calculate the mass of sediment in suspension at the time. The physical basis of the correction method is as follows. Neglecting any error of the type described in (a) above (which is generally unknowable), it is assumed that the sediment is fully stirred with uniform spatial distribution of both sediment density and size (or settling velocity) characteristics. This condition is assumed to exist at time t=0. The Oden theory for sedimentation of a polydisperse suspension (Oden 1924) given for example in Lovell and Rose (1988), describes the settling of an initially uniform sediment suspension of depth L. After settling for time t there are two classes of particles: (a) those that have completely deposited on the bottom of the container, and (b) those for which only some of the particles have deposited. Using equation (6) in Lovell and Rose (1988) an expression follows relating the actual mass of sediment collected from the tank at time t, Ms(t), to the correct total mass of sediment in the tank (M). This ratio of M to Ms(t), which is of the nature of a correction factor, is given by,