A comparison between modified splash-cup and flume techniques in differentiating between soil loss and detachability as a result of rainfall detachment and deposition

Abstract

Temporal changes in soil loss rates as a result of rainfall detachment were measured in modified splash-cups (kc) for two contrasting soil types with 5 mm depth of surface water at two constant rainfall rates (56 and 100 mm h-1). Results were compared with those from a flume (kf) for the same rainfall duration, rainfall rates, soil types and water depth. Reasons are given why soil loss rate commonly measured from splash-cups is not a true measure of soil detachment by rainfall when surface water is present. In order to yield the true rate of soil detachment, the measured net rate of soil loss must be augmented by a correction accounting for the rate of deposition. Theory for the net outcome of rainfall detachment and sediment deposition was used to interpret net soil loss data at equilibrium from splash-cups to yield true soil detachment rates (eTc), and compared those from a flume (eTf ). The two soil types were a cracking clay (black earth or Vertisol) and a slightly dispersive sandy clay loam (solonchak or Aridisol). Splash-cup modification allowed the proportion of sediment lost as airsplash (and therefore not deposited within the splash-cup) to be quantified to allow calculation of true soil detachment rates, and hence true soil detachabilities. Under constant rainfall rates and water depth, kc decreased significantly (5% level) with time until an equilibrium detachment rate was reached. This decrease was attributed to the development of a deposited layer on the soil surface, coarser in texture than the original soil. Values of kc were higher for the solonchak than the black earth, and increased with rainfall rate. At equilibrium, eTc and qf were approximately three orders of magnitude greater than kcand kf, illustrating the importance of recognizing the deposition process in determining true rates of soil detachment and soil detachabilities. There was no significant difference (5% level) between kc and kf at equilibrium for the black earth, but values of kc were significantly higher (5% level) than kf for the solonchak. There were no significant differences (5% level) between qc and eTf for both soil types at the low rainfall rate, but eTc were significantly lower than eTf for both the black earth (5% level) and solonchak (0.1% level) at the high rate.