Erosion of termite mounds under natural rainfall in semiarid tropical Northeastern Australia

Abstract

The early stages of erosion of six unoccupied termite mounds at a site in northeastern Australia with a semiarid tropical climate have been studied in order to determine overall erosion rates. An attempt has been made to assess which rainfal parameters are best related to the measured sediment losses from the mounds. The conical mounds, built by Amitermes vitiosus Hill, varied in basal diameter from 0.36 m to 0.47 m and in mass from 35.5 kg to 61.6 kg. Over the two summer wet seasons studied, they lost on an individual mound basis between 4.3% and 10.2% of their masses (average 7.1%). At this rate the mounds would be completely destroyed after the order of 30 years of exposure to natural rainfall, although the actual erosion rates over such a time-span may be non-linear. Kinetic energy and momentum estimates, rainfall erosion indices (EI 5-EI 60), and the maximum amounts of rain falling continuously in 5-minute intervals up to 1 hour long were the most reliable predictors of sediment yields from the mounds. Total rainfall during each sediment sampling period is almost as good a predictor and is much more readily determined from daily rainfall data. The occurrence of high intensity rain (equivalent hourly intensities of 90 mm hr −1 or greater) of short duration during the summer wet season was found to be critically important in the termite mound erosion process. The main effect of such rain would seem to lie in the ability of the large drops to break down the hard-setting, dense, organically-cemented surfaces of the mounds to initiate sediment removal. The finer drops of the less intense rains have a much reduced ability to transport sediment from the mounds.