EROSION AND HYDROLOGY OF STEEPLANDS UNDER COMMERCIAL PINEAPPLE PRODUCTION

The pineapple growing industry of south-east Queensland began as a ''p ioneer'' industry because farmers could purchase and develop steep slo ping land with a small amount of capital. Over a period of 50 years, c onsiderable areas have been abandoned while other new areas have been developed. Such commercial farming is a source of agricultural polluta nts that could endanger the quality of coastal habitats and the expand ing tourism industry. This paper reports 3 years of soil erosion and h ydrology results from one pineapple-growing farm. Field experiments we re established to find out row lengths that would allow sustainable pr oduction from steep sloping land (33-38%). The farmer's up-and-down sl ope row lengths of 7 m were investigated, but also increased to 12 m a nd 22 m for comparison. It was found that the hydrology of the shaley regosol was influenced by exfiltration from small areas on the lower s lopes. ''Talc-like'' lines of soil had high rates of transmissivity in parts of the midslope and water moved in these sub-surface lenses as hillslope throughflow before exfiltrating down slope, When events infl uenced by throughflow were deleted and runoff was separated into thund erstorms and rain depressions, it was found that row length affected p eak runoff rates but not total depth of runoff. Soil erosion of 7 m an d 12 m long rows was very similar but increased by 4 times due to rill ing in the 22 m long rows, Multiple regression analysis showed that pr ecipitation-related variables were important in un-rilled rows while r unoff type variables were significant in rilling. Cumulative El(30) wa s found to be a useful surrogate for soil surface armouring and consol idation in the prediction of soil loss and sediment concentrations for events. Sediment concentrations were strongly influenced also by even t type (thunderstorms versus rain depressions). Beta values (used as a n index of soil erodibility) were also affected by event type, but sho wed a decline through time as the soil consolidated. The study establi shed maximum permissible row lengths for the farming system studied an d provided guidelines for improvement of the system.