Especially on sloping land in the tropics, and under permanent rather than the traditional form of shifting cultivation, soil erosion by water presents a constant threat to the sustainability of crop production. The change from shifting to permanent cultivation, and the continual cultivation of lands previously considered too steep for cultivation, are outcomes of increased population pressure and other factors. During a cropping season, there can be considerable change in soil condition and in the protection of the soil against erosion provided by growing vegetation or by mulch. Therefore, there has been increased interest in methodologies of erosion study which allow an interpretation of erosion outcomes on an event-by-event basis ( Rose, 1993, 1994). The meth­odology illustrated by the Universal Soil Loss Equation (Wischmeier and Smith, 1978) recognised the importance of rainfall rate. However, more recent research has also established the significance of runoff rate. The measurement and recording of such rates has been facilitated by electronic data loggers, and the interpretation of such data assisted by the use of computer programmes.