Suspended-sediment characteristics in four humid tropical watersheds of contrasting land use, Puerto Rico

A hypothetical model of sediment production, delivery, and transport in the humid-tropics of Puerto Rico was developed through statistical analysis of storm-generated sediment loads and hydrologic characteristics from three time categories: (1) the current storm-generated event, (2) previous storm-generated events, and (3) minor yet localized rainfall and runoff between the storm-generated events. The effect of previous storms in reducing sediment availability for the current event operates differently as sediment supply increases in a basin. In sediment-limited systems, such as the forested Rio Icacos basin, a dominant cycle of sediment production and delivery exists, where large previous storm events mobilize sediment on hillslopes and in low-order channels, and transport the sediment to the main channel where it is deposited. In the next event, this deposited sediment becomes the main source of sediment and clockwise hysteresis loops are more common. Small rainfall events that occur between large flow events in forested basins flush some of this deposited sediment and reduce the sediment load of the next large runoff event. Quebrada Blanca, a basin with small sediment sources, pasture (54%) and cropland (8%), has a cycle of sediment production and delivery where large previous storm events and localized rainfall and runoff between major runoff events mobilize and make sediment available for the next large runoff event. Depending on the magnitude of previous events, the mobilized sediment may be transported varying distances in the basin. Larger previous storm events mobilize sediment which is deposited further along the fluvial system, making sediment available to the rising limb portion of the next runoff event. The increased availability of sediment on the rising limb leads to hydrographs where clockwise hysteresis loops dominate. As the previous events lessen in magnitude, less sediment is delivered to the main channel, and sediment in the next event is transported to the collection station after the highest peak flow; counterclockwise loops are common. In the agricultural Rio Caguitas basin, sediment supply increases and a cycle of sediment production and delivery exists where previous rainfall events supply sediment to the system which is flushed by the corresponding large previous flow event. Rainfall between large runoff events also supplies sediment to the system which is flushed by higher runoff between events. In the highly disturbed Rio Piedras, which is undergoing urbanization, sediment is so abundant that large previous runoff and rainfall events and rainfall between events act to remove sediment and reduce sediment loads of the next runoff event. As sediment supply in a basin increases, channel hysteresis loops trend from clockwise to counterclockwise. Disturbance in watersheds draining fine-grain soils will more likely exhibit counterclockwise hysteresis loops as sediment that is entrained remains in suspension longer, often peaking after the peak flow. Flushing sediment is more important as disturbance in a basin increases.