Partitioning of rainfall and flowpath processes in small oil palm catchments

Abstract

Oil palm area forms the second largest land-use fraction in Malaysia, after forest. Therefore, scientific understanding of hydrological processes in oil palm catchment is crucial for an improved catchment and water resources management. This important issue was examined, involving three small catchments, namely C1, C2 and C3 in Sedenak, Johore. Rainfall and streamflow were continuously recorded in all catchments. Throughfall and stemflow were monitored for interception loss computation. Neutron scattering and resistivity imaging system were applied for measuring soil moisture and generate stratigraphy pattern. Storm water samples were analysed for oxygen-18 and silica concentration for hydrograph separation to determine event and pre-event water. The revised Gash‘s model was also applied to estimate interception loss. On an average, 28.9% of the gross rainfall was intercepted by the canopy and 71.1% reaches the ground surface in C3. Throughfall and stemflow account for 66.9% and 4.2% of the rainfall, respectively. About 14.5% of the rainfall was infiltrated into the soil and recharge the soil moisture. During storm events, stormflow and baseflow constituted up to 56.6% of the total rainfall. The hydrographs were dominated by event water during wet seasons and by pre-event water during dry seasons. The stratigraphy patterns revealed that streamflow leakage to deeper layer might contribute to lower stormflow response factors and runoff coefficient in C1. The four components of the interception loss in C3, i.e. canopy storage capacity, trunk storage capacity, evaporation and evapotranspiration were estimated at 8.8%, 0.3%, 8.5% and 11.3%, respectively. The predicted interception loss derived from the revised Leyton and Gash methods were 13% and 11%, respectively. Both models were found to underestimate the measured interception loss and therefore not applicable in oil palm catchments. Radioisotope technique is a better approach for establishing time of concentration as it is able to determine the actual travelling times of both overland and channel flow in the field. The water budgets in the study catchments seem comparable with forested catchments. However, detailed study on the long term flow regime and water quality is still necessary to formulate management guidelines for protecting water resources in oil palm catchment.