Rainfall‐runoff responses and hillslope moisture thresholds for an upland tropical catchment in Eastern Madagascar subject to long‐term slash‐and‐burn practices

• Published in: Hydrological processes Vol. 37; no. 8
• Main Authors: Zwartendijk, B. W., van Meerveld, H. J., Teuling, A. J., Ghimire, C. P., Bruijnzeel, L. A.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01.08.2023; Wiley Subscription Services, Inc
• Subjects: Agriculture; Catchment scale; Catchments; Charcoal; Eucalyptus; Grasslands; highlands; Hydrology; Land cover; Land use; Madagascar; Moisture effects; Mosaics; Overland flow; Perennial streams; Physical properties; Precipitation; rain; Rainfall; Regrowth; rice; Rice fields; Runoff; runoff generation processes; Saturation; saturation‐excess overland flow; shifting cultivation; shrubs; Soil; Soil moisture; Soil physical properties; Soil properties; soil water; Stable isotopes; stormflow; Storms; Stream discharge; Stream flow; streams; Surface runoff; swidden agriculture; threshold response; topographic slope; topsoil; trees; Tropical forests; Valleys; Water resources; Water yield; Watersheds; Wetlands; Madagascar
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.14937

Slash‐and‐burn agriculture is an important driver of tropical forest loss and typically results in a mosaic of land uses. However, there is little quantitative information about the hydrological effects of long‐term slash‐and‐burn agriculture and how such mosaics affect the rainfall‐runoff response at the catchment scale. We monitored streamflow responses at two points along a perennial stream in a 31.7 ha catchment in eastern Madagascar that was monitored previously between 1963 and 1972. Land cover in 2015 consisted of degraded grasslands, shrub and tree fallows at various stages of regrowth (64%), eucalypt stands for charcoal production (30%), and rice paddies and wetlands in the valley‐bottom (3%). For the majority (60%) of the events during the study period, the ratio between the total amount of stormflow and rainfall was <3%, suggesting that for these events runoff was generated in the valley‐bottom only. Events for which an antecedent soil moisture storage plus rainfall (ASI + P) threshold was exceeded had much higher stormflow ratios (up to 50%), indicating that a certain wetness was required for the hillslopes to contribute to stormflow. Stable isotope sampling for four small to moderate events indicated that stormflow was dominated by pre‐event water. Total stormflow and annual water yield in 2015 were higher than in the 1960s, despite much lower rainfall in 2015. We attribute these differences to changes in soil physical properties caused by the repeated burning and loss of top‐soil, which has resulted in a reduction in the depth to the impeding layer. The changed runoff‐processes (less infiltration, more saturation‐excess overland flow) thus affect local water resources. Small rainfall events: only the stream surface and valley bottom contribute to stormflow, leading to a small streamflow response. Intermediate rainfall events or moist antecedent conditions: perched water tables develop on the hillslopes, leading to subsurface stormflow and saturation‐excess overland flow at degraded sites and in the riparian zone; stormflow is dominated by pre‐event water. Large rainfall events or wet antecedent conditions: the catchment is hydrologically connected via subsurface stormflow; saturation‐excess overland flow also occurs on up‐slope areas; event‐water dominates stormflow.