资源环境科技发展态势分析平台

Reforestation of degraded grasslands can increase the soil hydraulic conductivity and number of preferential flow pathways. However, it is not clear to what extent these changes affect streamflow responses and whether this depends on the event size. We, therefore, studied the hydrological response of two small catchments near Tacloban, Leyte (the Philippines): a degraded Imperata grassland catchment and a catchment that was reforested 23years prior to our study. Precipitation, stream stage, and electrical conductivity were measured continuously from June to November 2013. Samples were taken from streamflow, precipitation, groundwater, and soil water for geochemical and stable isotope analyses. Streamflow and electrical conductivity changed rapidly during almost every event in the grassland catchment, but in the reforested catchment, these responses were much smaller and only occurred during large events. Streamflow was a mixture of groundwater and precipitation for both catchments, but the maximum event water contributions to streamflow were much larger for the degraded grassland than for the reforested catchment. The differences in the event water contributions and timing of the streamflow responses were observed across all event sizes, including a large tropical storm. Together with the low saturated hydraulic conductivity in the degraded catchment, these results suggest that overland flow occurred more frequently and was much more widespread in the degraded grassland than in the reforested catchment. We, therefore, conclude that reforestation of a degraded grassland can change the dominant flow pathways and restore the hydrological functioning if the forest soil is allowed to develop over a sufficiently long period.

Plain Language Summary It is not clear yet to what extent reforestation of degraded tropical grasslands changes the response of streams to rainfall events and whether this depends on the size of the event. We, therefore, studied two small catchments near Tacloban, Leyte (the Philippines): a degraded grassland catchment and a catchment that was reforested 23years prior to our study. Streamflow and stream water chemistry changed rapidly during almost every rainfall event in the grassland catchment, while in the reforested catchment, these responses were much smaller and only occurred during large rainfall events. Together with the slow rate with which water can infiltrate into the soil, these results suggest that water flowed more frequently over the soil surface and this overland flow was much more widespread in the grassland catchment than in the reforested catchment. The differences in the maximum fractions of rainfall in stream water and the timing of the streamflow responses were observed for all events, including a large tropical storm. This indicates that the pathways that the rain takes toward the stream have changed as a result of reforestation. We, therefore, conclude that reforestation of a degraded grassland can improve streamflow regulation if the forest soil is allowed to develop sufficiently.