Effects of reforesting a degraded Imperata grassland on dominant flow pathways and streamflow responses in Leyte, the Philippines

Increases in soil hydraulic conductivity after reforestation of degraded grasslands due to increases in soil organic carbon content and the number of macropores may increase the amount of precipitation that infiltrates into the soil and thus reduce overland flow. However, it remains unclear to what extent such changes in dominant runoff generation mechanisms affect streamflow responses to rainfall and how this effect depends on the size of the rainfall event. We, therefore, used a space-for-time approach and instrumented two small catchments near Tacloban, Leyte (Philippines) to study their hydrological response to a range of rainfall events. The 3.20 ha Basper catchment is a degraded Imperata grassland, with shrubs and small trees along the streambanks. The 8.75 ha Manobo catchment used to be a degraded grassland but was reforested 23 years prior to the study. The average canopy height is 7.5 m. Precipitation was measured at two places in each catchment using tipping bucket rain gauges. Stream stage and electrical conductivity (EC) were measured continuously behind a V-notch weir from June until November 2013 when the catchments were severely damaged by typhoon Haiyan. Samples were taken from streamflow, precipitation, soil water and groundwater for geochemical and stable isotope analyses. The streamflow response of the degraded Basper catchment was much more flashy and rapid than that of the reforested Manobo catchment. Streamflow and EC changed rapidly during almost every rainfall event in the grassland catchment while in the reforested catchment, the streamflow- and EC-responses were much smaller and occurred only during larger events. Stormflow was largely a mixture of groundwater and precipitation for both catchments, but the fractions of rainfall and groundwater differed significantly. Minimum pre-event water contributions to stormflow were much smaller for the Basper grassland catchment than for the reforested Manobo catchment (medians based on the EC data of 26 and 81%, respectively). This suggests that overland flow occurred frequently and was much more widespread in the grassland catchment than in the reforested catchment. The differences in the timing of the streamflow responses and pre-event water contributions to streamflow were observed across all event sizes, including a large (>150 mm) tropical storm. These results indicate that the dominant flow pathways have changed as a result of reforestation, leading to less overland flow and smaller event-water contributions to stormflow, and thus show that reforestation of degraded land can restore its hydrological functioning if the forest soil is allowed to develop over a sufficiently long period.