Contrasting headwater runoff generating mechanisms in fire-climax grassland and semi-mature secondary forest, Leyte, Philippines
Thursday, 9 June 2016
Jun Zhang1, Cecile M.O. Quiñones2, Roger Tripoli3, Ilja H.J. van Meerveld4, Victor B. Asio2 and L.A. (Sampurno) Bruijnzeel5, (1)VU University Amsterdam, Earth and Life Sciences, Amsterdam, Netherlands, (2)Visayas State University, Agriculture and soil science, Baybay City, Leyte, Philippines, (3)Visayas State University, ACIAR project, Forestry Department, Baybay City, Leyte, Philippines, (4)University of Zurich, Zurich, Switzerland, (5)King's College London, Department of Geography, London, United Kingdom
Abstract:
Although secondary forests have become a dominant land cover across the humid tropics, their storm runoff generating behaviour has received little study. The same holds for degraded fire-climax grasslands which occupy large tracts of land in South-east Asia. This poster documents and explains contrasting patterns and mechanisms of stormflow (Qs) generation based on one year of data from two headwater catchments near Tacloban, eastern Leyte, Philippines. The Manobo catchment (9.9 ha) has a semi-mature (22 yr-old) secondary forest, while the Basper catchment (3.6 ha) is dominated by landslide infested grassland and young (<5 yr) regrowth. Both catchments are underlain by ultramafic rock. Median saturated soil hydraulic conductivity (Ksat) of the top 25 cm at Manobo (53 mm h-1, range 0.7–190 mm h-1) was much higher than at Basper (2.4 mm h-1, range 0.1–9.1 mm h-1). Subsoil Ksat decreased to 0.2 mm h-1 at 50–90 cm depth at Basper vs. 7.4 mm h-1 at Manobo. Five-minute rainfall intensities exceeded Amoozemeter based topsoil Ksat for most of the time at Basper vs. for ~5% at Manobo, suggesting frequent Hortonian overland flow (HOF) at Basper but significant HOF at Manobo too. Storm runoff at Basper was ‘flashier’ than at Manobo (R-B flashiness indices of 0.12 and 0.03, respectively) with average stormflow coefficients (Qs/P) of 13% (±14% SD) and 4% (±8% SD), respectively. After the passage of mega-typhoon Haiyan in November 2013, which intensified landsliding at Basper and severely damaged trees at Manobo, Qs/P increased to 22% (±21% SD) and 18%(±25% SD), respectively. Interpreting observed Qs/P values in terms of the minimum contributing area (MCA) concept suggested landslides in the Basper grassland (occupying 8.2% and 13.2% of the catchment before and after the typhoon) to be a major supplier of HOF. The contrasting behaviour of the two catchments was confirmed by chemical and stable isotope tracer based hydrograph separation. We conclude that forest regrowth had a strongly moderating effect on Qs generation.
