Published on February 19, 2014
Modelling the effects of adopting agroforestry on basin scale surface runoff and sediment yield in the Gabayan watershed, Bohol, Philippines. David M Wilson1, 2 & Rodel D Lasco1, 1World Agroforestry Centre (ICRAF) Philippines 2SESAM, University of the Philippines Los Baños Presented to the World Congress on Agroforestry, New Delhi, India February 12th 2014 Breakout session: 6.5 Agroforestry, water quality and nutrient export 1
Study Site Gabayan watershed, Carood basin - Bohol Characteristic Weather Station Description Watershed Area 52.05km2 (5205 ha) Sub basins 14 Elevation Range 7m – 797m (790m) (Relief) Mean slope 17.1% Drainage Pattern Mean Annual Rainfall Dendritic 1677mm Soil (dominant) Ubay Clay loam (31%clay/59%Silt/40 %Sand) pH 5.9 2
Current land use 3
Objectives To quantify the effects of land use on ecosystem services: Domain Ecosystem Service Indicator Provision Water supply Water balance Gradual release Q min: mthly mean P Soil stabilisation Sediment yield (t ha-1 yr-1) Water quality Sediment concentration (mg l-1) Regulating …..under two land use scenarios vs baseline (S0): 1. S1 - degraded 2. S2 - Conservation Agriculture with Agroforestry (CA + AFS) 4
Methods: Soil and Water Assessment Tool (SWAT) 1. Model input preparation i. Time series climatic data (25yrs): rh, wind, s.rad, P&T ii. Land cover iii. Soils - FAO iv. 30m DEM 3. Parametisation, calibration & validation 4. Scenario development 5. 3 runs – SO, S1, S2 ArcSWAT 2012 Model inputs Topography Land Cover Soils Climate Water Balance Components + Sed. Yld & concentration S1 (Degraded) S2(CA + AFS) 5
Methods: Land Cover Change analysis 1990 Land Cover (S0) 2010 Land Cover (S1) 6
Methods: CA + AFS scenario (S2) S1: degraded S2: CA + AFS Reduced Tillage Mixed annual agriculture (maize, cassava, cash crops) Degraded grasslands AFS: Jackfruit (Artocarpus sp.) + mango (Mangifera indica) Cowpea (Vigna unguiculata) Contour planting and naturally vegetated strips Rubber – Cacao – Coffee Ipil Ipil (Leucaena leucocephala) Riparian planting (15m wide buffers in critical sub-basins) Shrubs & grasses 7
Results: supply indicator Water balance 100% 90% 196.14 (12%) 155.19 (10%) 1007.32 (60%) 1108.66 (67%) 427.31 (26%) 271.91 (16%) 368.95 (22%) 80% 70% 60% 50% 1017.39 (61%) 40% 30% 20% 10% 338 (22%) 0% S0 Surface Runoff ET S1 Lateral Flow Baseflow S2 8
Results: Gradual release indicator 1 1 (a) 0.8 0.8 0.6 0.6 0.4 0.4 0.2 (b) 0.2 0 0 0 5 10 15 Simulated year 20 25 0 5 10 15 20 25 Simulated Year Lowest monthly discharge relative to mean monthly rainfall* in each hydro. year (a) – S1: degraded (b) S2: CA & AFS See van Noordwijk et al. 2006 & 2011 for more indicators 9
Results: Soil stabilisation Sediment yield S0: 1990 S1: degraded Total Watershed Mean sed. yld. (t ha-1 yr-1) % change vs. baseline S2: CA + AFS Baseline (S0) Degraded (S1) CA + AFS (S2) 17.8 45.5 14.19 n/a 155 -20 10
Results: water quality Sediment concentration Sed. Conc (mg/l) 2500 2000 1500 1000 500 0 1 2 6 8 Sub Basin S0 S1 S2 Mean monthly in-stream sediment concentration Riparian Buffer: 15m either side of streams in critical sub-basins (1,2, 6, & 8) Fast growing Ipil Ipil (Leucaena leucocephala) – fuelwood, charcoal and mulch Bank stabilisation and sediment trap 11
Summary Indicator S1: Degraded S2: CA + AFS + 19.80% + 3.43% Gradual release Shallow ∆ in monthly low flow Steeper ∆ in monthly low flow Sediment Yield + 155% -20% Sediment conc. in critical sub-basins + 165% -35% Surface runoff Key messages: 1. SWAT provides a reasonable estimation of hydrologic function at basin scale 2. Ecosystem services are under threat based on current land use 3. Combining Conservation Agriculture with strategically located agroforestry systems could improve water quality and reduce sediment yield 12
Acknowledgements This research was conducted as part of an Environmental Science MSc at University of the Philippines, Los Banos and is generously supported by ICRAF, Philippines via an Associate Graduate Fellowship. Special thanks to the members of the Carood Watershed Model Forest Management Council and local communities for their help, warmth and guidance. David Wilson. Associate Graduate Fellow, ICRAF Philippines email@example.com 13
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