Download - Session 6.5 modelling the effects of adopting agroforestry on basin scale runoff, philippines
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 export1
Gabayan watershed, Carood basin - Bohol
Characteristic Description
Watershed Area 52.05km2 (5205 ha)
Sub basins 14
Elevation Range
(Relief)
7m – 797m (790m)
Mean slope 17.1%
Drainage
Pattern
Dendritic
Mean Annual
Rainfall
1677mm
Soil (dominant) Ubay Clay loam
(31%clay/59%Silt/40
%Sand) pH 5.92
Study Site
Weather Station
3
Current land use
To quantify the effects of land use on ecosystem services:
…..under two land use scenarios vs baseline (S0):
1. S1 - degraded
2. S2 - Conservation Agriculture with Agroforestry (CA + AFS)4
Objectives
Domain Ecosystem Service Indicator
Provision Water supply Water balance
Regulating
Gradual release Q min: mthly mean P
Soil stabilisation Sediment yield(t ha-1 yr-1)
Water quality Sediment concentration(mg l-1)
5
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
Water Balance Components + Sed. Yld & concentration
S1 (Degraded) S2(CA + AFS)
Climate
Soils
Land Cover
Topog-raphy
ArcSWAT 2012 Model inputs
6
Methods: Land Cover Change analysis
1990 Land Cover (S0) 2010 Land Cover (S1)
Mixed annual agriculture (maize, cassava, cash
crops)
Reduced Tillage
AFS: Jackfruit (Artocarpus sp.) + mango (Mangifera indica)
Methods: CA + AFS scenario (S2)
Cowpea (Vigna unguiculata)
Degraded grasslands
Contour planting and naturally vegetated strips
Rubber – Cacao – Coffee
Riparian planting(15m wide buffers in critical sub-basins)
Ipil Ipil (Leucaena leucocephala)
Shrubs & grasses
S1: degraded S2: CA + AFS
7
8
Results: supply indicatorWater balance
338(22%)
427.31(26%)
368.95(22%)
1017.39(61%)
1007.32(60%)
1108.66(67%)
271.91(16%)
196.14(12%)
155.19(10%)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
S0 S1 S2Surface Runoff ET Lateral Flow Baseflow
9
Results: Gradual release indicator
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25Simulated year
(a)
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25
Simulated Year
(b)
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
Results: Soil stabilisationSediment yield
Total Watershed Baseline (S0) Degraded (S1) CA + AFS (S2)
Mean sed. yld.
(t ha-1 yr-1)
17.8 45.5 14.19
% change vs.
baseline
n/a 155 -20
10
S0: 1990 S1: degraded S2: CA + AFS
11
Results: water quality Sediment concentration
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
0
500
1000
1500
2000
2500
1 2 6 8
Sed
. Co
nc
(mg/
l)
Sub Basin
S0 S1 S2
Summary
Indicator S1: Degraded S2: CA + AFS
Surface runoff + 19.80% + 3.43%
Gradual release Shallow ∆ in monthly low flow
Steeper ∆ in monthly low flow
Sediment Yield + 155% -20%
Sediment conc. incritical sub-basins
+ 165% -35%
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
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 CaroodWatershed Model Forest Management Council and local communities for their help, warmth and guidance.
13
Acknowledgements
David Wilson. Associate Graduate Fellow, ICRAF [email protected]