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

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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

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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

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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.

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Acknowledgements

David Wilson. Associate Graduate Fellow, ICRAF Philippinesd.m.wilson@cgiar.org