Tropical forests and water flows: from small watersheds to the pantropics

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Tropical forests and water flows from small
watersheds to the pantropics

• Functional Value of Biodiversity Project
• with support from
• World Bank Netherlands Partnership Program
• FVOB is a component of ASBs crosscutting
  assessment
• Forest and Agroecosystem Tradeoffs in the Humid
  Tropics
• a sub-global component of
• the Millennium Ecosystem Assessment (MA)

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Team members

• Jeffrey Richey, UW
• Ellen Douglas Charles Vörösmarty, UNH
• Kate Sebastian Stanley Wood, IFPRI
• Kenneth Chomitz, DECRG
• Meine van Noordwijk Thomas Tomich, ICRAF
• plus other team members not here today
• plus contributions of others not directly
  involved in this project

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Overview

• Introduction
• Integrated assessment of real concerns
• Framework
• Micro scale
• Meso scale
• Pantropic scale
• Conclusions

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The ASB Matrix
TP Tomich
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ASB Matrix for the Forest Margins of Sumatra
TP Tomich
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Strong coincidence of watershed functions,
biodiversity and poverty alleviation agendas
Little overlap, separate attention for
watershed_functionspoverty and biodiversity
poverty
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Tropical Forest Biomes
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River Basins Containing Tropical Forest Biomes
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Population density within the pan-tropic basins
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Share of pre-industrial land cover converted by
1992/93 (Contemporary)
Conversion is the sum of agricultural (cropland
and pasture rainfed irrigated) and urban land
cover contained in the contemporary land cover.
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Table 1. Measurability of land use impacts by
basin size (Kiersch and Tognetti, 2002) x
Measurable impact No measurable
impact ___________________________________________
________ Impact Type Basin size km2 0.1 1
10 102 103 104 105 __________________________
__________________________________________________
_________ Thermal regime x x Pathogens
x x x Average flow x x x x
Peak flow x x x x Base flow x
x x x Groundwater recharge x x x
x Organic matter x x x x
Sediment load x x x x
Nutrients x x x x x Salinity
x x x x x x x Pesticides x x x x x
x x Heavy metals x x x x x x x
1.Is there simply a lack of data here? 2. Are LU
impacts on this group of functions really
restricted to small areas? 3. Do we understand
why this could be so? 4. What does it mean for
upland-lowland interactions?
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Watershedfunctions
Site cha- racteristics
Relevant for

• 1. Transmit water
• 2. Buffer peak rain events
• 3. Release gradually
• 4. Maintain quality
• 5. Reduce mass wasting

• Rainfall
• Land form
• Soil type
• Rooting depth (natural vegetation)

• Downstream water users,
• esp. living in floodplains river beds,
• w.o. storage
• or purification

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cloud interception
What matters most in a forest
rainfall
canopy water evaporation
Forest
transpiration
surface evaporation
the trees
through-fall
the landscape
stem-flow

infiltration
the soil
quick- flow
recharge
?
lateral outflow
uptake
base flow
percolation
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Looking at the water balance in summary terms
and understanding it from a summation of
event-level processes
We need models to keep track of the various
interaction terms.
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Hydrological null-model
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Nested, Overlapping Hydrologic Model Capabilities
Space (km2)
100,000
WBM
10,000
VIC
1,000
FALLOW
GENRIVER, DHSVM WANULCAS
100
10
1
Time
Hours
Days
Weeks
Months
Years
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River flow Rainfall
Way Besai (Indonesia), 2.5 m rainfall,
100 persons km-2
Mae Chaem (Thailand), 1.5 m rainfall, 10 persons
km-2
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Mae Chaem
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Basic properties transforming rainfall signal to
stream and riverflow signals
Exceedance probability
Area under the curve Rainfall ET River
?storage
0
rainfall
stream
river
Importance declines with time of consideration
1
mm day-1
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Way Besai Rain and Riverflow exceedance for two
types of rain
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Measurement point
Plot-level buffering gets lost. But the internal
floodplain replaces its role
Tentative interpretation..
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Macropore distribution
1.5 m
Coffee 1 yr
Forest
Coffee 3 yr
Coffee 10 yr
Coffee 7 yr
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Geospatially-explicit process-based models
provide fundamental new insight
MEKONG VIC (Variable Infiltration
Capacity) Meso/Macroscale Landscape/Hydrologic
Model (Daily, 1-10 km)
MAE CHAEM DHSVM (Distributed Hydrology
Soil Vegetation Model) Micro/Mesoscale
Landscape/Hydrologic Model (4h, 150m)
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Mae Chaem SCENARIOS OF HYDROLOGIC RESPONSES
Crops (7/9/99)
Veg89 (7/9/99)
1500
Veg2000irr
ET
1000
500
SM
0
N95
N96
N97
N98
N99
N00
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MUN RIVER Scenarios
All Forest
Model
No Forest
Obs
5000
1500
Ban Chot
Yasothon
4000
1200
900
3000
2000
600
300
1000
0
0
5000
10000
Ubon
Rasi Salai
4000
8000
3000
6000
2000
4000
1000
2000
0
0
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MEKONG Daily/10 km Resolution
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Precipitation
Discharge Hydrographs
Evapo- transpiration
Channel Topology
WBM/WTM
Runoff
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Scenario 1
N/A
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N/A
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Tropical forests and water flows

• Deforestation increases total water supply.
• Reforestation does not cause dry rivers to flow
  again.
• Upland deforestation increases risks of lowland
  flooding (1-20 year return period).
• Upland deforestation is a small factor in the
  most devastating floods.
• Forest degradation produces many possible
  trajectories of change in biodiversity and
  watershed function these policy objectives are
  not tightly linked and there are a wide range of
  options.