Ir Henk Ritzema, Alterra-ILRI

Dr Henk Wösten, Alterra

Ing Tom Grobbe, Larenstein

EU INCO-DEV STRAPEAT PROJECT

STRATEGIES FOR IMPLEMENTING SUSTAINABLE MANAGEMENT OF PEATLAND IN BORNEO

REclamation Policy Evaluation and Analysis Tool

Decision Support System for Water Management Strategies in Tropical

Peatlands

REPEAT:

RESEARCH OBJECTIVE

“To develop, calibrate and verify a Decision Support System that predicts the consequences of various land use options for tropical peat swamp in Sarawak (Malaysian Borneo) on the groundwater regime and subsidence of the land surface”

METHODOLOGY

• Selection of a study area

• Selection of a groundwater model

• Model set-up and calibration

• Selection of scenarios

• Groundwater modelling

• Calculation of peat subsidence

• Visualisation of the results in Arc View

STUDY AREA: BALINGIAN

(PS Konsultants, 1998)

STUDY AREA: BALINGIAN

• Area: approx. 10,000 ha

• Predominantly Anderson 3 soils (7,970 ha)

• Identified as a Rural Growth Centre (RGC) by the Government of Sarawak in 1994

• Main development: oil palm plantations

• Total population: about 3,000 (1992)

GROUNDWATER MODEL

• Model: Processing Modflow for Windows (PMWIN) 5.0.79

• Steady state calculations

• Input data: level of peat dome and mineral subsoil from grid survey, k-values from literature, …

• Recharge determined through inverse modelling

• The calculated amount of recharge - as a % of precipitation - differed (lower) from current flow components in TPS based on water balance studies

GROUNDWATER MODEL: SCENARIOS

Scenarios based on:

1. Location: on the top (A) or along the edge (B) of the peat dome

2. Drainage needs: deep (0.8 m) for oil palm and shallow (0.4m) for sago

GROUNDWATER MODEL: SCENARIOS

Scenarios:

0. Reference scenario (calibration)

1. Deep Drainage (0.8 m) on top of the peat dome

2. Shallow Drainage (0.4m) on top of the peat dome

3. Deep Drainage (0.8m) along the edge of the peat dome

4. Shallow Drainage (0.4m) along the edge of the peat dome

5. Combination of scenarios 2 and 3

GROUNDWATER MODEL: RESULTS

1 2

3

4 5

PEAT SUBSIDENCE INITIATED BY GROUNDWATER DRAWDOWN

• Example: subsidence caused by scenario 1 (drainage 0.8 m; on top of the peat dome) and scenario 3 (drainage 0.8 m; along the edge of the peat dome).

Subsidence rate (Sarawak; m/year) = 0.1 * groundwater depth (m-sel)

• Peat subsidence calculations:

• Soil elevation level calculated 0, 1, 2, 5, 10, 20 and 50 years after having lowered the groundwater tables

RESULTS - COMPARISON OF SCENARIOS 1 AND 3

Soil elevation level vs. time

0.5

1.5

2.5

3.5

4.5

5.5

6.5

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

Chainage (m)

Soil e

levat

ion

level

(m)

A A'

T = 1

T = 2

T = 5

T = 10

T = 20

T = 50

T = 0

Soil elevation level vs. time

0.5

1.5

2.5

3.5

4.5

5.5

6.5

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000

Chainage (m)

Soil

elev

atio

n le

vel (

m)

A A'

T = 1

T = 2

T = 5

T = 10

T = 20

T = 50

T = 0

DECISION SUPPORT SYSTEMS

“A Decision Support System (DSS) is a computer-based system that helps decision-makers to deal with semi- and non-structured problems in each part of the decision-making process through direct interaction with data and analysis models” (Schakel, 1998)

CAPABILITIES OF THE REPEAT DSS

RE PEAT can be used to identify the optimal water management strategy for the peat dome in question:

• to show the impacts of a changed groundwater regime caused by reclamation;

• to increase insight in the geohydrological behaviour of these fragile systems;

• to enable determination of the drain deepening regime

• to predict the point of time at which the peat has disappeared, or gravity drainage is no longer possible

RESEARCH NEEDS

• Further refinement of the groundwater model, enabling determination of water management strategies on a smaller scale. Required: additional data

• Additional verification of the small amount of recharge found in this study. Required: additional data (e.g. simultaneous measurements of groundwater tables, conductivity, precipitation and discharge)

• Additional PMWIN simulations on the impacts of water conservation strategies

RESEARCH NEEDS (2)

• further research after quantification of soil subsidence in tropical peatlands (subsidence equations)

• extension of the DSS with additional features (e.g. CO2

emissions and ecological impacts) and a user-interface

EU INCO-DEV STRAPEAT PROJECT

STRATEGIES FOR IMPLEMENTING SUSTAINABLE MANAGEMENT OF

PEATLAND IN BORNEO

Thank you for listening!