1. 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:

2. 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”

3. 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

4. STUDY AREA: BALINGIAN (PS Konsultants, 1998)

5. 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)

6. 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

7. 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

8. 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

9. GROUNDWATER MODEL: RESULTS 1 1 2 2 3 3 4 4 5 5

10. 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

11. RESULTS - COMPARISON OF SCENARIOS 1 AND 3

12. 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)

13. 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

14. 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

15. RESEARCH NEEDS (2) further research after quantification of soil subsidence in tropical peatlands (subsidence equations) extension of the DSS with additional features (e.g. CO 2 emissions and ecological impacts) and a user-interface

16. EU INCO-DEV STRAPEAT PROJECT STRATEGIES FOR IMPLEMENTING SUSTAINABLE MANAGEMENT OF PEATLAND IN BORNEO Thank you for listening!