A process-based model for quantifying the effects of canal blocking on water table and CO₂ emissions in restored tropical peatlands

Abstract

Abstract. Drainage in tropical peatlands increases CO2 emissions, the rate of subsidence, and the risk of forest fires, among other negative environmental impacts. These effects can be mitigated by raising the water table depth (WTD) using canal or ditch blocks. The performance of canal blocks in raising WTD is, however, poorly understood, because the WTD monitoring data is limited and spatially concentrated around canals and canal blocks. This raises the following question: how effective are canal blocks in raising the WTD over large areas? In this work we composed a process-based hydrological model to assess the rewetting performance of 168 canal blocks in a 22000 ha peatland area in Sumatra, Indonesia. We simulated daily WTD over one year using an existing canal block setup and compared it to the situation without blocks. The study was performed across two El-Niño Southern Oscillation (ENSO) scenarios, and four different peat hydraulic properties. Our simulations revealed that while canal blocks had a net positive impact on WTD rise, they lowered WTD in some areas, and the extent of their effect over one year was limited to a distance of about 600 m around the canals. We also show that canal blocks are most effective during dry periods and in peatlands with high hydraulic conductivity. Averaging over all modelled scenarios, blocks raised the annual mean WTD by only 0.9 cm. This value was 2.78 times larger in the dry year than in the wet year (1.39cm versus 0.50 cm), and there was a 2.76 fold difference between the scenarios with the maximum and minimum hydraulic conductivity (1.50 cm versus 0.54 cm). Using a linear relationship between WTD and CO2 emissions, we estimated that, averaging over peat hydraulic properties, canal blocks prevented the emission of 1.03 Mg ha-1 CO2 in the dry year and 0.37 Mg ha-1 CO2 in the wet year.