Modelling the performance of bunds and ditch dams in the hydrological restoration of tropical peatlands

Abstract

Many tropical peatlands are subjected to artificial drainage that leads to degradation.Hence, hydrological restoration has recently been prioritized. Nevertheless, as field monitoring data are limited, little is known about how restoration measures, such as ditch dams and bunds, can regulate tropical peatland water tables. We used a hydrodynamic model -DigiBog_Hydroto simulate the effectiveness of ditch dams and bunds across three El Niño-Southern Oscillation (ENSO) scenarios, which are El Niño, La Niña and Neutral, in three typical sites. The sites were moderately degraded (Mod-Dgr) and severely degraded (Sev-Dgr) peatland plots (each 0.2 km 2 ), representing typical peatland conditions in Sebangau National Park, Kalimantan, Indonesia. Our fine-scale (1 m  1 m spatial resolution) modelling revealed that in the dry season of any ENSO scenario, the significant effects of ditch-dams alone on peatland water-level were limited to lateral distances of 26 m (in Mod-Dgr) and 12 m (in Sev-Dgr) from the ditch. In the dry season of an El Niño year, the combination of ditch dams and bunds helped maintain water levels up to 72 cm (in Mod-Dgr) and 69 cm (in Sev-Dgr) higher than in the no-restoration condition. During the extremedry period of an El Niño year, the bunds reduced the number of days when the water table was deeper than 40 cm in Mod-Dgr and in Sev-Dgr by 50% and 73%, respectively. We suggest that bunds used in combination with ditch dams are a practical restoration measure for tropical peatlands, providing critical extra water storage and helping maintain water tables near the peatland surface in dry periods. We also demonstrate how fine-scale hydrodynamic modelling is beneficial for planning and assessment of restoration measures in tropical peatlands.

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

“…In turn, assuming a strictly positive impact of WTD rise in CO 2 emission reduction, this implies that dams must reduce CO 2 emissions overall. However, in line with previous studies (Putra et al, 2021(Putra et al, , 2022, dams were not enough to maintain WTD above the −40 cm limit set by the Indonesian Peatland Regulation Agency everywhere in the peatland during the extremely dry year (see Republic of Second, despite raising the WTD on average, blocks did not do so everywhere in the study area. In some areas downstream from the dams, the WTD was systematically lower in the blocked scenario than in its non-blocked counterpart.…”

“…However, neither of the two works analyzed the effect of different peat hydraulic properties or weather scenarios. The only study that is comparable in scope to ours is Putra et al (2022). Nevertheless, their study area was several orders of magnitude smaller (20 ha), and their CWL was not dynamic, but manually fixed to field measurements.…”

“…The WTD difference between the blocked and non-blocked scenarios was, on average, 3 cm at 400 m from the canals, and it was 1 mm at a 1 km distance (see Figure 8). Several studies support this claim (Sutikno et al, 2019(Sutikno et al, , 2020Evans et al, 2019;Putra et al, 2021Putra et al, , 2022Ishii et al, 2016;Jaenicke et al, 2010). Sutikno et al (2019), using dipwell measurements, claimed that the radius of action of dams in tropical peatlands is around 170 m, and Ishii et al (2016) found the modelled WTD rise due to blocks to be about 10 cm at a distance of 400 m. This result suggests that naively extrapolating WTD measurements performed in the vicinity of the canals may lead to an incorrect assessment of the ability of blocks to raise WTD throughout the study area.…”

“…Process based models have been applied to simulate WTD in tropical peatlands in multiple studies (Wösten et al, 2006;Cobb et al, 2017;Baird et al, 2017;Urzainki et al, 2020). Only few of those have dealt with the question of block performance (Jaenicke et al, 2010;Ishii et al, 2016;Putra et al, 2022). The studies by Ishii et al (2016) and Jaenicke et al (2010) did not consider different peat hydraulic properties or weather scenarios, therefore limiting the generalizability of their results.…”

“…The studies by Ishii et al (2016) and Jaenicke et al (2010) did not consider different peat hydraulic properties or weather scenarios, therefore limiting the generalizability of their results. Putra et al (2022), on the other hand, presented a good experimental setup to analyze block efficiency, but their simulations were confined to an area of 20 ha. Notwithstanding the usefulness of their approach to plan small-scale mitigation strategies and to understand restored peatland WTD dynamics, such small scales are insufficient for assessing the rewetting abilities of blocks over regional scales.…”