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Wetlands are important reservoirs of water, carbon and biodiversity. They are typical landscapes of lowland regions that have high potential for water retention. However, the hydrology of these wetlands in tropical regions is often studied in isolation from the processes taking place at the catchment scale. This contribution offers a comprehensive analysis of the hydrological dynamics of two neighbouring poorly gauged tropical basins; the Kapuas basin (98,700 km²) in West Kalimantan and the Mahakam basin (77,100 km²) in East Kalimantan, Indonesia. Both basins are characterized by vast areas of inland lowlands. The hydro-climatological data described herein were obtained during fieldwork campaigns carried out in the Kapuas over the period 2013-2015 and in the Mahakam over the period 2008-2010. Additionally, we used the Tropical Rainfall Measuring Mission (TRMM) rainfall estimates over the period 1998-2015 for analysing the distribution of rainfall and the influence of El-Niño - Southern Oscillation. Flood occurrence maps were obtained from the analysis of the Phase Array L-band Synthetic Aperture Radar (PALSAR) images of 2007 through 2010. Drought events were derived from time-series of simulated groundwater recharge using time series of TRMM rainfall estimates, potential evapotranspiration estimates and the threshold level approach. The Kapuas and the Mahakam lakes region are vast reservoirs of water of about 1000 km² and 1500 km² that can store as much as 3 billion m³ and 6.5 billion m³ of water, respectively. These storage capacity values can be doubled considering the area of flooding under vegetation cover. Discharge time-series show that backwater effects are highly influential in the wetland regions, which can be partly explained by inundation dynamics shown by flood occurrence maps obtained from PALSAR images. In contrast to their nature as a wetland, both lowland areas have frequent periods with low soil moisture conditions and low groundwater recharge. The Mahakam wetland area regularly exhibits low groundwater recharges, which may lead to prolonged drought events that can last up to 13 months. It appears that the Mahakam lowland is more vulnerable to hydrological drought leading to fire occurrence than the Kapuas basin. [ABSTRACT FROM AUTHOR]

Conversion of tropical peatlands to agriculture leads to a release of carbon from previously stable, long-term storage, resulting in land subsidence that can be a surrogate measure of CO2 emissions to the atmosphere. We present an analysis of recent large-scale subsidence monitoring studies in Acacia and oil palm plantations on peatland in SE Asia, and compare the findings with previous studies. Subsidence in the first 5 years after drainage was found to be 142 cm, of which 75 cm occurred in the first year. After 5 years, the subsidence rate in both plantation types, at average water table depths of 0.7 m, remained constant at around 5 cm yr-1. Bulk density profiles indicate that consolidation contributes only 7% to total subsidence, in the first year after drainage, and that the role of compaction is also reduced quickly and becomes negligible after 5 years. Over 18 years after drainage, 92% of cumulative subsidence was caused by peat oxidation. The average rate of carbon loss over the first 5 years was 178 t ha-1 yr-1 CO2eq, which reduced to 73 t ha-1 yr-1 CO2eq over subsequent years, resulting in an average loss of 100 t ha-1 yr-1 CO2eq annualized over 25 years. Part of the observed range in subsidence and carbon loss values is explained by differences in water table depth, but vegetation cover and addition of fertilizers also influence peat oxidation. A relationship with groundwater table depth shows that subsidence and carbon loss are still considerable even at the highest water table levels theoretically possible in plantations. This implies that improved water management will reduce these impacts by only 20% at most, relative to current conditions, and that high rates of carbon loss and land subsidence should be accepted as inevitable consequences of conversion of forested tropical peatlands to other land uses. [ABSTRACT FROM AUTHOR]

Degradation of tropical peats is a global concern due to large Carbon emission and loss of biodiversity. The degradation of tropical peats usually starts when the government clears closed peat forests into open and drained peatlands for agricultural uses. Tropical peats have high values of Water Contents (WC), Organic Matters (OM) and Total Organic Carbon (TOC), and low values of Total Nitrogen (TN) and Total Sulphur (TS). Dry Bulk Density (DBD) is commonly less than 0.1 g cm-3. Decline of concentration values of OM (<90%) and TOC (<40%) indicate peat degradation. In disturbed peats, TN concentration tends to decrease and the concentration of TS slightly increases. Changes in OM, TOC, TN and TS are potentially important indicators for assessing peat degradation in the tropics. [ABSTRACT FROM AUTHOR]

There are limited data for greenhouse gas (GHG) emissions from smallholder agricultural systems in tropical peatlands, with data for non-CO 2 emissions from human-influenced tropical peatlands particularly scarce. The aim of this study was to quantify soil CH 4 and N 2 O fluxes from smallholder agricultural systems on tropical peatlands in Southeast Asia and assess their environmental controls. The study was carried out in four regions in Malaysia and Indonesia. CH 4 and N 2 O fluxes and environmental parameters were measured in cropland, oil palm plantation, tree plantation and forest. Annual CH 4 emissions (in kg CH 4 ha -1  year -1 ) were: 70.7 ± 29.5, 2.1 ± 1.2, 2.1 ± 0.6 and 6.2 ± 1.9 at the forest, tree plantation, oil palm and cropland land-use classes, respectively. Annual N 2 O emissions (in kg N 2 O ha -1  year -1 ) were: 6.5 ± 2.8, 3.2 ± 1.2, 21.9 ± 11.4 and 33.6 ± 7.3 in the same order as above, respectively. Annual CH 4 emissions were strongly determined by water table depth (WTD) and increased exponentially when annual WTD was above -25 cm. In contrast, annual N 2 O emissions were strongly correlated with mean total dissolved nitrogen (TDN) in soil water, following a sigmoidal relationship, up to an apparent threshold of 10 mg N L -1 beyond which TDN seemingly ceased to be limiting for N 2 O production. The new emissions data for CH 4 and N 2 O presented here should help to develop more robust country level 'emission factors' for the quantification of national GHG inventory reporting. The impact of TDN on N 2 O emissions suggests that soil nutrient status strongly impacts emissions, and therefore, policies which reduce N-fertilisation inputs might contribute to emissions mitigation from agricultural peat landscapes. However, the most important policy intervention for reducing emissions is one that reduces the conversion of peat swamp forest to agriculture on peatlands in the first place., (© 2023 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)