Many calibration techniques have been developed for the Soil and Water Assessment Tool (SWAT). Among them, the SWAT calibration and uncertainty program (SWAT-CUP) with sequential uncertainty fitting 2 (SUFI-2) algorithm is widely used and several objective functions have been implemented in its calibration process. In this study, eight different objective functions were used in a calibration of stream flow of the Pursat River Basin of Cambodia, a tropical monsoon and forested watershed, to examine their influences on the calibration results, parameter optimizations, and water resources estimations. As results, many objective functions performed better than satisfactory in calibrating the SWAT model. However, different objective functions defined different fitted values and sensitivity rank of the calibrated parameters, except Nash–Sutcliffe efficiency (NSE) and ratio of standard deviation of observations to root mean square error (RSR) which are equivalent and produced quite identical simulation results including parameter sensitivity and fitted parameter values, leading to the same water balance components and water yields estimations. As they generated reasonable fitted parameter values, either NSE or RSR gave better estimation results of annual average water yield and other water balance components such as annual average evapotranspiration, groundwater flow, surface runoff, and lateral flow according to the characteristics of the river basin and the results and data of previous studies. Moreover, either of them was also better in calibrating base flow, falling limb, and overall the entire flow phases of the hydrograph in this area.

The Mekong River Basin (MRB) in Southeast Asia is among the world’s ten largest rivers, both in terms of its discharge and sediment load. The spatial and temporal resolution to accurately determine the sediment load/yield from tributaries and sub-basin that enters the Mekong mainstream still lacks from the large-scale model. In this study, the SWAT model was applied to the MRB to assess long-term basin hydrology and to quantify the sediment load and spatial sediment yield in the MRB. The model was calibrated and validated (1985–2016) at a monthly time step. The overall proportions of streamflow in the Mekong River were 34% from surface runoff, 21% from lateral flow, 45% from groundwater contribution. The average annual sediments yield presented 1295 t/km²/year in the upper part of the basin, 218 t/km²/year in the middle, 78 t/km²/year in the intensive agricultural area and 138 t/km²/year in the highland area in the lower part. The annual average sediment yield for the Mekong River was 310 t/km²/year from upper 80% of the total MRB before entering the delta. The derived sediment yield and a spatial soil erosion map can explicitly illustrate the identification and prioritization of the critical soil erosion-prone areas of the MR sub-basins.

Microplastics are widespread in coastal and marine environments, and sediments serve as a sink for microplastics. In this study, four sediment cores were collected from the Kuala Gula Mangrove area. The abundance of microplastics in mangrove sediments ranged from 25–130 items/kg dry weight. The highest abundance of microplastic was observed at the KG04 site near the mouth of the river. The number of microplastics by sampling site was significantly different (p < 0.01), with station KG04 having the highest fiber content. The dominant color of microplastics was blue, and the main shape was fiber. Rayon, cotton, Polyethylene terephthalate (PET), and azlon were observed from FT-IR, indicating that the potential sources of microplastic and fiber could come from household laundry wastewater. Microplastics tended to accumulate in deeper depths at KG01 (p < 0.01), whereas other stations showed no significant difference (p > 0.05). However, this study provides evidence that mangroves can be a trap for microplastics and can be used as baseline data for future studies.

An integrated modeling approach analyzing water demand and supply balances under management options in a river basin is essential for the management and adaptive measures of water resources in the future. This study evaluated the impacts of climate change on the hydrological regime by predicting the change in both monthly and seasonal streamflow, and identified water supply and demand relations under supply management options and environmental flow maintenance. To reach a better understanding of the consequences of possible climate change scenarios and adaptive management options on water supply, an integrated modeling approach was conducted by using the soil and water assessment tool (SWAT) and water evaluation and planning model (WEAP). Future scenarios were developed for the future period: 2060s (2051–2070), using an ensemble of three general circulation model (GCM) simulations: GFDL-CM3, GISS-E2-R-CC, and IPSL-CM5A-MR, driven by the climate projection for representative concentration pathways (RCPs): 6.0 (medium emission scenario). The results indicated that, firstly, the future streamflow will decrease, resulting in a decline of future water availability. Secondly, water supply under natural flow conditions would support 46,167 ha of irrigation schemes and the water shortages will be more noticeable when environmental flow maintenance was considered. The study concludes that reservoir construction would be necessary for agriculture mitigation and adaptation to climate change. Furthermore, the water resources management options considering both supply and demand management are more effective and useful than supply management only, particularly in dealing with climate change impacts.

Mangroves are typically found in tropical coastal areas, and these ecosystems face deterioration and loss due to threats from climate and human factors. In this study, sediment cores were collected from human-planted mangroves in sub-tropical Ximen Island, China, and were determined for sedimentary phosphorus (P) species. The objective was to investigate the ability of mangroves planted in a zone bordering their temperature limit to preserve and regulate P. Our results showed that bioavailable P (BAP), which includes exchangeable-P (Ex-P), iron-bound P (Fe-P), and organic P (OP), accounted for approximately 64% of total P (TP). Apatite P (Ca-P), which accounted for 24% of TP, most likely originated from aquaculture activities surrounding the island. The vertical distribution of sedimentary P species along the sediment cores showed a rather constant trend along the salt marsh stand but considerable fluctuations for the mangroves and bare mudflat. These results indicate that mangroves accumulated P when there was a high P discharge event, and that this P was eventually released during organic matter decomposition and contributed to Ca-P formation. Nevertheless, old and young mangroves accumulated higher sedimentary P species, OP, and BAP compared to the salt marsh stand and bare mudflat areas. This study showed the potential of mangroves planted outside their suitable climate zone to preserve and regulate P.

Mangroves are highly productive blue carbon ecosystems that preserve high organic carbon concentrations in soils. In this study, particle size, bulk elemental composition and stable carbon isotope were determined for the sediment cores collected from the landward and seaward sides of two mangrove forests of different ages (M1, ca. 60; M2, ca. 4 years old) to determine the effects of geomorphic setting and age (L1 = old mangrove and S1 = salt marsh stand in M1; L2 = young mangrove and S2 = bare mudflat in M2) on sediments and organic carbon accumulation. The objective of this study was to determine the feasibility of the northernmost human-planted mangroves in China to accumulate sediment and carbon. Our results showed that fine-grained materials were preserved well in the interior part of the mangroves, and the capacity to capture fine-grained materials increased as the forest aged. The biogeochemical properties (C/N: 5.9 to 10.8; δ¹³C: −21.60‰ to −26.07‰) indicated that the local organic carbon pool was composed of a mixture of autochthonous and allochthonous sources. Moreover, the accumulation of organic carbon increased with the forest age. The interior part of the old mangrove had the highest organic carbon stock (81.93 Mg C_org ha⁻¹). These findings revealed that mangrove reforestation had positive effects on sediments and organic carbon accretion.