Based on a 2015 vegetation survey in the AHNFR [13], in the PSF—which is defined by the presence of organic soils—there are 69 species and 30 families of plants. The five most abundant species found were Syzygium cerinum var. cerinum (called Kelat, from family Myrtaceae), Syzygium inophyllum var. bernaddi (Kelat), Palaquium hexandrum (Nyatoh jambak), Santiria rubiginosa (Kedondong), and Stemonurus secundiflorus (Sampul keris). The vulnerable species Gonystylus bancanus (Ramin melawis) and the endangered species Shorea platycarpa (Meranti paya) were also identified.

The presence of mineral soil and a slightly higher elevation than the PSF characterized the lowland forest (Figure 2). Additionally, the low water table did not influence the lowland forest floor, as is peat swamp forest. Seventy-three species and 33 families of plants were found. The five most abundant were Syzygium inophyllum (Kelat) followed by Macarangga lowii (Mahang hutan), Aglaia palembanica (Bekak), Dipterocarpus crinitus (Keruing mempelas), and Aidia densiflora (Menterbang). The number of species recorded in the lowland forest is slightly higher than that in the PSF.

PSF develops on waterlogged sites after the formation of the peat layer from organic matter. The decomposition occurs under anaerobic conditions, which retard microbial degradation; because the partly decomposed organic matter builds up to form a layer of peat over the clay soil. It takes many years for substantial levels of peat to form, with estimated deposits of only 2.5 - 4.5 mm per year [14].

The 2017 soil survey in AHNFR [15], determined that the parent material is organic deposits over sulfuric marine clay. The soil pit at the edge of AHNFR extending 1.5 m below the soil surface revealed that the dominant material in the subsurface layer was sapric, with undecomposed wood (Table 1). The actual depth in this area is 8 metres; peat thicker than 3 metres falls into the classification called the Teraja series. Overall, peat levels up to 9 metres can be found (Figure 3). The normal concave shape of the peat depth is defined because the peatland areas extend outside the forest reserve.

Five physico-chemical water quality parameters were observed at the study sites before and after the dam was constructed. Compared to water quality of freshwater rivers in lowland forest the black water found is acidic (pH 3.04), with low total dissolved solids (TDS) and dissolved oxygen (DO), but greater conductivity. Table 2 shows the water quality parameters before and after the check dam was installed. Aluminium (Al) concentration was reduced, which produces better conditions for the oil palm trees. DO value increased, improving the water quality, and conductivity and TDS also increased. The pH values are still acidic.

The AHNFR is developed between three river systems that are the Muar River, whose upper catchment originates in the northwestern parts of the hilly areas, while the Batu Pahat River and Sarang Buaya River are in the southeast and southern areas. The Sarang Buaya flows into the Straits of Malacca. The upper catchment of the Sarang Buaya River is delineated by GIS and literally the distinct drainages were not found at the location. The slope of the peat swamp land surface is considered gentle, except for some higher ground primarily located in the western areas, where the maximum height is 40 m. Small highland freshwater streams extend into the edges of the eastern parts of the forest area. Field observations show that surface water flows in a south western direction in small canals. These canals were constructed mainly for water management in an oil palm plantation and other agricultural activities surrounding the AHNFR. Apart from that, some side ditches constructed along the Tenaga National Berhad (TNB) power line access road fall within the forest reserve area.

In January 2016, a rainfall station was set up at Kompleks Penghulu (KP), Mukim Ayer Hitam using a SEBA Hydrometrie RG60 gauge with a tipping bucket

of 1.0 mm/tip that was connected to HB event logger (SFS89424). Additionally, a conventional rainfall recorder was also set up for back-up.

There were six points for the groundwater level (GWL) monitoring as shown in Figure 1. In June 2015, two piezometers equipped with Van Essen Cera-Diver well data logger sensors were set up for continuously measuring and recording the groundwater levels in Compartments 10 (W2) and 14 (W4). In May 2016, another Cera-Diver was installed in Compartment 18 (W6). This device measures the pressure (piezometric head) of groundwater. The data were recorded in 30-minute intervals. Water levels were also directly measured manually from PVC tube wells in Compartments 9 (W1), 12 (W3), 16 (W5). Compartments 1, 21, and 22 were excluded from the monitoring because the land surfaces were covered with mineral soils. The barometer was also set up to measure the atmospheric pressure at W2.

Groundwater levels were monitored to determine the changes corresponding to the monsoon and to check if there are any differences in groundwater levels along the forest boundaries that correlate with the gradient in that area.

A survey was conducted to identify the locations of peat drainage outflow near the border of the forest reserve. The establishment of oil palm plantations requires a drainage system needs before planting activities. The networking of drainage system established along the border of the forest reserve describes the major drainage system. From the forest reserve border, points 1 and 3 only 40 m and 60 m, respectively. The check dams, a simple design made from concrete, were constructed at points 1 and 3 because they are the nearest to the forest border. Van Essen Cera-Diver water level sensor measured continuously in the drain at both locations before and after the construction of the check dams. The check dam at point 1 was installed in December 2016, and one at point 3 in 2017. These dams block the flow of water out of the forest, thus maintaining the groundwater level in the forest while also serving as part of the water management system in the oil palm plantation.

Total rainfall monitored by RG60 from March to December 2016 was 1941 mm (Figure 4). Data for January 28 to March 7 were not available due to technical error. The amount of rain collected using the conventional method for this lost period was 277.5 mm. Combining these two measurements, the total amount of rainfall observed in 2016 was 2218.5 mm. In 2017, total rainfall was 2436 mm. There were no extreme monthly quantities of rainfall recorded during this period.

A comparison of the monthly rainfall at KP station and the Malaysian Meteorological Department’s (MMD) data on 16 years (2000-2015) of monthly average rainfall at Felda Ayer Hitam (FAH) station shows a similar pattern. In this bimodal pattern, November and April had the highest points, while February and July had the lowest points. However, in May 2016, 479-mm rain was recorded at KP station, which is high compared with the monthly average at the FAH station. The average annual rainfall at FAH is 2027 mm.

Observations by piezometers at W2, W4, and W6 show that the monthly average GWL at the three locations demonstrated the same fluctuation, but with different rates, which indicated their response to rainfall events. Less rainfall was observed from May to September during the southeast monsoon and it was found that the GWL at W4 dropped to the lowest at 90 cm below the surface. Groundwater levels were closed at the soil surface during the wet season from December to February the following year. GWLs at W2, W4, and W6 from January to December 2016 indicated that the groundwater level at W2 was closer to the surface compared to W4 and W6 (Figure 5). The GWL at W2 ranged from 0 - 46.9 cm below the surface; the GWLs at W6 and W4 ranged from 1.6 - 64.1 cm and 2 - 90.6 cm below the surface, respectively. The different groundwater levels at the three locations showed that the slope of the ground surface is higher toward the north and west of the forest reserve than at the southeast.

The GWLs were almost at the surface in December 2015, and January and February 2016 (the period of the NE monsoon). The GWL also increased in June 2016 following a large amount of rainfall in May 2016, which indicates that the GWL in the forest reserve remains at the same level for up to 30 days. The lowest points occurred in April and October 2016. The ground surface at W2 was wetter than that at other locations, but the results also show that the GWL at W6 was sometimes higher than at W2 and W4. The lower ground surface at this site as evidence of headwater catchment delineated from the satellite imaginary map in Compt. 18. From January to June 2017 the GWL at W4 remains the lowest compared to W2 and W6.

A correlation between GWL and rainfall has occurred. The GWL decreases with less rainfall; a large amount of rainfall in a particular month results in a slow but continuous rise of the GWL toward the surface. In this situation, the maximum water level will be detected the following month as shown in May 2016 that after heavy rain received this month and the GWL reached its maximum in June 2016.

In 2017, the GWL at W2 and W6 was close to the surface from January to April. In May, the GWL at W6 was 17 cm above the surface. At C10 and C18 levels were above the surface from May to December (Figure 6). The water level differs from 2016, in which the GWL was below the surface at all three locations.

Monthly GWLs were observed manually at points W1, W2, W3, W4, W5, and W6. Figure 7 and Figure 8 show the same fluctuations measured by the automatic sensors. The GWLs at W1 and W2 were closer to the surface compared to the other points. The lowest water level was in October, while the highest was in December. In 2016, the highest values ranged from 4 - 34.8 cm, and the lowest from 41.5 - 81.5 mm. In 2017, GWLs ranged from 1.2 - 49.2 cm and were above the surface in May at W5 and W6.

The check dam used has a rectangular outlet, which is effective for use at a shallow drain. It blocks water flow until the maximum level is reached, then the water breaches the top edge of the outlet (Figure 9).

At Parit Gantung, which is Point 3, the water level was continuously monitored from December 2016 through the completion of the check dam in March 2017, to the end of 2017 (Figure 12). Additionally, the profile of the drain was measured for calculating the flow rate.

As expected, water levels increased in April 2017 after the completion of the dam. Observed water levels increased from 17.5 cm in February to 65.5 cm in April 2017 and remained above 60 cm in May and June 2017. The changes in the surface water level were ranged from 10.8 mm to 68.4 mm. The greatest water level increased was 58.5 cm.

The average before the dam was constructed from December 2016 to February 2020 was 12.7 cm and the average water level of 57.6 cm after the establishment of the check dam from Mac to December 2020. Hence, the average water level increased was 44.9 cm. The lower WL was recorded in December because of water flow diversion by the plantation.

Flow rate measured from the drain before construction:

Q = 0. 144m 2 × 0. 486m ⋅ s − 1 = 0.0 699m 3 ⋅ s − 1

The yearly discharge estimated from the forest reserve is 2,204,366 m³.

Discharge from the check dam’s outlet:

Q = 0. 59 × 0. 667 × 19. 6   ms − 1 ( 0. 93m + 0.0 275m ) ( 0.0 3m + 0.00 1m ) 3 / 2 = 0. 3935 ( 4 . 428ms − 1 ) ( 0. 9575m ) ( 0.00 5167m ) = 0.00 8619m 3 ⋅ s − 1

The flow rate decreased from 0.0699 m³·s⁻¹ before the construction of the check dam to 0.00862 m³·s⁻¹ afterward. The directly calibrated flow rate at the check dam’s outlet is 0.00667 m³·s⁻¹, which is not much different from the outlet of the check dams.

Based on Sarang Buaya River head water catchment of 267.4 hectares, the yearly estimation of water replenished with net increment of water level of 449 mm below:

V = 449 × 267.4 × 10 , 000 1000 = 1200626   m 3

The result shows that 1.201 million m³ of water was conserved from leaving the forest reserve with the implementation of check dam at Parit Gantung.

Besides the Coronavirus 2019 (COVID-19) pandemic, only check dam at Parit Pulai maintenance manage to conduct at the end of 2019 and the measurement continued in 2020. The observation shows that the ground water level increment was higher after redesigning of the check dam from rectangular to V-notch shape. Indirectly, it also means that more water has been retained in the forest reserve. Based on net water level increment of 18.4 cm, 1,689,120 m³ of water was replenished with reference to head water catchment of Batu Pahat River in 2020. In terms of flow rate, it was reduced from 0.252 m³·sec⁻¹ to 0.1057 m³·sec⁻¹.