Adaptive water management in response to tidal inundation and sea level rise on tidal agricultural lowlands of Katingan, Central Kalimantan

Decreased water management systems and sea level rise cause inundation in the Katingan tidal agricultural lowland. Excessive and prolonged inundation can decrease rice production or even loss of rice land. A study was conducted to provide adaptive water management for controlling water levels in canals and rice crop fields inundation. For this purpose, a combined one- and two-dimensional unsteady flow method was used in HEC-RAS. The scenario of water level control uses sluice and flap gates in secondary canals. Under spring tide conditions, water level control with gates decreases the inundation area by 95.62%. Likewise, in the 25 and 50-year sea level rise scenarios, inundation decreased by 88.51% and 82.25%. In the spring tide condition, the duration of inundation is decreased from 3.5 to 0.9 days. Under 25 and 50-year sea level rise scenarios, the duration of inundation was decreased from 3.7 to 3 days and from 4.3 to 4.1 days. The use of gates significantly decreases inundation in the spring tide condition, but under sea level rise in 25 and 50-year, the depth and duration of inundation are still dangerous for rice crops; therefore, it is necessary to advance water management control for the sustainability of rice fields.


Introduction
After decades of reclamation projects, there are still problems in Indonesia's lowlands, such as acidity in water and soil, flooding and drought, sedimentation of canals, and decreased function of gates infrastructure [1][2][3][4][5].Problems arise due to the lack of maintenance of canals and water structures.This problem was found in the Katingan tidal lowlands, where there was no maintenance after 40 years of reclamation projects, causing the water system to decrease.Many canals found sedimentation and nonfunctioning gates that reduces the water flow capacity [6], and even secondary canals are overgrown by wild plants.The influence of tides is due to the land being in hydro-topography categories A and B [7].Land with hydro-topography category A will be inundated at least four times in one tidal cycle in the wet or dry season, while land with hydro-topography category B will be inundated at least four times in one tidal cycle in the wet season only [8], the comparison of land to tidal water level can be seen in Figure 1.Current water management conditions and hydro-topography factors have resulted in excessive and prolonged inundation of rice fields.
The location of tidal lowland close to the coast means that the impact of sea level rise will potentially occur on agricultural land.With an annual sea level rise in Indonesia of 4.9 mm, in the next 25 and 50-IOP Publishing doi:10.1088/1755-1315/1343/1/012035 2 years, sea level rise will reach 0.12 m and 0.24 m [9].Due to sea level rise, inundation will worsen, resulting in decreased rice production or even loss of rice land.
Sea level rise impacts coastal areas around the world.In the Gambia, which has resulted in the loss of wetlands and mangrove forests and damage to buildings in coastal areas, with significant material impacts [10].Sea level rise has resulted in continuous flooding of land around the Mekong and Red Rivers and damage to coastal agricultural areas in Vietnam [11].In the coastal area of Upazila, Bangladesh, sea level rise increases the vulnerability of coastal lands to degradation.The highest vulnerability occurs in agricultural land production; therefore, adaptive measures are needed to decrease land vulnerability to sea level rise [12].

Figure 1.
Hydro-topography classification of tidal lowlands area [8,13] In Indonesia, sea level rise impacts industrial areas, settlements, and agricultural lowlands [14].In agricultural lowlands, sea level rise results in inundation from fresh or brackish water, which can reduce crop productivity [15].Changes in land hydro-topography also occur due to sea level rise, from previously type B to type A or type B and C to type A [16].Hydro-topography changes can be seen in the irrigated lowlands area of Tabunganen, South Kalimantan, causing changes in land suitability for lowland rice [17].Changes in the tidal lowland hydro-topography reduce the drainage capacity of land with hydro-topography A and B, where water is difficult to flow so that the inundation will be deeper and longer.Considering the current condition and the impact of sea level rise on the Katingan tidal agricultural lowlands, a study was conducted to provide adaptive water management for controlling water levels in canals and fields to increase the resilience of agricultural land.

Study Area
The Katingan tidal lowland area in Katingan Regency, Central Kalimantan Province, is shown in Figure 2. Katingan is a tidal lowland for rice cultivation with an area of 8,083 hectares.This research was conducted on the Katingan sub-area of 1,871 hectares at the intersection of the left and right primary canals.Katingan is in the coastal region of Central Kalimantan with hydro-topography A and B and partly C and D [7].The water source for rice fields in Katingan comes from the Katingan River and rainfall.The water management system on the land was built through the Tidal Agricultural Land Opening Project (P4S) in 1984 [18].The water system is a comb with three primary canals as water supply and two collector canals as drainage.However, the primary and drainage canals currently have the same function as supply at high tide and drainage at low tide.The micro water system has secondary and tertiary canals that supply and drain rice fields.Currently, improvement of the water system is being carried out by the Kalimantan II River Basin Organization.The work aims to increase the flow capacity of the canal, water control with gates, and build infrastructure to increase rice productivity.

Inundation Condition
The goal of water management improvement is to control the water level elevation in the canals to decrease the depth, area, and duration of inundation.The maximum inundation depth in rice crops is 0.32 m for the vegetative phase, while in the generative phase, it is only 0.085 m [19].Under spring tide and sea level rise, Katingan rice fields are inundated with a depth of more than 0.32 m [20], as seen in Table 1.In [20] presents a similar table to Table 1 of the simulation results of inundation conditions in the Katingan tidal lowlands.However, in this study, a re-simulation was carried out with improved data on channel and field geometry and boundary conditions.
The depth and duration of inundation trigger rice crop damage and failure.Rice crops tolerate inundation, but crop failure can occur if it takes more than three days [21].Under spring tide conditions, 25 and 50-year sea level rise, the inundation duration is more than three days [20], potentially damaging rice crops and crop failure.Therefore, controlling the water level in the canal and rice fields is necessary to decrease the impact of excessive inundation.One of the controls is carried out by using gates.

Unsteady Flow Modeling
Unsteady flow modeling of tide and inundation control using HEC-RAS.The analysis was conducted by modeling the water system in the primary, secondary, and lateral structures and gates.Unsteady flow modeling was used to simulate tidal flow, where the water level in the canal changes gradually with time.Tidal boundary conditions were used at the downstream ends of the middle, the left, and the right primary canals.There were three boundary condition tides: the currently recorded spring tides and the 25 and 50-year sea level rise scenarios.In the upstream of the secondary canal, minor flow discharge data of 0.01 m 3 s -1 is used because, in field conditions, there is no discharge from upstream of the secondary canal and more dominant tidal discharge from downstream.
The use of gates is aimed to lower the water level in the secondary canals.The gates used are sluice gates on secondary canals one left and secondary canals one right, while other secondary canals use flap gates.The gates modeling scheme can be seen in Figure 3.The gate is modeled using an inline structure with dimensions of the sluice gates used are 2.0x1.5 m for four units in one secondary canal.The base elevation of the sluice gates is +1.0 m and is located 25 m from the mouth of the canal.Meanwhile, the dimensions of the flap gates are 1.5x1.5 m for two units in one secondary canal with a base elevation of +1.0 m.
The operation method of the sluice gates is based on the water level downstream of the gates or the mouth of the secondary canal.When the elevation downstream of the gates reaches +3.50 m, the sluice gates will be closed and opened if the water level drops to +3.00 m.Scenarios for minimum gate opening were conducted at 0 m, 0.10 m, and 0.20 m to analyze the effect on the water level in the canal and the inundation of the rice fields.The minimum gate opening of 0 m means that the gate is fully closed during the closed condition, while the minimum gate opening of 0.10 m and 0.20 m means that the gate is always open with a height of 0.10 and 0.20 m during the closed condition.The gate opening and closing speed is 0.20 m min -1 , and the maximum gate opening is 1.50 m.

Water Level Elevation in the Canal
Under spring tide conditions, gates can lower the water level in the secondary canal.Of the three scenarios of the minimum gate opening, the lowest water level is obtained in the first scenario with a minimum gate opening of 0 m.However, in this condition, the water in the canal at low tide cannot flow into the primary canal, so the drainage process does not occur.The water level in the canal is stagnant when the gate is fully closed, as shown in Figure 4.This gate operation is not suitable for the drainage process of the canals.
In the second scenario, the water level in the channel fluctuates with the water level downstream of the gates, indicating that the drainage process can occur.In the third scenario, with a minimum gate opening of 0.20 m, the water level in the canal also fluctuates with the water level downstream of the gates, similar to the second scenario.However, the elevation of the water level in the channel is higher than in the second scenario; therefore, the second scenario is used to control the water level in the channel in the subsequent discussion.The 25 and 50-year sea level increases the secondary canal's water level.The gates can decrease the water level in the canals, as seen in Figure 5 and Figure 6.Under the 25-year sea level rise scenario, the maximum water level in the canal decreased from +3.95 m to +3.84 m.In the 50-year sea level rise scenario, decrease the maximum water level elevation from +3.97 m to +3.84 m.The minimum gate opening of 0.10 m is the most suitable for lowering the water level in the canal, among other scenarios.However, the minimum gate opening will result in high flow velocities below the gate.The highest velocity occurs when the gate is maximally closed; this will cause scouring downstream of the gate and can cause erosion, so it is necessary to strengthen the gate to sustain the gate operation process.

Inundation Area
The maximum tolerable inundation depth for rice plants is 0.32m [19]; therefore, the effectiveness of reducing inundation is based on an inundation depth of more than 0.32 m.The use and operation of gates on secondary canals decrease tidal overflow so that the inundation area in rice fields is decreased; this can be seen in Figure 7.In the spring tide condition, the gate can decrease the inundation area from 487.26 ha to 21.31 ha, or the effectiveness of the decrease reaches 95.62% so that only 4.38% of the rice fields are still inundated.The 25-year sea level rise increased the inundation areas to 569.95 ha, but the gates decreased to 65.51 ha.The effectiveness of decreasing the inundation area reached 88.51%, so only 11.49% of rice fields are inundated due to sea level rise in 25-year.Likewise, for the 50-year sea level rise scenario, the inundation area decreased from 601.31 ha to 106.72 ha with the effectiveness of decreasing the inundation area by 82.25%, which means that only 17.75% of the rice fields that are inundated.
The total inundation area did not significantly decrease, as shown in Figure 7d, 7e, and 7f.However, the inundation area is more dominant, with a minimum depth of 0.32 m, so it does not threaten rice crops.This inundation indicates overflow still occurs in the secondary canal, but the overflow discharge is lower than before the gate.Under spring tide conditions, the overflow discharge reached 51,700 m 3, while with the gate, the overflow discharge was reduced to 19,540 m 3 .In the 25-year sea level rise scenario, the total overflow discharge reached 146,750 m 3 ; after the gate, the overflow discharge was reduced to 33,560 m 3 .The 50-year sea level rise scenario, the overflow discharge was reduced from 359,060 m 3 to 52,380 m 3 .

Depth of Inundation
The inundation depth in the rice fields can be controlled with gates.For example, the rice fields around the secondary one-left canal can be seen in Figure 8, where the maximum inundation depth reached 0.52 m in the spring tide condition.With the gates, the maximum inundation depth was decreased to 0.30 m.In the 25-year sea level rise scenario, the maximum inundation depth decreased from 0.55 m to 0.37 m.In contrast, in the 50-year sea level rise scenario, the maximum inundation depth decreased from 0.56 m to 0.40 m.In the spring tide conditions, the operation of gates can decrease the depth of inundation to below the maximum depth for rice crops.However, it differs from the rising sea levels in 25 and 50 years.In the 25-year sea level rise scenario on February 20-28, 2023, the inundation depth was more than 0.32 m.Likewise, for the 50-year sea level rise scenario from February 19-28, 2023, the inundation depth was more than 0.32 m.Over three days of inundation will cause rice crop failure [21].Therefore, the use of gates is still insufficient to overcome the inundation problem in the fields due to rising sea levels in 25 and 50-year.The inundation area of more than 0.32 m due to sea level rise in 25 and 50-years is not as significant as before gates can be seen in Figure 7.However, there is still a need for other alternatives to overcome these problems, such as the use of pumps on some fields that experience inundation so that the sustainability of agricultural land can be maintained.

Duration of Inundation
Using gates and proper operation techniques can decrease the duration of inundation under spring tide conditions and the rise of 25 and 50-year sea levels.Inundation duration is computed from the average inundation area on fields with an inundation depth of more than 0.32 m.Under spring tide conditions, the duration of inundation with a depth of more than 0.32 m is 3.5 days, while with gates, the duration is decreased to 0.9 days.The 25-year sea level rise increases the duration of inundation on the fields.With gates, the duration of inundation can be decreased from 3.7 days to 3 days.Under a 50-year sea level rise scenario after gates, the duration of inundation is decreased from 4.3 days to 4.1 days.Under spring tide conditions, gates can reduce the duration of inundation below three days.However, under the 25 and 50-year sea level rise, the duration of inundation is still above the maximum duration on rice fields, in line with the example discussed in Figure 8, where this condition still requires further treatment to decrease the duration of inundation on the fields due to sea level rise.

Proposed Sustainable Water Management Anticipating Sea Level Rise
The use of gates significantly impacts inundation control under spring tide and sea level rise.Table 2 shows the decrease in the water level elevation in the secondary canals and inundation in the fields.Inundation conditions such as total area, area with a depth of more than 0.32 m, duration, and depth of inundation on rice fields have decreased.Under spring tide conditions, the use of gates can control the elevation of the water level in the canals, the depth of inundation in the fields is below 0.32 m, and the duration of inundation is below three days, so it is no longer dangerous for rice crops.However, under sea level rise in 25 and 50-year, 65.51 ha and 106.72 ha of rice fields are still inundated with depths above 0.32 m and an inundation duration of more than three days.There is a need for advanced water management control to minimize inundation conditions in rice fields after gates due to sea level rise.Advance water management controls can be in the form of using pumps on land with low contours.Control can also be done by making dikes around rice fields that are often inundated so that overflows can be decreased and can also use drainage pipes from the fields to the secondary canal to decrease the depth of inundation.

Conclusions
The decrease of the water management system in Katingan's tidal lowlands due to sedimentation in the channels, non-functional water structures, and low topography conditions resulted in excessive inundation of rice fields.Sea level rise causes inundation to worsen, so there is necessary to control inundation with adaptive water management, one of which is with gates.
The use of gates can decrease the elevation of the water level in the canal, the area of inundation, and the depth and duration of inundation.In the spring tide condition, the inundation area decreased from 487.26 ha to 21.31 ha or decreased by 95.62%, and the inundation duration decreased from 3.5 days to 0.9 days.Under the 25-year sea level rise condition, the inundation area decreased from 569.95 ha to 65.51 ha, or a decrease of 88.51%, while the inundation duration decreased from 3.7 days to 3.0 days.The gates can also decrease the inundation area at the 50-year sea level rise, from 601.31 ha to 106.72 ha or a decrease of 82.25%, while the duration of inundation is decreased from 4.3 days to 4.1 days.
The use of gates can overcome inundation problems in spring tide conditions, but under sea level rise in 25 and 50-year, 65.51 ha and 106.72 ha of rice fields are still inundated with depths above 0.32 m and duration above three days.This condition is still dangerous for rice crops; therefore, it is necessary to advance water management control for the sustainability of rice fields, such as using pumps, dikes, and drainage pipes.

Figure 2 .
Figure 2. Study site in the tidal lowlands area of Katingan, Central Kalimantan Province (Base Map from OSM Humanitarian Data Model in QGIS 3.28.0)

Figure 3 .
Figure 3. Schematic modeling of gates on secondary canals (Base Map from OSM Humanitarian Data Model in QGIS 3.28.0)Flap gates operate automatically using water energy to open and close according to water flow regulation criteria.The flap gate will close if the water level downstream of the gate or the mouth of the secondary canal is 0.20 m higher than the water level upstream of the gate.The gate is automatically open if the water level upstream and downstream of the gate is the same.The gate opening and closing speed is 0.20 m min -1 with a maximum opening of 1.50 m.To evaluate the gate operation effect on the water level in the canal and inundation in the rice fields, minimum gate opening scenarios were conducted at 0 m, 0.10 m, and 0.20 m.

6 Figure 4 .
Figure 4. Comparison of water level elevation in the secondary canal due to spring tide condition without and with gates scenario

Figure 5 .
Figure 5.Comparison of water level elevation in the secondary canal due to 25-year sea level rise without and with gates scenario

Figure 7 .
Comparison of inundation area in (a) spring tide without gates, (b) 25-year sea level rise without gates, (c) 50-year sea level rise without gates, (d) spring tide with gates, (e) 25-year sea level rise with gates, (f) 50-year sea level rise with gates (dark blue color represents inundation of more than 0.32 m)

Figure 8 .
Figure 8.Comparison of inundation depth in rice fields due to spring tide, 25 and 50-year sea level rise without and with gates

Table 2 .
Comparison of inundation conditions without and with gates at spring tide, 25 and 50-year of sea level rise