The study of water salinity distribution for supporting agriculture in Katingan I lowland irrigation area, Central Kalimantan

The Katingan I tidal lowland irrigation area (DIR Katingan I) is a sensitive area to salinity changes. This study aims to identify and estimate the spatial distribution of salinity in DIR Katingan I. The method used in this study is field measurement for collecting tidal and salinity data in the Katingan River as well as in several locations of primary and secondary canals. The measured data at the upstream canal, Katingan River, and primary canal as boundary conditions and calibration for hydraulic and water quality simulations. The simulations used HEC-RAS software to estimate salinity spatial distribution under tidal conditions at primary and secondary canals. The results showed that the distribution of water salinity can reach lower primary canals. It is necessary to take anticipatory actions to reduce the salinity intrusion. To overcome the problem, alternative recommended solutions that can be done are sluice gates installation on the lower primary canal, right primary canal, and left primary canal about 1 to 3 km from the mouth of the primary canal. The prediction of salinity spatial distribution is expected to be a useful recommendation for water management in DIR Katingan I, and it is expected to increase agricultural productivity.


Introduction
Seawater intrusion is a natural phenomenon in estuary ecosystems.This is the movement of seawater to freshwater bodies or coastal areas toward land.An estuary is a semi-enclosed body of water with a free connection to the open sea, and in an estuary, the sea water is gradually diluted with fresh water from the upper reaches of the river.In estuaries, seawater intrusion is one of the important physical phenomena that affect the quality of surface water and groundwater.In estuaries, the mechanism of water mixing and the timing of salt intrusion is determined by tidal dynamics and freshwater discharge from the estuary.However, if seawater intrusion moves further upstream, it could significantly reduce water quality and freshwater availability.It is a very important resource for natural ecosystems and human activities around estuaries.One of the most common activities is agriculture.Especially in tropical countries, many lowlands are used as rice fields [1], [2].1311 (2024) 012041 IOP Publishing doi:10.1088/1755-1315/1311/1/012041 2 One of the tropical countries that has a lot of lowlands is Indonesia.The lowland potential in Indonesia is enormous, with a total area of 33,393,570 ha; 60% consists of tidal lowlands and 40% swamps [3].The Indonesian government has used lowlands with potential for agriculture since 1969 to implement a food self-sufficiency program.Tidal lowland areas in Indonesia are spread across three major islands, namely Sumatra, Kalimantan, and Papua [4].One of the tidal lowlands that is of concern to the Indonesian government for development is the Katingan I Lowland Irrigation Area (DIR Katingan I) located in Central Kalimantan Province.
DIR Katingan I is a tidal lowland irrigation area that is close to the sea.Water sources originating from the Katingan River are affected by tides and seawater intrusion.The intrusion of saltwater entering rivers and irrigation systems is the result of tidal dispersion from the sea.Given these conditions, DIR Katingan I is a sensitive area to changes in salinity.Salinity changes over a period of time in response to changes in tidal currents and over longer periods in response to inflows of fresh water [5].The uncontrolled spread of salinity can negatively affect plant growth and agricultural productivity.Salinity intrusion will negatively affect plant growth if left in the field for more than three months.Areas exposed to tides and salinity intrusion for less than three months can still be planted twice a year without inhibiting growth [6].So far in DIR Katingan I there has been no study of the distribution of salinity in the existing water system, especially in the primary and secondary canals.Therefore, this study aims to identify and estimate the spatial distribution of salinity in DIR Katingan I.The prediction of spatial salinity distribution is expected to be a useful recommendation for water management in DIR Katingan I, and it is expected to increase agricultural productivity.

Material and Method
2.1.Study area DIR Katingan I is administratively located in the Katingan Kuala District, Katingan Regency, Central Kalimantan Province, and has an existing area of around 4970 ha.Some of these areas have been used by the community as land for food crops, plantations and land for settlements and settlements.The people's main livelihood is agriculture by growing rice and crops [7], [8].The location map of the DIR Katingan I can be seen in Figure 1.

Data collection
Primary data is obtained directly by measuring in the field using two portable data logger installation devices.One device is installed in the Katingan River (location 1) and the other device is installed in the Primary Canals (location 2).The output generated from this tool is data on water level and electrical conductivity (EC) in μS/cm.The portable data logger installation device and the installation location for the device can be seen in Figure 2 and Figure 3.

Data analysis
The analysis was carried out by observing and predicting the distribution of salinity in the DIR Katingan I water system using HEC-RAS version 5.0.7 software to model flow and water quality (salinity) simulations.The canal cross-section scheme will be analyzed in a one-dimensional form that appears after the canal geometry data is entered into the application.Geometry data of primary and secondary canals are made in accordance with the network scheme and canal cross-sectional dimensions are shown in Figure 4. Flow simulation analysis using the unsteady flow module.For input data on the downstream part of the canal using hourly water level data for 15 days from 1 January 2023 at 00:00 to 15 January 2023 at 23:00 with boundary conditions on the HEC-RAS using a stage hydrograph from the measurement results using the installation of a portable data logger on the location 1, Katingan River (as shown in Figure 3).In the upstream part of the canal, a flow hydrograph is used with boundary conditions in the form of discharge data from measurements upstream of the canal of 2.17 m 3 /s.
Water quality modeling uses the water quality analysis module in the form of arbitrary constituents in mg/l or ppm equivalent with the input water quality data from the conversion data of salinity values from EC measurements.To get the salinity value, it is necessary to convert using the EC value approach (μS/cm) by multiplying a factor of 0.55 in ppm (mg/l) [9].The boundary condition in the upstream primary canal is a constant value using momentary measurement data of 15 mg/l and in the upstream secondary canal, it is 37 mg/l.In the downstream boundary condition, the conversion of EC measurement results is used for the same period, namely for 15 days from 1 January 2023 at 00:00 to 15 January 2023 at 23:00.The initial condition data uses a salinity value of 15 ppm.The dispersion coefficient values use the computed values of the options in the water quality data module.

Calibration
Flow data calibration is required to obtain flow simulation results that approximate flow conditions in the field.The flow data used for calibration is water level data as a result of measurements using a portable data logger installation installed at location 2, lower primary canal (as shown in Figure 3).The measurement period for calibrating the water level used is the same as the boundary conditions, namely January 1, 2023, at 00:00 to January 15, 2023, at 23:00.In modeling the canal geometry, the location of the tool is at the river station (RS) 5000, so that the water level calibration uses the results of calculations at the RS.Calibration is carried out based on the value of the manning roughness coefficient (n) according to the canal conditions Table 1 After comparing the results of the flow simulation with the measurement data, a more representative manning value of 0.016 was chosen as shown in Figure 5. Analysis of the Root Mean Squared Error (RMSE) indicator was carried out to see the accuracy of the calibration.The results of the analysis obtained an RMSE value of 0.23 which indicates that the Manning value can be used.Calibration accuracy depends on the quality of the measurement data.Calibration data for water quality modeling for salinity uses conversion data from EC measurement results from a portable data logger installed at location 2, lower primary canal (as shown in Figure 3).The measurement period for salinity calibration used is the same as the boundary  conditions, namely January 1, 2023, at 00:00 to January 15, 2023, at 23:00.Approach to simulated water quality measurement results by changing the multiplier factor of the Fischer equation.After comparing the results of the flow simulation with the measurement data, a more representative multiplier factor of 10 is chosen as shown in Figure 6.
Figure 6 The result of calibration salinity at primary canal RS 5000

Hydraulic and water quality (salinity) simulation result
A comparison of simulation results and measurements on the primary canal at RS 5000 (as shown in Figure 7) shows that the maximum water level that enters the lower primary canal is 4 m, while the minimum water level is 2 m.This shows that when the tide occurs, the water does not pass through the primary and secondary canal embankments which are at a height of 5 m.At low tide, the primary canal still has flow because the bottom of the primary canal is at a height of 1 m, while in the secondary canal, there is no flow (dry) because the bottom of the secondary canal is at a height of 2.25 m.A comparison of the simulation results and salinity measurements in the primary canal shows that the maximum salinity value is 1,600 ppm or equivalent to 1.6 ppt.The recommended salinity level for irrigation water ranges from 0.5 to 1.5 ppt, so it is necessary to treat the lower primary canal so that the salinity entering the lower primary canal can be reduced [1].

Water salinity distribution
To find out the spatial distribution of salinity in the canal, the results of the salinity modeling that can be used as a basis are the maximum salinity that occurs during the simulation period.Based on the simulation and measurement results, the maximum salinity occurred on January 7, 2023, at 23:00, so this period can be used to see and predict the spatial distribution of salinity.The results of the spatial distribution of salinity can be seen in Figure 8.The relatively high level of salinity of around 2,090 ppm enters from the Katingan River only to the lower primary canal, right primary canal, and left primary canal about 4 km from the mouth of the primary canal, while the secondary and upper primary canals are not so affected by salinity.From the results of this spatial distribution of salinity, action can be taken to reduce the level of salinity entering the canal are sluice gates installation.The operation can be placed on the lower primary canal, right primary canal and left primary canal about 1 to 3 km from the mouth of the primary canal.

Conclusions
The results showed that the distribution of water salinity in DIR Katingan I can reach lower primary canals.It is necessary to take anticipatory actions to reduce the salinity intrusion.To overcome the problem, alternative recommended solutions that can be done are sluice gates installation (operation can be placed on the lower primary canal, right primary canal, and left primary canal about 1 to 3 km from the mouth of the primary canal).The prediction of water salinity distribution is expected to be a useful recommendation for water management in DIR Katingan I, and it is expected to increase agricultural productivity.For further research, it is best to calculate the dimensions of the sluice gates and simulate them to find out the correct sluice gate operation pattern.It is recommended that salinity modeling is also carried out on agricultural land so that the level of salinity that enters paddy fields can be predicted so that the plants do not die because the level of salinity is too high.

Figure 1 .
Figure 1.Location of DIR Katingan I

Figure 2 .
Figure 2. The portable data logger installation device

Figure 3 .
Figure 3.The installation location for the deviceOther primary data is obtained from instantaneous measurements using a handheld device to determine the salinity value and using a float to determine the discharge upstream of the canals.The measurement period was carried out between December 2022 and March 2023.Secondary data was obtained from information from scientific publications and data from government agencies, namely

Figure 4 .
Figure 4. Geometry of existing primary and secondary canals

Figure 5 .
Figure 5. Calibration result at primary canal RS 5000

Figure 7 .Figure 8 .
Figure 7.Comparison between measurement and simulation data results at primary canal RS 5000