The Threat of Saltwater Intrusion on Rice Farming in Katingan I Lowland Irrigation Area, Central Kalimantan

The Katingan I Lowlands Irrigation Area (LIA) is one of tidal lowlands irrigation area in Central Kalimantan, Indonesia. Because the Katingan I LIA utilizes water from the estuary, the occurence of saltwater intrusion is likely to occur. Assessing water salinity due to saltwater intrusion provides essential information for farmers to manage water appropriately. In addition, this information can also be used by the government to develop policies and management strategies for Katingan I LIA. Saltwater intrusion that occurs when sea water flows into irrigation canals and agricultural land can lead to increased salinity. High salinity levels can affect agricultural productivity, especially in rice crops. Therefore, a thorough salinity study is needed to support the development of rice farming. This study aims to evaluate salinity levels due to saltwater intrusion and understand the factors affecting them. The study measured tidal (water level elevation), electrical conductivity (EC), and salinity data of the Katingan River and several locations in the primary and secondary channels. Data collection methods included continuous and instantaneous measurements using portable data logger installations and handheld devices. The existing water salinity conditions at Katingan I LIA obtained the maximum water salinity value from the three locations of 14,536 ppm and the lowest water salinity value of 3.02 ppm. The maximum water salinity value that enters the primary canal is 5,547 ppm, higher than the thershold salinity value for rice farming (5,120 ppm). Therefore, anticipatory measuers must be conducted to reduce salinity intrusion, such as optitmizing water channels, making water gates, and managing the water use efficiently. It is also recommended that measurements be carried out during the dry season to see the comparison of results and the magnitude of the maximum water salinity value that occurs because salinity problems occur more during the dry season


Introduction
In Indonesia, the agricultural sector has a vital role in the national economy, especially as a food provider and industrial raw materials, earning foreign exchange, creating jobs, and is the primary source of income for families in rural areas.In addition, the agricultural sector also has a vital role in the regional economy, strengthening food security, the economy, culture, and local institutions [1].
IOP Publishing doi:10.1088/1755-1315/1313/1/012002 2 One of the main factors needed is potential paddy fields to improve and strengthen the agricultural sector.Still, on the other hand, the area of possible paddy fields is shrinking due to changes in land use to meet the needs of housing, industry, and related facilities.Data from the Ministry of Agrarian and Spatial Planning/National Land Agency revealed that, during the 2013-2019 period, there was a reduction in paddy fields in Indonesia by 287,000 hectares.In 2013, the size of paddy fields was around 7.75 million hectares, while in 2019, the area of paddy fields was around 7.46 million hectares.This condition resulted in the expansion area of the planting to sub-optimal land, such as tidal lowlands, with various constraints that caused lower land productivity compared to potential ground [2,3].
The area of lowlands in Indonesia reaches 36 to 39 million hectares [4].Around 20 million hectares are tidal lowlands in Sumatera, Kalimantan, and Papua.About 4 million hectares of these lowlands have been reclaimed, mainly by local people (over 2.5 million hectares) and the national government (approximately 1.3 million hectares).One of the existing tidal lowland irrigation areas is Katingan I LIA, which covers an area of around 4,970 hectares [4].
Katingan I LIA is a tidal lowland irrigation area that utilizes water sources from the Katingan River and is close to the sea estuary, so it is likely that the water that enters the Katingan I LIA is affected by seawater.Saltwater intrusion that occurs when seawater flows into irrigation primary channels and agricultural land can cause an increase in salinity.High levels of salinity can affect agricultural productivity, especially in rice crops.Salinity intrusion adversely affects crop growth when left in the field for over three months [5].The study that focuses on discussing the condition of water salinity in the tidal lowland irrigation system for rice farming has not been carried out much, even for the location in Katingan I LIA has never been done.Therefore, a thorough salinity study is needed in this location to support the development of rice farming.This study aims to evaluate the salinity level due to saltwater intrusion and understand the factors that influence it.

Location of study
The study location is Katingan I LIA, Katingan Kuala District, Katingan Regency, Central Kalimantan Province.Katingan I LIA is in the Mentaya-Katingan River area and is included in the Katingan watershed area.Katingan I LIA's water source comes from the Katingan River.The location of Katingan I LIA can be seen on the map in Figure 1.The Katingan I LIA area is administratively located in four villages.The villages are Bumi Subur, Subur Indah, Jaya Makmur, and Makmur Utama.Katingan I LIA is a potential area because it is one of the rice granaries in Central Kalimantan Province.

Data collection
Primary data collection was carried out using instantaneous manual measurement methods and automatic measurements.Instantaneous manual measurements use a handheld tool to measure electrical conductivity (EC) and salinity were carried out at several locations of the primary and secondary canals to get an overview of the existing EC value and Salinity at the location.Handheld tool and instantaneous measurement locations can be seen in Figure 2 and Figure 3. Automatic measurements use the installation of a portable data logger to measure water level and EC values.The water level and EC values will be read automatically every minute in millimeters (mm) and microsiemens per centimeter (μS/cm).The measurement data output is in the form of an excel file which is automatically stored in the memory card.Two units of portable data loggers are installed.One tool is placed in a fixed location on the Katingan River (location 1) to compare the results of water level and EC with other location.One other device is placed in primary canal (location 2).Installation of portable data loggers in location 2 by moving in the other location in primary canal (location 3) after getting data for 15 days.Installation of portable data loggers and their placement locations can be seen in Figure 4 and Figure 5.

Data analysis
This study used quantitative methods with descriptive analysis to describe the existing conditions of tides and water salinity in Katingan I LIA.Data analysis was carried out by comparing the water level elevation and water salinity values obtained from measurements and conversion results of EC values to see the effect of tides, to the water salinity value.Data analysis was also carried out by comparing the importance of water salinity with rainfall to see the impact of rain falling at the local location on the value of water salinity.

Tidal dynamic in Katingan River and primary channel
The water level has the same pattern at the three measurement locations and can be seen during spring tides and neap tides.The primary canal embankment, average land, and bottom of the immediate canal is located at +5 m, +3.9 m, and +1 m elevations, respectively [6].During the highest tide, the water level is below the primary canal embankment elevation, which is +4.4 m.This means that the water entering the canal from the Katingan River does not overflow the embankment and the water level is above the average land height so that water can flow to agricultural land.At the lowest ebb point, the water level is still above the base elevation of the primary canal (+1.8 m).This means that in the primary canal there is still water in it.The results of tidal water level measurements at the three locations of the portable data logger device can be seen in Figure 6.

Water salinity condition
Salinity is the leading indicator in estuaries because estuaries are a mixing area between fresh and salt water.The water in estuaries is classified as brackish water with salinity levels ranging from 500 to 3,500 ppm [7].The suitability of saline water for irrigation depends heavily on the conditions of use, including crops, climate, soil, irrigation method, and management practices.The following is a classification scheme to identify the level of water salinity can be seen in Table 1 [8].In general, irrigation water has a salinity level between 500 to 1,500 ppm (slightly saline).Each type of plant has different salinity tolerance limits.One of the factors causing soil salinization is salt carried by irrigation water.Namely, when the irrigation water evaporates, salt crystallization occur and thus remain in the soil [7].Based on the land suitability class for rice plants, the inappropriate salinity value is more than 8 dS/m or the equivalent of 5,120 ppm for rice swamps [9,10].
Salinity intrusion will negatively affect plant growth if left in the field for more than three months [5].Areas affected by tides and salinity intrusion for less than three months can still be sown with twice the amount of rice in a year without inhibiting growth.The resulting salt intrusion will also change the water treatment area in the future.Long salt intrusion times and vegetation types should also be considered when determining future water treatment areas [5].To measure water quality from a portable data logger, the EC value (μS/cm) is obtained, so to obtain a salinity value, a conversion is required using the EC value approach (μS/cm) by multiplying a factor of 0.55 in ppm [11].
The measurement results from the portable data logger at location 1 show an unfavorable value.The maximum salinity value that is read exceeds 60,000 ppm, while seawater generally has a salinity between 34,000 ppm and 37,000 ppm [12].So these results need to be corrected by using other comparison tools (handheld) which are considered to be in accordance with the standard.
After correcting the water salinity values in location 1 and, the maximum and minimum water salinity values reached 14,536 ppm and 3.02 ppm, respectively.The maximum and minimum water salinity value that enters the primary canal is 5,547 ppm and 14.5 ppm, respectively.The maximum values salinity in primary canal exceeding the salinity threshold value for rice farming, which is 5,120 ppm.Water salinity measurements were also carried out intermittently at several locations of the primary and secondary canals of Katingan I LIA using a handheld device.It can be seen that the overall range of water salinity values at low tide conditions has a smaller value than during high tide conditions.Still, the opposite is true for the secondary canals.This happens because the water entering the secondary canals is slower than the primary and collector canals, which flow directly into the Katingan River.The result of the comparison of water level and salinity can be seen in Figure 6.

Tidal effect on water salinity
To see the effect of tidal conditions during spring tides and neap tides on the water salinity value in Katingan I LIA, it is necessary to make comparative data in the same period and location.The results of a comparison of tidal data and water salinity at the three measurement locations can be seen in Figure 6.The salinity decrease insignificantly during neap tides, whereas the salinity significantly increase during spring tides.This shows that tidal conditions have a major influence on the importance of water salinity in Katingan I LIA.

Rainfall effect on water salinity
To see the effect of local rainfall on the water salinity value in Katingan I LIA, it is necessary to make comparative data in the same period and location area.The results of comparing daily rainfall data and water salinity at the three measurement locations can be seen in Figure 7.The rainfall data available at the local location is the maximum daily rainfall data obtained from the Agriculture Technical Executive Unit (TEU) of the Katingan Kuala District using a rainfall gauge and an ombrometer.The rainfall data results from daily recording, which is recorded every 7.00 in the morning by TEU officers.To see the comparison results, the rainfall data is assumed to occur at 0.00 on the same day when it rains.These data show that the local rainfall significantly affect the water salinity value at the Katingan I LIA location.After the rains, the salinity decreases in February.This may also be influenced by the fresh water discharge of the Katingan River which has increased from upstream due to rain in the upstream.

Conclusion
The existing water salinity conditions at Katingan I LIA obtained the maximum water salinity value from the three locations of 14,536 ppm and the lowest water salinity value of 3.02 ppm.The maximum water salinity value that enters the primary canal is 5,547 ppm, exceeding the salinity threshold value for rice farming, which is 5,120 ppm, so there is a need for anticipatory measures that can be taken to reduce salinity intrusion, among others, by making water gates, managing water use efficiently, or by optimizing drainage canals.

Suggestion
In this study, the measurement of water salinity was carried out during the rainy season; it is better if the size is also carried out during the dry season to see the comparison of results and the magnitude of the maximum water salinity value that occurs because salinity problems occur more during the dry season.

Figure 1
Figure 1 Location of the Katingan I LIA

Figure 2
Figure 2 Handheld tool Figure 3 Location of instantaneous manual measurements

4 Figure 4
Figure 4 Installation of portable data logger Figure 5 Location of the portable data logger installation

6 Figure 6
Figure 6  The results of the comparison of water level elevation (tide) and salinity

Figure 7
Figure 7 The results of the comparison of maximum daily rainfall and salinity