Study of irrigation aspect in the interconnection system of Telaga Tanjung and Singin Ponds in Tabanan Regency

Interconnection is a channel that links multiple water channel systems with those downstream of a river area. The proposed system will utilize existing canals in Gadungan, Matan, and Unun Irrigation Areas, extending to the Lambuk Reservoir dam system. The problem is that irrigation areas downstream of the Lambuk Reservoir are Lambuk (726 ha), Babakan Anyar (42 ha), and Lanyah (281 ha) experience water shortages (planting intensity of 170% to 200%) during the dry season. The study utilized water balance methods in rice fields, the basic year method for main discharge at weirs, and interconnection channel analysis for water needs calculation. From the water balance analysis, the Singin System cannot make a new interconnection channel to the Lambuk Reservoir because the available water in each reservoir can only irrigate the Matan and Unun Irrigation Areas. From the Telaga Tunjung System, there is still a lot of water available, especially during the rainy season, so that it can supply the reservoir for the Lambuk Reservoir. This means that the interconnection system from this reservoir still needs to be created from the primary channel of the Gadungan Irrigation Area.


Introduction
The continuously decreasing land area for irrigated wet fields in the Province of Bali, particularly in Tabanan Regency, requires efforts of land and water resource management to increase agricultural land area optimally.Specifically in Tabanan, the land area for rice paddies was 32,475 ha in 2018, which decreased to 26,607 ha in 2019 by 10% of the existing land area.Meanwhile, rice production which amounted to 188,446 tons in 2018 decreased to 158,757 tons in 2019 [1].
Tabanan Regency, as one of the rice-producing areas in the Province of Bali, has a sufficiently great potential for water resources in the form of rivers with continuous flow, such as the Yeh Ho River and Yeh Matan River.There is a very striking fluctuation in the discharge for these two rivers, where in the rainy season water continues to run off, while in the dry season, the discharge is small and there is a lack of water.As such, the presence of a containment (reservoir) is very much necessary to fulfill the requirements of supplying irrigation and drinking water throughout the year.
The effort to develop the potential of these two rivers has been conducted with the execution of the construction of Telaga Tunjung Weir at Tukad Yeh Ho and technical planning (detailed design) for the Singin Reservoir at the Yeh Matan River.However, the obstacle faced at the two rivers is the relatively steep topographic conditions, which cause the containment volume to be relatively small and the utilization to be less optimal [2].The Telaga Tunjung Weir is planned to fulfill irrigation water requirements for 1,335 ha of land, covering the irrigation areas of Gadungan Lambuk and Sungsang.Singin Reservoir is planned to contain the flowing water of the Yeh Matan River, which will, later on, be utilized to fulfill water needs for the Yeh Matan and Yeh Unun irrigation areas, 407 ha in total.
The Yeh Matan River possesses an annual inflow that is quite large, particularly in the rainy season.The alternative to exploit the existing discharge of the Yeh Matan River and the discharge of the Yeh Ho River is by providing a new containment at the Lambuk River, which is an affluent of the Yeh Ho River and is located between the Yeh Ho River and the Yeh Matan River.The containment is created with an interconnection system and utilized to overcome the lack of water in the downstream irrigation areas, such as the Lambuk Irrigation Area and the Lanyah Irrigation Area.The development effort for the potential of water resources possessed by the Yeh Ho River and the Yeh Matan River is with an interconnection concept between Telaga Tunjung Weir and Singin Weir, to fulfill the water requirements for downstream areas of the Lambuk River.
This study is intended to find out and obtain detailed information regarding the irrigation aspect, particularly for irrigation areas downstream of the Lambuk Weir concerning the scope of the necessity of this interconnection system.The main problem downstream of the Lambuk weir is that the three existing irrigation areas, namely Lambuk (726 ha), Babakan Anyar (42 ha), and Lanyah (281 ha), experience water shortages during the dry season so that the planting intensity in the three irrigation areas can only be between 170% to 200% with only two planting periods a year.From the background above, in this study, it is necessary to carry out an analysis from the irrigation aspect, namely (a) is an interconnection channel needed from the Singin system and the Telaga Tunjung system to Lambuk Pond.

Research Location
In this study, the research location was carried out in an irrigation area covered by 3 (three) systems, namely : a.The Tunjung Lake system includes the Gadungan Dam which irrigates the Gadungan Irrigation Area.b.The Singin system includes the Matan Dam which irrigates the Matan Irrigation Area and the Unun Weir in the Unun Irrigation Area.c.The Lambuk system includes the Lambuk Dam which irrigates the Lambuk Irrigation Area, the Babakan Anyar Dam which irrigates the Babakan Anyar Irrigation Area, and the Lanyah Weir which irrigates the Lanyah Irrigation Area.All of the irrigation areas above are irrigation areas located in the Selemadeg Timur Water Observation Area.The location of the study area is at a distance of ± 35 km to the west of the city of Denpasar.To reach the study location, you can travel by a relatively good road.The location of the study area can be seen in Figure 1.

Coverage of the Interconnection System
In the case of water utilization from several sources of water (rivers), the first phase in irrigation engineering that must be conducted is the design of water diversion toward a certain point of storage location to result in the maximal supply of water [3].In this study, the coverage of the river system in the interconnection design contains three systems, as follows: 1.The Singin System covers the Matan Weir and the Unun Weir with the channel entering into Lambuk with all its hydrological analytics.2. The Telaga Tunjung System covers the Gadungan Weir with its suppletion channel that comprises the existing irrigation channel to the Gadungan Lambuk Irrigation Area. 3. The Lambuk System covers the Lambuk Weir, which leads to the Babakan Anyar Weir and the Lanyah Weir downstream, and for which containment will be created later on for the supplying of irrigation water to all irrigation areas downstream of the Lambuk Weir.Simply stated, the three systems may be described as the following : Below is a picture of the existing irrigation channel layout which is used for interconnection channels from the Singin system and the Telaga Tunjung system to the Lambuk system.The Singin System interconnection channel is planned to use the Singin and Unun primary irrigation channels.Meanwhile, the Telaga Tunjung system interconnection channel uses the Gadungan and Lambuk primary irrigation channels.
Figure 2 shows a picture of the existing irrigation channel layout which is used for interconnection channels from the Singin system and the Telaga Tunjung system to the Lambuk system.Figure 3 shows the two interconnection channel systems mentioned above can be described simply as below and can be explained as follows: 1.The interconnection channel plan for the Matan system is planned to use the DI Matan primary channel from the Matan weir on the Matan River to the Unun weir on the Unun River and be connected from the DI Unun primary channel and a new channel will be created to the Lambuk storage system.2. The interconnection channel plan for the Telaga Tunjung system is planned to use the DI Gadungan primary channel from the Gadungan dam on the Yeh Ho River to the Lambuk reservoir system.Along this interconnection channel, there are 3 natural channels, namely Pk1, Pk2, and Pk3. 3.This Lambuk reservoir is an upgrade of the Lambung Dam to the Lambuk Dam which will supply irrigation water to the Lambuk, Babakan Anyar, and Lanyah irrigation areas.c.If the water balance at the downstream Weirs, as the Matan, Unun, and or Gadungan Weirs are at a deficit, then suppletion cannot be performed to the Lambuk, Babakan Anyar, and Lanyah Irrigation Areas (interconnection is not needed), because of the reason that the development of irrigation in the respective Irrigation Areas must be prioritized.
If the interconnection is made, then Irrigation Areas downstream of the planned containment at Lambuk, as the Lambuk, Babakan Anyar, and Lanyah Irrigation Areas, may be developed optimally according to the standard wet field area.

Research Steps
Following the title above, from the irrigation aspect, the research steps carried out can be explained as follows: 1. Inventory of primary data and secondary data.Some primary data was collected, such as data on the condition of the irrigation network including existing irrigation canals and buildings, the condition of the planned interconnection canal site that will be used from both the Singin System (primary canal from the Singin weir to the Unun primary canal and ending at the (new) canal site in the Lambuk system Meanwhile, secondary data collected includes climate data for at least the last 5 years, rain data for at least the last 10 years, irrigation network scheme data, discharge data on all related weirs, data on planting patterns that have been applied to all related irrigation areas, and others.2. Conduct consistency test analysis of rain data using the Outlier, Trend, and RAPS methods and continue by calculating effective rainfall for rice and secondary crops 3. Analyze climatological data to obtain potential evapotranspiration values and determine irrigation water requirement parameters.4. Calculate irrigation water requirements in each irrigation area, namely in the Singin system (Singin and Unun irrigation areas, in the Telaga Tunjung system (Gadungan Lambuk irrigation area), and the Lambuk system (Lambuk, Babakan Anyar, and Lanyah irrigation areas).5. Conduct water balance analysis at each weir in the three interconnected systems, namely Singin weir, Unun weir, Gadungan weir, Lambuk weir, Babakan Anyar weir, and Lanyah weir.6.In this study, it has been determined to optimize water utilization in each irrigation area, meaning that the provision of irrigation water in each irrigation area must be prioritized.7. From the results of the water balance analysis at each weir, if the water balance at the weir in each system results in a negative water balance then there is no interconnection allowed from that system to the Lambuk system.On the other hand, if the water balance is positive then from that system it is permissible to make interconnection channels to the Lambuk system.

Analysis Method
The analytical method used in this study is a quantitative method which is based on study data in the form of numbers such as climate data, rain data, weir discharge data, and other technical data.For the analysis of irrigation parameters in this study, the following formula was used :

A. Evapotranspiration Analysis
For technical irrigation planning, assessing the amount of water needed for an irrigation area does not separate evaporation and transpiration.In this case, the process of evaporation and transpiration can occur at the same time so it can be called evapotranspiration.
The amount of evapotranspiration is influenced by several climate factors such as temperature, relative humidity, solar radiation, and wind speed.Besides that, evapotranspiration is also influenced by regional geographical factors such as elevation and regional latitude.
In irrigation engineering, the evapotranspiration value is considered for consumptive use for crops, for which the value is considered equivalent to the value of potential evapotranspiration [4].
Evapotranspiration is the evaporation that occurs from the surface of land that is covered by crops (vegetated areas).If the water content in the soil is indefinite, then the term potential evapotranspiration is used [5].
The following is the calculation of potential evapotranspiration, with the formula for calculating the amount of evapotranspiration by crops [4] : Where: Etc is evapotranspiration (mm/day), Kc is crop coefficient and Eto is potential evapotranspiration (mm/day) Penman (1977) has made the most complete theoretical approach, where it is stated that potential evapotranspiration cannot be separated from incoming solar radiation.This formulation is very well known as the "Modified Mock Method" with the following formula [4]: Where: Eto is potential evapotranspiration (mm/day), C is adjustment factors influenced by day and night weather conditions, W is the weighing factor depends on the average temperature and altitude, Rn is net solar radiation reaching the earth (mm/day) ed is saturated vapor pressure at average air temperature (mbar), and ea is Saturated vapor pressure at average air temperature (mbar).

B. Rainfall Analysis
The rainfall analysis required in this study is in the form of monthly average rainfall and semi-monthly effective rainfall according to the irrigation period.Average monthly rainfall is calculated to determine alternative planting patterns and planting schedules for each irrigation system related to the interconnected system.Meanwhile, effective rainfall calculations are needed to calculate irrigation water needs in each irrigation area.

1). Effective Rainfall Analysis
Effective rainfall is the rainfall that is truly utilized for growing crops.The value of effective rainfall for this study was adjusted to the kinds of crops to be planted, such as rice and secondary crops or groups of crops that require little water.
The Department of Public Works through the Directorate of Water Resources provided a guideline to use a value of effective rainfall with a probability of 80 %. (R80) [6].
In practice, for the calculation of effective rainfall, the following formula is used [6]: a. Rice : Re = 0.70.R80/15 (3) b. 2 nd crops : Re = 0.40.R80/15 (4) Where Re is effective rainfall (mm), and R80 is rainfall for 15 days with a probability of 80% (mm) Effective rainfall with a probability of 80% is determined based on the "basic year planning" method with the following formula: R80 = n/5 + 1 (5) Where n is the number of years of data recording.

2). Mean Areal Rainfall Analysis
Rain that falls on the rainfall measuring tool is called the point rainfall, while the rainfall that is taken into account for the planning of water structures is the mean areal rainfall, which is the mean value of the point rainfall that is recorded at the rain station [7].
The method for obtaining data on the maximum mean annual rainfall involved the method of Thiessen Polygons [8].The following is the calculation formula: Where d is the height of mean areal rainfall (mm), d1,2,.. dn is height of rainfall at rain station 1, 2, …, n (mm) and A1,2,..An is area of influence of rain station 1, 2, …, n (mm)

C. Irrigation Water Requirement Analysis
Irrigation water requirements are the amount of water (discharge) given to an irrigation plot (rice field) which is calculated through irrigation intake and the amount is different for each type of plant [9].Irrigation water needs can be divided into 2 (two) terms, namely [9] : 1.The need for irrigation water in rice fields (field irrigation) is calculated without including the water loss factor in the network.
2. The need for irrigation water at the intake (irrigation requirement) is calculated based on the amount of water needed in the rice fields after taking into account irrigation efficiency (water loss in the network).
There are several factors affecting the amount of required irrigation water, such as cropping patterns, evapotranspiration, water for seedling cultivation, inundation and percolation, rainfall, replacement water, and irrigation efficiency in the channels.The height of inundation has a very close relationship to the amount of percolation.The evapotranspiration value is equivalent to the value for crop requirement, for which the specific value for rice is 8.6 mm/day [8].Meanwhile, the best mean percolation value for growing rice plants is 10-20 cm/hr [7].
Irrigation water requirement may be calculated based on the best conditions, for which the height of minimum inundation in the rice fields is taken into account and the calculation is based on the weekly water balance [4].
Where WR is irrigation water requirement (l/s/ha), ER is effective rainfall (mm/day), and S is additional groundwater refilling (mm/day).The Department of Public Works has provided formulas for calculating irrigation water requirements for rice and secondary crops, as the following [10]: Where KAT Pd is water requirement, rice (l/s/ha), KAT Pal is water requirement, secondary crops (l/s/ha), Kt.Eto is crop evapotranspiration (Etc) (mm/day), Kt is crop coefficient, P is percolation (mm/day), HE is effective rainfall (mm/day), GAL is inundation replacing water (mm/day), and EI is irrigation efficiency (%)

D. Dependable Discharge Analysis on the Weir
Dependable discharge is the minimum mean discharge that occurs continuously in the river.In this study, the calculation of dependable discharge utilized the method of basic year planning.The basic year for planning is the year where the dependable discharge occurs with a dependability percentage of 80%.Its determination is conducted by ordering the annual rainfall data from smallest to largest.
Calculation of the dependable discharge may be performed by using the method of basic year planning, with the formula [11]: Where Q80 is discharge with a probability of 80 % (m 3 /s), and n is the length of the discharge data series Dependable discharge may also be calculated by the Weibull equation : Where m is the rank number of the data, and n is the value of the data series.

E. Water Balance Analysis on the Weir
The parameter that is the greatest determinant of the dependable discharge value is the total discharge available at the Weir and the amount of required irrigation water withdrawn from the intake structure for irrigation.Simply described, the formulation of the water balance at the Weir is stated by the following equation [12]: Qo = Qa -Qi (12) Where Qo is discharge surplus or deficit (m 3 /s), Qa is dependable discharge at the weir (m 3 /s), and Qi is discharge for irrigation at the intake (m 3 /s).

Result A. Analysis of Mean Areal Rainfall
Here, the mean annual rainfall refers to the mean monthly rainfall from the three existing rain stations, which was utilized as the basis for calculating the value of irrigation water requirement.The method of calculation utilized the Polygon Thiessen Method because the rain stations are outside the river basin.The results of calculating the monthly mean area rainfall are presented in the following tables.

1). Mean Monthly Rainfall Analysis of the Matan, Unun, and Lambuk River Basin System
For the Matan, Unun, and Lambuk River Basins, three rain stations affect the river flow and irrigation water requirements for each Irrigation Area, which are the Meliling, Bajera, and Pupuan Rain Stations.Rainfall data available at all rain stations for 15 years (2005-2020).Average monthly rainfall is calculated using the method with results as shown in Table 1 and the monthly rainfall histogram image in Figure 4.    From the histogram of mean monthly rainfall, it is known that the period of water sufficiency is the same for both systems from October to April, while the period of water insufficiency is from May to September.
The establishment of the pattern of water availability for an irrigated field may be based on the consideration that rain greater than 200 mm represents a period of water sufficiency, while rain less than 200 mm represents a period of water insufficiency.

B. Analysis of Cropping Patterns
Following the rain patterns that occur at the two irrigation systems of Gadungan and Telaga Tunjung as well as Matan and Unun, the cropping pattern of Rice I-Rice II-Secondary Crops was selected with a cropping intensity of 280%.The establishment of the cropping pattern utilized an alternative system based on the pattern of water availability in the soil, which is represented by the rain pattern that occurs throughout the year or cropping season.

C. Analysis of Irrigation Water Requirements at the Intake
The irrigation water requirements at the intake structure were calculated for the two Irrigation Area systems for the entirety of the field areas being irrigated.The maximum provision of irrigation water in each irrigation area is 0.661 m3/s for Gadungan, 1.455 m3/s for Lambuk, 0.563 m3/s for Lanyah, 0.768 m3/s for Unun and 0.224 m3/s for Matan.
The amount of irrigation water requirements at the intake is presented in the following Table 3.

D. Analysis of Dependable Discharge at the Weir
The discharge at the weir is the amount of discharge that enters the irrigation channel and spills over the crest.The dependable discharge at the irrigation Weirs in this interconnection system was calculated with the Basic Year method with a probability of 80%.Table 4 presents the results of calculating the dependable discharge for all weirs.

Analysis of Water Balance at the Weir
The calculation of the water balance at the intake is intended to find out the deficit or surplus of water at the weir as considered from the availability of surface water.The cropping pattern for which water insufficiency never occurs may be selected as the alternative cropping pattern.In general, from the analysis of the water balance at all weirs, it can be seen that during the rainy season, there is always excess water and irrigation is sufficient to meet the rainfall that occurs.However, in the dry season at each weir, there is a shortage of water during only a few irrigation periods.The results of the water balance for all weirs in this interconnected system are presented in the following Table 5.The results of the water balance study at each weir can be explained as follows: 1.In the Gadungan irrigation area (Telaga Tunjung system), from the analysis of the water balance in the Gadungan weir, it can be seen that there is a water deficit for only a few periods (semi-monthly), namely April II, May II, June I, August I, II and November 1 (on dry season), so it doesn't have much of an effect on drought in the rice fields because it is accompanied by a water surplus (there has been rain).2. In the Unun and Matan irrigation areas (Matan System), from the results of the water balance at the Unun and Matan weirs, it can be seen that there is also a water deficit in several periods (semimonthly), namely May, June, August (dry season) but this also does not affect drought. in the rice fields because it is also interspersed with rainfall.Irrigation water in this system is prioritized to optimize the supply of irrigation water for the Unun and Matan irrigation areas only.3.In the Lambuk, Babakan Anyar, and Lanyah irrigation areas (Lambuk System), from the results of the water balance analysis there is also a water shortage in each period during the dry season.Because Tukad Lambuk water availability is limited, a storage system with interconnecting channels from Gadungan Weir (downstream of Telaga Tunjung Dam) is needed.For the condition of the biannual water balance, a deficit of water occurred for all Weirs in certain months.

Conclusions
According to the basic design concept above, the following conclusions may be made: (a) From the Singin System (Matan and Unun Weirs), a new interconnection channel cannot be created to the planned Lambuk Weir because the water available at each Weir can only irrigate the irrigation area being serviced, as the Matan and Unun Irrigation Areas, (b) From the Telaga Tunjung System (from Gadungan Weir) plenty of water is still available, particularly in the rainy season, which will thusly be supplied to the Lambuk Reservoir.This means that the interconnection system from this Weir will still be implemented, but it will use the existing channel (without creating a new channel), which is the primary channel of the Gadungan Irrigation Area and (c) The Lambuk System (the Lambuk, Babakan Anyar, and Lanyah Weirs) still requires suppletion, which in this case is taken from the Gadungan Irrigation Area interconnection to increase the cropping intensity of the three Irrigation Areas that are serviced by the Weirs.

Figure 1 .
Figure 1.Map of the Study Location

Figure 2 .
Figure 2. Lay Out of Interconnection System of Singin Weir, Telaga Tunjung Dam, and Lambuk Reservoir Plan

Figure 4 .
Figure 4. Histogram of the Mean Monthly Rainfall Matan, Unun, and Lambuk River Basin Area

Figure 5 .
Figure 5. Histogram of the Mean Monthly Rainfall of the Gadungan and Telaga Tunjung River Bazin Area

Table 1 .
The Mean Monthly Rainfall for the Matan, Unun, and Lambuk River Basin Area

Table 2
shows the Mean Monthly Rainfall Analysis of the Gadungan and Telaga Tunjung River Basin System for the Telaga Tunjung and Gadungan River Basins, three rain stations affect the river flow and irrigation water requirements for each Irrigation Area of each River Basin, which are the Baturiti, Penebel, and Bajera rain stations.And monthly rainfall histogram image as Figure2.

Table 2 .
The Mean Monthly Rainfall for the Gadungan and Telaga Tunjung River Basin Area

Table 3 .
Irrigation Requirement at Intake

Table 4 .
Analysis of the Dependable Discharge at the Weir

Table 5 .
Analysis of the Water Balance at the Weir Overview of Water Balance at the WeirsFor the development condition with the application of the rice-rice-secondary crops cropping pattern with a cropping intensity of 280%, the following was obtained : a. Annual Water Balance For the condition of the annual water balance, a surplus of water occurred for all weirs : 1. Gadungan Weir, surplus of 110.95 m 3 /s.2. Matan Weir, a surplus of 18.389 m 3 /s.3. Unun Weir, surplus of 3.475 m 3 /s.4. Lambuk Weir, surplus of 11.265 m 3 /s. 5. Babakan Anyar Weir, surplus of 2.112 m 3 /s.6. Lanyah Weir, surplus of 19.424 m 3 /s.b.Seasonal (Biannual) Water Balance