Analysis of Potential Water Availability for Irrigation Needs at Lekopancing Dam, Maros

. Lekopancing Dam is an infrastructure that has the function of providing water supply for irrigation demand. To prevent a water availability deficit, a water balance analysis is conducted. This analysis will compare the need and availability of water, especially for irrigation. The purpose of this study is to analyze and determine the level of water availability at Lekopancing Dam for agricultural purposes in Maros Regency. Analysis of water availability with the water balance method is based on effective rainfall data, actual evapotranspiration, potential evapotranspiration, mainstay discharge, NFR of rice fields and irrigation water needs in the range of 2013-2022. The results showed that water availability in the December to July period I, tended to be greater than water demand, with the highest water availability of 22.02 m 3 /s. The highest water demand occurs in the August-November range and a deficit of 11.35 m 3 / s. The potential availability of water for irrigation needs at Lekopancing Dam is quite large because the water deficit is small and occurs at the end of the growth period or entering the harvest period.


Introduction
Water is a basic need for humans to meet their daily needs.Especially in the agricultural sector, water plays a very important role in determining optimal crop yields [1].The quality of crop yields is highly dependent on the availability of sufficient water on agricultural land.The availability of water in the irrigation system will affect the cropping pattern in agricultural land.The cropping pattern in the rainy season is certainly different from the cropping pattern in the dry season.However, currently, many agricultural areas are facing serious challenges related to water availability.Climate change, including unstable rainfall patterns, rising temperatures, and longer periods of drought, is further exacerbating the situation.
The water balance in a dam is very important in agricultural activities, especially in areas that rely on irrigation water for agriculture.Dams can be a source of irrigation water for agriculture in the surrounding area.However, to ensure the continuity of water supply for agriculture, it is important to consider the balance between water availability and water demand.According to [2] the availability of water in a dam is influenced by factors such as rainfall, river discharge, and evaporation.Meanwhile, the water demand for agriculture can vary depending on the type of crop planted, the planting season, and other factors such as soil moisture.
The Lekopancing Irrigation Area is located in Lekopancing Village, Tanralili District, Maros Regency, South Sulawesi Province, which is approximately 31.3km from Makassar City.This irrigation area has a potential area of 3.626 Ha and a functional area of 3.578 Ha which is supplied by water from the Maros River.The Irrigation Network has a length of 35.205 m [3].The Lekopancing Dam is a water control infrastructure that has great potential to supply water for the irrigation needs of agricultural land in the surrounding area.However, efficient and optimal water management at the Lekopancing Dam is still a challenge.Unstable water availability, climate change, and changes in rainfall patterns can affect the water supply needed for agriculture.Therefore, a comprehensive analysis is needed to understand the existing water potential and its development to ensure the sustainability of agriculture.
In the analysis of water potential and water balance in a dam, it is necessary to consider factors such as the dam's storage capacity, the use of water for other purposes, the discharge of water into and out of the dam, rainfall patterns, and water use in different planting seasons.The results of this analysis can provide important information for water resources management and the sustainable development of agricultural land around the dam [4].
If water availability is not in balance with water demand, then there may be water shortages or even excess water that can cause damage to plants.Based on the problems mentioned above, it is important to carry out an analysis of the potential water availability in a dam to ensure that the supply of irrigation water can be met consistently and according to agricultural needs.

Material and Methods
In general, data can be divided into two types primary data and secondary data.Primary data is data that is collected directly in the field, such as surveys of water availability conditions, interviews with employees of the Lekopancing PU Office, and documentation.Secondary data is data that is collected from relevant institutions, such as data from the Balai Besar Wilayah Sungai (BBWS) Pompengang Jeneberang the Maros Regency Agriculture Office, and the Lekopancing Irrigation PU Office.The secondary data in question are daily rainfall data for 10 years, climate data, flow data, the Lekopancing Irrigation Area map, agricultural commodity data, and PDAM water demand data.

Calculation of Evapotranspiration (Eto)
The calculation of evaporation and transpiration (evapotranspiration) is carried out by referring to Penman data and methods that have been modified to suit the conditions of the Indonesian region [5]:   (/) = 0,1 -0.9./ (8)  ()= 0.27.(1 + 0.864.) (9)  = . (10) ea =7.01 x 1.062T (11) d = 2(0.00738x Tc+0.8072 -0.0016 (12) y = 0.386 x L/P (13) P = 1013 -0.1055 x E (14) L = 595 -0.510 x T (15) where W is the temperature and height factor, Rs is the shortwave radiation (mm/day), Ra is the shortwave radiation reaching the outer boundary of the atmosphere, Rn1 is the net longwave radiation (mm/day), f(T) is the temperature function, f(ed) is the vapor pressure function, f(n/N) is the clearness function, f(u) is the function of wind speed at an altitude of 2m (m/s), (ea-ed) is the difference between saturated vapor pressure and actual vapor pressure, RH is relative humidity (%), and C is the Penman correction factor, which is based on day and night conditions.

Hydrological Analysis
The procedure or stage of hydrological analysis in this study consists of an analysis of water availability and demand at the dam by calculating effective rainfall.Effective rainfall for rice crops is 70% of the monthly rainfall that exceeds 80% of the time in a period [6]: Re = 0.7 x 1/10 R80 (16) where Re is the effective rainfall (mm/day) and R80 is the minimum monthly rainfall of 80% (mm/day).
The methods for analyzing irrigation water requirements, both during land preparation and during the growing season, can be seen as follows: IR= M.e K e K -1 (17) where IR is the Irrigation Water Demand during Land Preparation (mm/day), M is Water Loss (mm), M is Eo + P, Eo is Evaporation (mm), P is Percolation (mm), K is (M x T)/S, T is Duration of Crop Cultivation (days), S is Water Loss to Saturated Soil (200-250 mm) and e is the exponential number (2.718).
Then the crop water demand is calculated according to the planting time and type of crop.The calculation of crop water demand is determined based on the following equation [7].NFR = ETc + P + WLR -Re (18) Etc = Kc x ETo (19) where NFR is the water demand in the paddy field (mm/day), ETC is the crop water demand (mm/day), WLR is the water layer replacement (mm/day), which is 3.The irrigation water demand is calculated based on the area of the paddy field, so the irrigation water demand equation is as follows.Q = NFR x A (20) where Q is the irrigation water demand discharge (m 3 /sec) and A is the area of the paddy field (Ha).Furthermore, in the calculation of annual irrigation water demand, it is proposed to use the highest water demand in the paddy field divided by the irrigation efficiency.The irrigation efficiency is 0.65 [8].
The calculation of reliable discharge to determine water availability is carried out using the Weibull equation [9].
where P is the probability of occurrence of the expected set of values during the observation period (%), m is the event sequence number and n is the number of data.

Water Balance
Water balance analysis is calculated based on the difference between needs and water availability by the following equation [10].
If the water balance result is positive, it indicates that there is an excess of water (surplus), while if the water balance result is negative, it indicates that there is a water shortage in the location being studied (deficit).

General Condition of the Area
The Lekopancing Irrigation Area is located in Lekopancing Village, Tanralili District, Maros Regency, South Sulawesi Province, which is approximately 31.3 km from Makassar City.This irrigation area has a potential area of 3.626 Ha and a functional area of 3.578 Ha which receives water supply from the Maros River.The Irrigation Network has a length of 35.205 m [3].
The Lekopancing Dam was built right in the Maros River watershed in Pucak Village, Tompobulu District, Maros Regency.In 1989, the use of raw water from the Lekopancing Dam was increased to 1.000 l/d.After two years of being used as a source of raw water for the PDAM Kota Makassar, precisely in 1979, on an open channel about 1 km from the Lekopancing Dam, a water treatment plant was built to treat raw water from the Lekopancing Dam for drinking water [3].The Lekopancing dam has a sand pocket dam width of 25.6 meters, 13 dam gates, a tunnel, a drainage structure, 7 siphons, a 2 x 2.40 m intake width, and a planned intake discharge of 8.0 m3/s.In the process of developing the Maros River as the site for the construction of the Lekopancing Dam, the river was initially not as wide as it is now.However, due to the Lekopancing Dam construction project in the Maros River, the river was dredged and widened [11].

Evapotranspiration
The calculation of potential evapotranspiration (Eto) values using the modified Penman method.The calculation requires climate data in the form of temperature data, relative humidity, wind speed, and hours of sunshine.The evapotranspiration calculation is at the Maros climate station with a position of 4.93 o LS and 119.50 o BT with an elevation of 13 m.Based on the graph analysis results in Figure 2, it shows the variation of monthly evapotranspiration.From the calculation results using the Penman method, the average evapotranspiration value is 4.18 mm/day or 62.83 mm/month with a total evapotranspiration of 50.26 mm/day.The maximum evapotranspiration occurred in the first period of September, reaching 6.15 mm/day, while the minimum evapotranspiration occurred in the second period of June, reaching 3.06 mm/day.The value of evapotranspiration (Eto) varies every month due to climatic factors.This is in line with the statement of [12], which states that some factors that affect the value of Eto are the intensity of sunlight, temperature and air pressure, wind speed, and leaf area of vegetation

Effective Rainfall
Effective rainfall is rainfall that can be utilized by plants for their growth.With effective rainfall, the water needs of plants can be met and will also minimize the use of irrigation water, which is continuously decreasing, and the available water can be stored to meet the water needs of plants in the future.In the calculation of effective rainfall which was carried out using the Weibull probability analysis method with 15-day rainfall data for the last 10 years from 2013 to 2022, the data input is in the form of rainfall data per monthly middle period or 15 days.For rice plants, effective rainfall is usually predicted to be 70% of the monthly middle rainfall with a probability of 80% of the period [13].Based on Figure 3, the maximum effective rainfall for rice occurs in the first period of January, at 11.6 mm/day.The minimum effective rainfall occurs from the first period of August to the end of September, at 0 mm/day.Rice cannot be planted from the end of June to October due to the dry season or very low effective rainfall.This is in line with the statement [14], who stated that climate change can affect rainfall patterns in a region, resulting in a shift in the start of the rainy season.As a result, the abnormal rainy season is not fully utilized by plants, which often leads to natural disasters.

Raw Water Demand
Lekopancing weir is one of the raw water sources that is managed directly by the Maros Regency and Makassar City Regional Water Company (PDAM).After the availability of raw water, it is then distributed for agricultural irrigation.The water needs that are directly distributed for PDAM raw water are 1300 liters per second (l/s) or 1.3 cubic meters per second (m 3 /s) with an irrigation area of 3.626 hectares (BBWS Pompengang Jeneberang, Makassar).

Irrigation Water Requirements
One of the most important irrigation water requirements is the land preparation water requirement.In this study, the water requirement used was 250 mm, where the water requirement for saturation was added to a 50 mm layer of water.(1986).The amount of water required to replace the water layer is 50 mm/month (or 3.3 mm/day for half a month).The standard percolation rate is 2.0 mm/day, which is used to estimate the water requirement in rice production areas [15].Based on Figure 4, the highest NFR value was 12.74 mm/day in September of the first period.This also indicates that the water requirement during land preparation is greater than when the planting season begins.Based on the NFR value, it is seen that January I and February I have a very low water requirement of 0.00 mm/day and 0.07 mm/day, respectively.This is because these months are the months of harvest.
The water requirement in the rice fields for the Lekopancing dam is 1120 ha or 11200000 m2.Therefore, the water requirement in the rice fields can be seen in Table 2.The calculation of annual irrigation water requirements is proposed to use the highest water requirement in the field divided by an irrigation efficiency of 0.65 [8].The irrigation efficiency is 0.65, so the irrigation water requirements at the Lekopancing dam can be seen in Table 3. Irrigation water requirements are calculated to determine the amount of irrigation water demand, both during land preparation and planting.The factors that affect irrigation water requirements during land preparation and planting are climatic conditions such as evaporation, percolation, crop coefficient, planting pattern, and effective rainfall [16].The graph above shows the relationship between effective rainfall (Re) and crop evapotranspiration (Etc).The graph shows that from May to November in the second period, Re is lower than Etc.This occurs when the rainfall is insufficient to meet the water needs of the crop.This is in line with the opinion of [17], who stated that crop evapotranspiration is higher than effective rainfall due to several factors, including high rainfall, high air temperature, low air humidity, high solar radiation, and high wind speed.

Water Availability Analysis
The calculation of reliable discharge is performed using the Weibull method.To determine the reliable discharge at the observation point, the first step is to rank the results of the calculation of the average monthly discharge of the Lekopancing Irrigation from largest to smallest [9].
The summary of available discharge of 80% and 50% is calculated from January to December with a period of 10 years between 2013 and 2022 [18].The availability condition of the monthly reliable discharge with a probability of 80% and 50% can be displayed graphically to see in more detail the changes or fluctuations in the amount of water available at the Lekopancing Dam.The results of the water availability graph at the Lekopancing Dam are shown in Figure 6.The reliable discharge value with Q80 can be seen in Figure 10 above.It shows that the biweekly discharge has decreased significantly from March to October and increased again in November.Overall, the monthly maximum discharge occurred in February with a value of 22.01 m 3 /s and the minimum discharge occurred in the first period of October with a value of 0.13 m 3 /s based on the results of the 80% probability recapitulation.This is in line with the opinion that a probability of 80% is usually used for irrigation purposes [19].
Based on Figure 6, the shape of the graph has almost the same tendency.The minimum reliable discharge with Q80 and Q50 occurs in the middle of the 13th to 19th month and has the same value or magnitude, while in the middle of the 20th to 24th month and the middle of the 8th to 13th month has the same tendency.

Water Balance
The analysis was performed by calculating the difference between irrigation water demand and irrigation water availability based on grouping half a month or 15 days.In the water balance analysis, the amount of water availability uses a probability value of 80% of the average discharge of the Lekopancing dam.The results of the calculation between irrigation water demand and irrigation water availability for agriculture can be seen in Table 4. Water balance calculation is carried out based on the comparison between water availability (inflow) and water demand (outflow).Water availability (inflow) consists of reliable discharge with a probability of 80%, while water demand (outflow) consists of irrigation water demand and PDAM water demand of 1.3 m 3 /sec.
The table above shows the comparison between the available discharge and the amount of irrigation water demand in the Lekopancing irrigation area.The available discharge can be known in the dry season where the water available in the dam is less than the water needed.It can be seen from the table above that there is a water shortage (water deficit) in the period that cannot meet the water needs in the Lekopancing irrigation area, namely from July II to November.If the result of the water balance shows a positive value (+) then it is said to be a surplus and if the water balance shows a negative value (-) then it is said to be a deficit [4].

Conclusion
Based on the results of the study of the Potential Availability of Water for Irrigation Needs at the Lekopancing Dam in Maros Regency, the following conclusions can be drawn: 1. Water availability at the Lekopancing Dam increases in February of the first period and has the smallest discharge in October of the first period.2. Water availability in the range of December to July of the first period tends to be larger than water demand, with the highest water availability of 22.02 m 3 /s.The highest water demand occurs from August to November and there is a deficit of 11.35 m3/second.

Fig. 5 .
Fig. 5. Graph of the Relationship Between Effective Rainfall and Crop Evapotranspiration.

Table 1 .
Land preparation water requirements Lekopancing Irrigation Area in Maros Regency has two rice cropping patterns, with the first cropping season (MT I) starting in April to September and the second cropping season (MT-II) starting in October to March.The water requirement for replacing the water layer (WLR) is set by the Directorate General of Irrigation

Table 2 .
Water requirement of rice per hectare

Table 3 .
Irrigation water requirement of Lekopancing dam

Table 4 .
Water Balance for the first season of rice cultivation

Table 5 .
Water balance for the second season of rice cultivation