Analysis of water supply fulfillment for watering plants needs in potentially upcoming El Nino period using reliable discharge and planned rainfall discharge methods

Based on the statement from the Meteorology, Climatology, and Geophysics Agency (BMKG), Indonesia is potentially entering a period of El Nino and facing below-normal dry conditions that lead to drought. The Environmental Service of Surabaya City (DLH), that in charge of watering city parks every day is affected because this watering process utilizes water from existing water channels as its source. During the El Nino period, it is feared that the water channels will dry up, resulting in insufficient water supply for watering. The solution then is to analyze the water supply fulfillment. The method used is calculating the planned rainfall discharge to determine the water level in the rainy season and the reliable discharge to determine the water level in the dry season. Comparing the two water levels, the difference in water levels is then used to calculate the required water discharge. Through this analysis, it is found that the water demand during the dry season increases by 130 percent, reaching 676,000 liters. However, the water level in the existing channels decreased to 0 meters in July. Besides, the availability of alternative water from WWTP reaches 846,700 liters, which is sufficient to fulfill the watering water demand.


Introduction
Meteorology, Climatology and Geophysics Agency (BMKG) is one of the authority bodies authorized by law and responsible for managing data and information about weather, climate, and disasters in Indonesia [1].By 20 May 2023, BMKG has reported that Indonesia is potentially entering a period of El Nino with a 60 percent chance and even a 90 percent chance in August.Initially, the term "El Nino" referred to a gentle warm ocean current that flowed southward along the coasts of Peru and Ecuador around Christmas (thus named "the boy Christ-child" in Spanish, "Nino") [2].Later, it became linked to irregular and significant warming every few years, leading to alterations in the local and regional ecosystems [2], [3].
The most prominent pattern of climate changes has been the global set of anomalies referred to as El Nino, ENSO (El Nino and the Southern Oscillation), is enormous forest fire on Kalimantan spread a thick cloud of smoke over Southeast Asia and crippled air travel by shutting down airports in Singapore, Malaysia, and Indonesia [3], [4].This gentle warm ocean brings sea level warming chances and a dry season condition for a long time [5].In this case, it is feared that Indonesia will have some water issues affected by El Nino, such as water supply availability for watering plants needs.The Environmental 1311 (2024) 012039 IOP Publishing doi:10.1088/1755-1315/1311/1/012039 2 Service of Surabaya City is tasked with watering city parks every day to retain the Open Green Space [6].This watering process utilizes water from existing water channels that may dry up, resulting in insufficient water supply.Analyzing the water supply fulfillment by comparing the water demand during the rainy and dry seasons and the water availability in the existing channels during both seasons is needed.Information on how much flow discharge is available from a water source can be obtained from recorded river flow discharges or from high rainfall data recorded at a rainfall station.High rainfall data can be used when having timekeeping to record [7], [8].Then, it is used to calculate the water level on the existing channels in rainy seasons.As for the water level on the existing channels in dry seasons, the dependable flow as the design value of the discharge from the monthly average is calculated [7], [9].Compared to the two water levels, this analysis will provide the difference in water levels, which can be used to calculate the required water discharge.This analysis is then used to determine the needed water discharge, which becomes a consideration for alternative water supply sources.An alternative water supply plan is to utilize a Wastewater Treatment Plant (WWTP) which has a guaranteed daily wastewater discharge [10].

Literature Study
A literature study is needed to conduct and complete this research.It is an initial stage in collecting several theoretical references to ancillary activities related to the same field [11].This stage needs to be done to inventory the previous study's results and determine the data requirements related to the title of the same area in different locations [12].

Research study location
The selected locations within the scope of the research at this stage are shown in Figure 1.The focus is on Timur 2 (the purple area, the left side, and the second from the bottom) as shown.

Calculation of daily water discharge needs
To ensure the daily needs for watering plants, calculation by analyzing every route for each time in a day and its water discharge needs is needed.The output is to know the proper daily water discharge that DLH always spends in a day [13].

Calculation of planned rainfall discharge
There are several stages to obtain the value of the planned rainfall in an area to be planned or evaluated.
The rainfall data for this calculation used the last 12 years from the rain gauge stations in Gubeng, Surabaya.These rain gauge stations are spread throughout the Catchment Area in Gubeng, Surabaya.This calculation obtained the maximum annual rainfall that will be used in the following calculation and analysis [10], [14].

Calculation of dependable flow
Dependable flow is the minimum river flow at a certain opportunity level that can be used for water supply purposes [15].The dependable flow is calculated with 70% reliability.From 10-year discharge data, discharge data is sorted from largest to smallest value to get a 70% probability value of the data sequence [16].If the dependable flow is set for hydropower purposes at 70%, then the risk of failure is 30%, this happens at a pickup discharge smaller than the calculated discharge [17].For irrigation purposes, a commonly used probability of 80%.For drinking water and industrial purposes, of course, it is required a higher probability, which is 90% to 95% [18].

Calculation and comparison of each water level
Calculation of water level in the rainy season needs the planned rainfall discharge approach.On the other hand, calculating the dry season water level needs a dependable flow value.It also uses crosssection water channel data [19].

Analysis of an alternative water supply plan
An alternative source is needed to cover the less water existing on channels.One of the considerations is the availability of alternative water from a Wastewater Treatment Plant (WWTP), which has a guaranteed daily wastewater discharge [20].

Calculation and analysis of daily water discharge needs on Timur 2
The analysis was carried out within all routes, multiple time water uptake from the channels, and their timings.There are three timings and five car operations each time with a maximum capacity of 5,000 liters.But the exceptional condition at nighttime, only three cars operating.Data in range number 1-10 are from the morning shift, 11-17 are from the evening shift, and 18-25 are from the night shift, with the total value of cumulative liters spent in a day being 520,000 ltr.As a result, daily water discharge needs in a day is 520,000 ltr.

Calculation and Analysis of Planned Rainfall Discharge
In analyzing the water availability in the channel on Rayon Timur 2 a rainfall calculation plan (Rayon Gubeng Drainage).To see the maximum rainfall value in Rayon Gubeng Drainage can be seen in Table 2.
Table 2  The calculation of the basic statistical parameters of the normal method can be seen in Table 3 and Table 4. Basic statistical parameters results of the Gumbel method have the same value as the result of the normal method except Ck which has the final value of 2.32.The last, method used is Log Pearson III, resulting in Ck = 2.22 and Cs = -0.18.
Each method has terms such as the normal method, Cs = 0 and Ck = 3, and the Gumbel method, Cs < 1.1396 and Ck < 5.4001.After checking the terms and clarifying the suitability of the frequency distribution that will be used, the normal method does not qualify.In the precipitation calculation, the re-period will be selected using the Log Pearson III method.In the calculation of the Log Pearson Type III method, the value of the Cs is -0.18.For this reason, it is necessary to interpolate to obtain the value of K. Interpolation calculations can be seen in Table 5.A frequency distribution test will be carried out which is divided into two methods.Namely the Chi-Square and Smirnov-Kolmogorov Test methods.The calculation of the Chi-Square Test is as follows: Based on degrees of freedom (DK) of 3 and degrees of trust (a) = 5%.Thus, a theoretical Chi-Square value of 7.815 is obtained.By The Smirnov-Kolmogorov Test methods, Dmax = -0.014.With a value of the number of data (n) = 12 and the value of the degree of confidence (a) = 5 % then obtained a critical D value of 0.38 by interpolation.After conducting a frequency analysis of rain distribution under the condition of the data and testing the results of the analysis can be conveyed that planned precipitation values (CH) with repeat periods of 2, 5, 10, 25, 50, and 100 years that can be seen on Table 7.

Calculation and analysis of dependable flow
As explained before, dependable flow value comes from 10-year discharge data, discharge data is sorted from largest to smallest value to get a 70% probability value of the data sequence.The calculation results above are sorted from largest to smallest data.A dependable flow value of 70% is taken with the meaning of a probability of error of 30%.For example, table 8 is a 10-year discharge data in January.This data was then sorted from the largest to smallest value.To get the Q70, it is needed to pick the right 70% probability value.With a total of 10 data, the 70% probability value is on the 7th value that is already sorted from largest to smallest.From the results of the dependable flow of 70%, there is a potential for having zero discharge in four months (July -October) which is quite important to analyze the rest water discharge needs.

Calculate and compare each water level Surface and analyze the water discharge needs in the dry season.
The value of planned rainfall discharge combined with the cross-section of the channel gives a result of the water level, as shown in Table 10.
• I = 0.0002 • v = 0.00 m/sec • Q = 0.000 m³/sec Changes in water levels in seasons of rainy and dry seasons can be seen in Figure 2. Therefore, it can be concluded that water needs in the dry season are more than the rainy season.However, the availability of water in the channel decreased.For this reason, preparation is needed regarding water sources that can be utilized.For example, water coming from the Installation Wastewater Treatment (WWTP).

Analysis of an alternative water supply plan
To get an alternative source, there is research to have some data on the availability of WWTP, nearby from the route and the actual water channels.The research method is by getting information that came from an interview with a developer who has information about which WWTP has good conditions for watering plants and then mapping the WWTP location.That information is then used to create an WWTP map.As a result, a map with an overlay of the watering plant's route and its water channel uptake point with the potential WWTP location is obtained.The outcome from the potential WWTP location in Timur 2 reaches up to 846,700 m³ alternative water discharge.With the amount of existing water discharge and the Q70 discharge on Timur 2 as a sample as shown in Figure 4 that represented the discharge coming from the rain that will be decreased after being affected by El Nino, Utilizing WWTP as the alternative water source for watering plants is a valuable opportunity.With all these results, a comparison is made between water discharge needs (normal), water discharge needs during the dry season (upcoming El Nino), and the potential WWTP availability discharge as shown in Figure 5.This comparison is made to show the large opportunities in using WWTP water as an alternative water source in the upcoming El Nino.
Analysis data results on Timur 2 end up for a valuable opportunity as well as all six rayons in Surabaya.A comparison of water discharge needs (normal), water discharge needs during the dry season (upcoming El Nino), and the potential WWTP availability discharge as a form of consideration in all six rayons can be seen in Figure 6.

Conclusions
The total water discharge for watering plants in Surabaya City is 3,615,000 m³.However, after being affected by El Nino, the needs in the dry season will increase to 130% from the regular needs.Therefore, the need for water during the dry season reaches 4,699,500 m³.On the other hand, WWTP has potential alternative water discharge in cumulative 5,884,660 m³ which is sufficient for fulfilling the watering water demand.This means the soaring needs of water discharge demand in the upcoming El Nino can be overcome by taking advantage of potential water discharge from nearby WWTP.But in this opportunity, of course, there are still risks.Risks that occur such as additional fuel costs for extra accommodation to take to WWTP and the need for further study on whether WWTP water is considered safe in water conditions to be splashed to plants.But also, this risk is considered the better solution if looking up to another option such as making drilled wells that cost up to 500,000/m on the market.The need to water plants is also considered important in the El Nino situation regarding that losing plants will lead to a worse situation and having probability create a new environmental problem.

Figure 1 .
Figure 1.Distribution of research locations (watering plants route areas and existing channels) in all rayon in Surabaya

Figure 2 .
Figure 2. Graph of channel water level changes during the rainy season and dry Season

Figure 3 .
Figure 3. Overlay of the watering plant's route and its water channels uptake point with the potential WWTP location in Timur 2.

Figure 4 .
Figure 4. Discharge Q70 graph (water discharge during the dry season in upcoming El Nino) on Timur 2, Gubeng

Figure 5 . 10 Figure 6 .
Figure 5.Comparison of water discharge needs (normal), water discharge needs during the dry season (upcoming El Nino), and the potential WWTP availability on Timur 2.

Table 1 .
Calculation of daily water discharge spends for all routes

Table 3 .
Calculation of Basic Statistical Parameters Normal Method

Table 4 .
Basic Statistical Parameter Result Normal Method

Table 5 .
An interpolated calculation to get the deal of k

Table 7 .
Basic Statistical Parameter Result Gumbel Method

Table 8 .
Discharge data on January in 10-year

Table 9 .
Dependable flow calculation