Reliability of reservoir inflow for supporting the operation simulation model of multi reservoir

This research intends to analyze the inflow reliability of the Tugu-Bagong multi-reservoir. The utilization of Tugu Dam is as follows: for fulfilling the irrigation water need of Ngasinan Irrigation Area an area is 1,200 ha, supplying 12 l/s of raw water, controlling the yearly flood in Trenggalek Regency with the discharge is 76.21 m3/s, as the potency of mini-hydro electrical generation (PLTM) in the amount of 0.40 MW, and as the tourism means. However, Bagong Dam, besides being utilized for flood control, is also utilized for supplying raw water, irrigation, and domestic water; for water resources conservation; and recreation or tourism. The methodology consists of collecting the rainfall data and watershed parameters like watershed area, infiltration coefficient each for dry and rainy seasons, retention coefficient, SMC value, etc.; to analyze the dependable discharge due to the rainfall analysis input in 4 discharge conditions: dry season discharge, low water discharge, normal water discharge, and sufficient water discharge. The result of inflow reliability is hoped to be able to support the operation simulation modeling of the Tugu-Bagong multi-reservoir.


Introduction
The reservoir operational pattern is the period operational standard of a reservoir in which the outflow of the reservoir has to follow the requirement so the elevation is maintained regarding the design [1], [2].The reservoir operation pattern at least consists of the procedures of outflow from the reservoir regarding the volume condition and or reservoir water elevation and water need, and also the river capacity in the dam downstream [3], [4].
Jayadi [1] said that the optimization model is one of the alternatives in compiling the reservoir operation rule [5] by attending maximally to the aspects that are related to the aim of reservoir utilization, and involves all of the constraints that limit the effort of water resources utilization.There are three stages in preparing the optimization model that are to identify the objective function, quantitative decision variable, and boundary factor [6][7].Some optimization techniques that are used for reservoir operation are linear programming, non-linear programming, dynamic programming (deterministic and stochastic) [8], classification of provision zones by using a rule curve, and raw operation pattern (SOP) [1].
In the Trenggalek Regency, the development of water resources for water broke power control, and water resources utilization is still needed for utilizing the potency that is still available for increasing the society's prosperity by maintaining environmental sustainability.The development of Tugu Dam will be finished in 2021 and the ongoing development of the Bagong Dam is the implementation of the potency utilization of water resources development in Trenggalek Regency.
The downstream of Tugu and Bagong Dam met on the same main river that is Ngsinan River [9] so there is the intersection of the same utilization that is related to the reservoir utilization as flood control in Trenggalek Regency.Therefore, it is needed the operation pattern of the two reservoirs [10] related to flood control so it can reach the optimal benefit value by optimizing each reservoir as the water supply for irrigation, domestic water, and the potency of micro-hydro electrical generation.

Study Location and Methodology
The development of Tugu Dam is implemented in 2 stages because there is a design change of the dam when the implementation period and the change is significant enough that the cost change is more than 10% of the contract value.Therefore, it has to be implemented in 2 stages of development.However, the benefits of the Tugu dam implementation are as follows: 1) to fulfill the irrigation water need in the Ngasinan irrigation area with an area is 1,2oo ha; 2) to supply the raw water in an amount of 12 l/s; 3) To control the yearly flood in Trenggalek Regency with the discharge is 76.21 m 3 /s; 4) potential for mini-hydro electrical generation in amount of 0.40 MW; 5) to be as tourism facility.
The Bagong Dam is designed in the Bagong River and is located in the Trenggalek Regency.The Bagong River contributes big enough flood discharge so the flood control system of Ngrowo-Ngasinan-Parit Raya cannot be totally fulfilling the flood that is happening in Trenggalek City and surrounding it.
The location maps of Tugu and Bagong Dam are presented in Figures 1 and 2. However, the research method is as follows: 1) to collect the hydro-climatology data (Tugu area and Bagong area); 2) to carry out the quality test of data; 3) to analyze the dependable rainfall; and 4) to analyze the discharge rainfall.

Analysis of Dependable Rainfall
The effective rainfall is part of rainfall that is effective in a hydrology process which can be utilized for instance the utilization by crop (KP.01-2023).According to Department of General Work (PU) KP.01-2023, for irrigation of paddy: the monthly effective rainfall is taken from 70% return period of minimum half monthly rainfall due to 5 years return period.The formulation for analyzing the effective rainfall is as follows: where R e is effective rainfall in mm/day and R (half monthly) 15 is minimum half monthly rainfall with 5 years return period.

Analysis of Dependable Discharge
Dependable discharge is defined as the available discharge for a long year with a certain failure risk.
Based on the experience and observation [11], the dependable discharge for various interest is as follow: a. Drinking water 99% (frequently close to 100%) b.Industry 95 -98% c.Irrigation (half moist) 70 -85% d.Irrigation (dry) 80 -95% e. Hydro electrical power 85 -90% The basic year method is generally used in the design or management of irrigation [11].Generally, the irrigation field uses 80% dependable discharge, so the formula for determining the basic year is as follows: where n is the return period and R 80 is the discharge with the possibility of rain that is smaller than R 80 is 20% and the other is 80% with the possibility of rain not less than R 80.

Collecting of Hydro-Climatology Data
Collecting of Hydro-Climatology data is carried out in the Tugu and Bagong areas.The climatology data is presented in the monthly average that is as the evapotranspiration data in the Tugu reservoir area (as presented in Figure 1) and the Bagong reservoir area (as presented in Figure 2).4, it is seen that the monthly average evapotranspiration in the Bagong reservoir area has the highest value in November (about 5 mm/ day) is in the beginning of the rainy season and the lowest one is in June (about 3 mm/ day) that is happened during dry season.

Analysis of Dependable Rainfall
Analysis of dependable rainfall is carried out for analysis of irrigation water requirement.The dependable rainfall analysis is carried out in Tugu Reservoir (Figure 3) and Bagong Reservoir (Figure 4).

Analysis of Dependable Discharge
Analysis of dependable discharge is carried out by using the basic year method for irrigation needs.This analysis is carried out in Tugu Reservoir (Figure 5) and Bagong Reservoir (Figure 6).
Figure 7 is the graphic of the transformation result from rainfall to discharge during 35 years (1987-2022).By using the basic year method, it is obtained the 80% dependable discharge in 2015 (bold blue curve).

Conclusion
Based on the dependable analysis in Tugu and Bagong Reservoirs, there is a similar characteristic in the dry season period, so the reservoir storage in the rainy season has to be optimal.However, the characteristic of the flood peak period that is different can be utilized as the flood control of multireservoir in controlling the flood in the Trenggalek City area that is downstream of the two reservoirs.

Figure 1 .
Figure 1.Location map of Tugu Dam

Figure 2 .
Figure 2. Location map of Bagong Dam

Figure 3 .
Figure 3. Evapotranspiration in the Tugu Reservoir Area Based on Figure 3, it is seen that the monthly average evapotranspiration in the Tugu area has the highest value in October (about 7.5 mm/ day) at the beginning of the rainy season and the lowest one is in April (about 4.5 mm/ day) that is at the beginning of the dry season.

Figure 4 .
Figure 4. Evapotranspiration in the Bagong Reservoir Area

Figure 7 .Figure 8
Figure 7. Dependable Discharge Due to the Basic Year in Tugu Reservoir (Q 80 = is on 2015) Figure 8 is the graphic of the transformation result from rainfall to discharge during 21 years (2001-2022).By using the basic year method, it is obtained 80% dependable discharge in 2018 (bold blue curve).

Figure 8 .
Figure 8. Dependable Discharge Due to the Basic Year in Bagong Reservoir (Q 80 = is on 2018)