Assessing Agricultural Water Reliability in Jetis Irrigation Area Using the F.J. Mock Hydrological Model

. Jetis Irrigation area is located in Sidohardjo Subdistrict, Sragen Regency, Central Java, and is known for its rice crop production. This study aimed to assess water availability, estimate water requirements, and analyze irrigation expansion in the area. Data on discharge, evapotranspiration, rainfall, and flow area were collected and analyzed using the F.J. Mock method. This method enabled monthly calculations of water balance analysis and discharge rates. The study found that the availability of water at Q80% and Q90% discharge rates was sufficient to meet the water needs of the Jetis irrigation area. By assessing agricultural water reliability, policymakers can incorporate these findings into land-use planning and water allocation decisions.


Introduction
The recent extreme climatic conditions occurring in various parts of the world can have direct and indirect impacts on food availability in Indonesia.Food security is a vital condition for both the state and individuals to have access to sufficient, safe, diverse, nutritious, equitable, and affordable food that aligns with their religious, cultural, and community beliefs.It allows people to live healthy, active, and productive lives in a sustainable manner [1].Prolonged drought in various regions of the world as well as in Indonesia, especially in food production centers, greatly affects the availability of food which of course has an impact on the availability of these food products [2].The demand for freshwater will be higher to meet the growing demand for food crop production in the future.Also, the two goals of the Sustainable Development Goals (SDGs) are to end hunger and ensure access to water [3,4].The availability of water and sanitation management is one of the achievement targets of sustainable development or sustainable development goals (SDGs) to ensure the availability and sustainable management of water and sanitation for all.In order to maintain a sustainable increase in economic welfare for the community, development should prioritize the sustainability of community social life, the quality of the environment, and justice, as well as implementing governance that continuously improves the quality of life from generation to generation [5].Over the past few decades, water stress has become a severe issue in various parts of the world due to overexploitation, pollution, and climate change.Presently, a staggering 2.2 billion people lack access to safely managed drinking water, while over 4.2 billion people do not have access to safely Corresponding author: yureana.wijayanti@binus.ac.id managed sanitation.Climate change is making matters worse, leading to increased incidents of floods and droughts.The impact of water stress is profound and multi-faceted, with around 80 percent of the world's untreated wastewater flowing back into ecosystems and a loss of 70 percent of the world's natural wetlands, including a significant loss of freshwater species.To overcome this global crisis, urgent action is needed to tackle the issue from a social, economic, and environmental perspective [6].Globally, agricultural water consumption is 70% of all human water use [7].
In general, the problems that occur are due to unbalanced water availability and water needs [8].
Based on the hydrological cycle, the total volume of water in the world has a fixed amount.However, if water resources management is not carried out properly, there will be a water crisis or drought.Therefore, water resources must be managed properly so that the availability of water can be sufficient.Well-managed water resources can ensure the availability of water in water source networks such as rivers, lakes, ponds, dams, and swamps.The FJ Mock method is one of the methods used to show water balance analysis for monthly discharge calculations based on monthly rainfall data, evapotranspiration, soil moisture, and groundwater.The FJ Mock method has a principle that states that the rain that falls on the catchment area will partly disappear due to evapotranspiration, some will become direct runoff, and some will enter the ground [9].The FJ Mock method was chosen because it is the method used as a standard in irrigation planning.In addition, the FJ Mock method shows a relatively simple calculation method for various components based on the results of research on watersheds throughout Indonesia [10].Previous studies in Indonesia have implemented FJ Mock methods [11,12,13,14] where the results of the analysis show that the FJ Mock method is the simplest method and the FJ Mock method is closer to the results of the calculation of instantaneous discharge measurements in the field compared to other methods.Therefore, this study wanted to analyze the agricultural water needs, and reliability of irrigation water, and project the irrigation water reliability in the next 10, 20, and 30 years using the F.J. Mock method.

Study Area
The research location is located in the Jetis Irrigation area, Sidoharjo District, Sragen and Karanganyar Regencies, Central Java Province.The Jetis Irrigation Area lies between 7°34' 29.1"South Latitude -110°53' 7.8" East Longitude.The Jetis irrigation area is located in Sidohardjo District, Sragen Regency, and Karanganyar Regency, Central Java with an area of approximately 11,765 Ha.Sragen Regency for most of its secondary canals and Karanganyar Regency for a small portion of the irrigation area.Overall, it consists of three weirs namely Jetis weir, Sirap weir, and Sidowayah weir.Jetis weir dams the Sragen River, then the water is used for irrigation purposes in the order that the water enters the Jetis main channel and then into the Mojodoyong, Nguwok, Terban, Sirap, Sidowayah, Botok, and Sidodadi secondary canals.Sragen Regency is one of the rice-producing areas in Central Java.

Methods
Performing water reliability analysis, where this analysis is carried out to determine whether the availability of water meets the water needs of the Jetis irrigation area using the flow diagram in Figure 2.

Water Availability
The data that will be used to model the FJ Mock method are observational discharge data, rainfall data, temperature data, sunshine duration data, air humidity data, wind speed data, and data flow area.Observational discharge data, rainfall data, and evapotranspiration data used are data from 2008 -2020.The flow area data used has an area of 11,765 km 2 .The following is a summary of the results of discharge calculations using the FJ Mock method (Table 1).

Model Calibration
FJ Model Calibration of the Mock FJ model is carried out when analyzing the availability of water.Calibration is carried out in order to determine the accuracy of the model discharge when compared to the observed discharge.Calibration is done by checking the results of the Correlation Coefficient, NSE, and RVE.Table 2 shows the results of the calibration of the FJ Mock model.

Dependable flow
The dependable flow used is Q80 and Q90.The dependable flow Q80 and Q90 are used because discharge with high probability is needed to meet water needs such as irrigation.The dependable flow analysis is determined using the Weibull method.Tables 3 and 4 are the results of the Q80 and Q90 dependable flow.

Agricultural Water need
The water needs analysis calculated irrigation water needs.The standard of water needs used Indonesia Standard for Irrigation design [10].After analyzing the calculation of irrigation water needs for land preparation, calculations are carried out to determine irrigation water needs.The results of the calculations can be seen in Table 5.
Table 5. Irrigation water needs for land preparation

Water Reliability
Water reliability analysis is a comparison of water availability and water needs which serves to determine whether the water availability is sufficient for water needs.Based on Table 7, and Table 8, the water availability with dependable flow Q80 and Q90 still fulfills water needs in the Jetis irrigation area.In addition to the reliability of water with a dependable flow of Q80 and Q90, an analysis of the reliability of water with a dependable flow of Q80 and Q90 is half monthly. Figure 3 shows the comparison between the water availability both Q80 and Q90, with agricultural water needs.The fluctuation of water discharge of Q80 and Q90 from Table 4 was plotted in this graph.The agricultural water needs throughout the year are ranging between 0.04 m 3 /s to 0.59 m 3 /s.It shows that the irrigation water both Q80% and Q90% discharge are reliable for agricultural water needs in the paddy field area.
The findings help farmers in Jetis Irrigation Area to plan their crop planting and optimize yields based on reliable water availability.By understanding the water reliability patterns and potential risks, farmers can make informed decisions about the selection of crops, timing of planting, and irrigation scheduling.This can lead to improved crop productivity, reduced water waste, and minimized environmental impact through optimized resource use.Also, by understanding the reliability of agricultural water, stakeholders can develop appropriate strategies to mitigate risks associated with water scarcity or variability.This can involve implementing watersaving technologies, adopting climate-resilient crops, and implementing adaptive management practices.By proactively addressing water-related risks, the findings contribute to increased resilience of agricultural systems and support long-term environmental sustainability.By assessing agricultural water reliability, policymakers can incorporate these findings into land-use planning and water allocation decisions.This can help strike a balance between agricultural productivity and the preservation of natural resources, ensuring that the irrigation area can sustainably support agricultural

Conclusion
The Jetis irrigation system has water availability with dependable flow Q80% is 2.46 m 3 /s and Q90% is 1.67 m 3 /s .The agricultural water need of paddy fields is 0.60 m 3 /s.The water reliability analysis result shows that both Q80% and Q90% discharge meet the agricultural water needs.In this study, the calculation of water needs, using a cropping pattern that is used to follow the guidelines for rice planting patterns from the Ministry of Agriculture.Meanwhile, the cropping pattern carried out at the study site may have been modified according to the conditions of the local rainfall pattern.Thus, for future studies, field surveys can be carried out to obtain a more accurate calculation of plant water requirements.
Overall, the contributions of the findings in the assessment help enhance water management practices, optimize agricultural productivity, mitigate risks, and support sustainable development.These outcomes have positive implications for both environmental sustainability and society, fostering resilience, resource efficiency, and the long-term well-being of communities dependent on agriculture.

Fig. 2 .
Fig. 2. Research methodology[14] Data required for the analysis are rainfall data, and daily climate data over a period of 13 years (2008 -2020), daily discharge data from Botok Reservoir.

Table 2 .
Model calibration

Table 6 .
Results calculation of Irrigation water needs

Q Water Availability Agricultural Water Needs Water Balance
Fig. 3. Water Availability and Water Needs Monthly