Analyzing the primary hydrological components (rainfall and discharge) within the context of Cipunagara Watershed management, West Java

The Cipunagara River Basin is a component of the Citarum Watershed, necessitating a thorough analysis of hydrological data particularly rainfall and discharge for effective management. The watershed spans a moderate area of 1,363 km2 across three administrative divisions: Subang Regency, Indramayu Regency, and a segment of Sumedang Regency. Hydrological data analysis is imperative in watershed management. This study aims to analyze the key hydrological elements relevant to Cipunagara Watershed management. Rigorous consistency and homogeneity tests were conducted on rainfall data from each station. Changes in watershed flow patterns were scrutinized by examining shifts in runoff coefficient values and base flow magnitudes. The upward trend in runoff during the rainy season and the corresponding decline in base flow throughout the dry season may be attributed to land use alterations within the Cipunagara Watershed zone. To proactively address drought concerns in watershed management, the study calculated the 80% dependable flow to assess water availability during arid months, yielding an 80% dependable flow rate of 16.25 m3/s for the Cipunagara Watershed.


Introduction
Integrated watershed management necessitates the engagement of diverse stakeholders in natural resource management, alongside the requirement for inclusive, comprehensive planning.Astute watershed management hinges on a judicious assessment of the interplay between human necessities and the resource pool available to fulfill them.The intricacies of water resource challenges are mounting, driven by a burgeoning population, industrial advancement, urban expansion, and agricultural progress.Inadequate integrated planning and management within a watershed can engender environmental quandaries, precipitating ecosystem deterioration and the depletion of water reserves [1][2][3][4].
Effective river basin management necessitates the analysis of hydrological data.Such hydrological analyses are pivotal in watershed management initiatives, catering to diverse requirements and strategic planning, encompassing irrigation networks, drainage systems, and dam installations.The significance of hydrological data reverberates profoundly in the formulation and execution of water resource planning and management, with the watershed serving as the fundamental organizational entity for resource governance.These data are scrutinized to draw informed conclusions and facilitate decisionmaking vis-à-vis hydrological phenomena.The watershed is a valuable entity for physical assessments and a pertinent socio-economic-political facet in implementing management strategies [5][6][7][8][9][10]. *sitiai.nurhayati@students.itb.ac.id, sitiai.nurhayati@gmail.comIOP Publishing doi:10.1088/1755-1315/1263/1/012034 2 Hydrological data is gathered through field monitoring and necessitates scrutiny through a data examination phase.Hydrological phenomena manifest as variables such as rainfall, river discharge, temperature, evaporation, solar irradiation duration, water level, wind speed, flow velocity, and river sediment concentration, which undergo continual temporal fluctuations.Errors within hydrological analysis can exert a significant impact on hydraulic infrastructure planning, potentially yielding severe consequences [7,[10][11][12][13][14].
This study aims to analyze the primary hydrological elements, specifically rainfall and discharge, within the confines of the Cipunagara Watershed region, aiming to facilitate effective watershed management.The study's purview encompasses the Cipunagara Watershed, encompassing rain and discharge data pertinent to the said locale.

Location of study
The Cipunagara Watershed is integral to the Citarum River Basin, as designated by Regulation No. 4 of 2015 by the Minister of Public Works and Public Housing.The principal watercourse, the Cipunagara River, traverses the Subang Regency and Indramayu Regency regions within West Java.Encompassing an expanse of 1,363 km 2 , its geographical coordinates are positioned at 6°34'47.82'LS and 107°51'15.35'E. This area envelops Subang Regency, Indramayu Regency, and Sumedang Regency [15].This study's computed rainfall data were derived from five rain stations: Pusakanegara Rain Station, Karangasem Rain Station, Kroya Rain Station, Bentrahuni Rain Station, and Ciberes Rain Station.Additionally, discharge data was sourced from a single discharge post.Figure 1 illustrates the geographical placements of the rain station points within the Cipunagara Watershed in West Java.

Methods
In this study, the primary hydrological components under scrutiny were rainfall and discharge within the confines of the Cipunagara Watershed region.Rainfall data is paramount in hydrological analysis due to its dynamic nature and relevance in disaster assessment, constituting a fundamental aspect of the natural environment.The rainfall data encompass measurements of the quantity of precipitated rain, typically expressed in units of height (mm).These data were garnered through rain measurement devices positioned at specific locations, accumulating over defined time intervals.Notably, rainfall data serves as vital input for the design of flood control infrastructure, including river embankments and drainage channels [16,[12][13].Streamflow data stands out as a pivotal information source in water resources management.The capacity to measure discharge flow is essential for assessing the water resource potential within a given watershed area [17][18].
Consistency tests were conducted to validate observational data and ensure the closeness of the analyzed rainfall data to actual conditions.This process serves to mitigate errors in watershed area planning.The consistency test involved a dual mass curve analysis employing annual rainfall accumulation data.Through the dual mass curve, pairs of Xi and Yi values were observed, with their cumulative sums compared to ascertain congruity in the long-term trends of Xi and Yi variations.Cumulative variables were derived using Equation 1and Equation 2 [6][7]. (1) where: i = 1,...,n and j = 1,...,i-1.
The homogeneity test involved plotting points H (N, TR) on the homogeneity test curve.Rainfall data can be considered homogeneous if the H (N, TR) points fall within the confines of the curve's funnel [6].The average rainfall for the area can be calculated through three methods: the arithmetic or algebraic mean method, the Thiessen polygon method, and the Isohyet method.These approaches are widely employed for determining mean areal rainfall, with the Thiessen polygon and Isohyet methods being particularly prevalent in water resources engineering.The regional rainfall calculation method can be tailored to the specific watershed area, as detailed in Table 1 [11,[19][20].
Table 1.Selection of the method based on the watershed area [11].

Area of Watersheds Methods
Large watershed (>5000 km 2 ) Isohyet Medium watershed (500 to 5000 km 2 ) Thiessen Small watershed (<500 km 2 ) Algebraic mean The characteristics of rain patterns in Indonesia are evident through the contrasting periods of the rainy and dry seasons within a single year.This rain pattern is prominent in the southern regions of Indonesia, including Java, southern Sumatra, Bali, and southern Maluku.into the temporal trends of their fluctuations.The simple linear regression approach is represented by Equation 3 [22][23].
Discharge extremes can arise from alterations in hydrological regimes, which can be examined by applying the moving average method.This technique is a technical indicator for forecasting forthcoming data within time series analysis, aiming to mitigate the inherent randomness of discharge patterns.The method involves selecting a set of observed values and deriving their average as a predictive value for future periods [24][25].
Dependable flow refers to specific discharges whose occurrence is associated with a defined probability or recurrence interval.Calculating dependable flow is vital in determining water availability, as it aids in identifying the minimum river discharge.The dependable flow is typically established with an 80% probability, ensuring that the likelihood of river discharge falling below this threshold remains at 20% or lower.The discharge reliability level can be ascertained by applying the Weibull formula, as outlined in Equation 4. The discharge calculation results are then presented as a graph known as the Flow Duration Curve (FDC), illustrating the discharge duration against probability.This graphical representation depicts the relationship between probability (%) and discharge (m³/sec) [26][27][28].
Where: P (X ≥ x) = probability of the occurrence of variable x (discharge) equal to or more excellent x m 3 /sec; m = data rank; n = amount of data; X = discharge data series; x = mainstay discharge if the probability matches its designation, e.g., P (X ≥ Q80%) = 0.8.

Results and Discussions
Watershed management systematically coordinates and directs land, water, and other natural resources to meet essential needs and minimize their impact on land and watershed ecosystems.This endeavor is integral to providing requisite goods and services while safeguarding these vital resources.Water resources management is intrinsically intertwined with water management since watersheds constitute a fundamental aspect of the hydrological cycle.Understanding watershed hydrological behavior is essential to enhance water resources management practices.The principal constituents of the hydrological cycle within a watershed encompass the precipitation within its confines and the river water discharge that traverses its expanse.Inadequate watershed management can disrupt the functional equilibrium of the watershed, potentially culminating in ecosystem degradation.One discernible consequence of watershed degradation is the fluctuation in river flow, notably pronounced during the rainy and dry seasons.Furthermore, precipitation is the preeminent variable for prognosticating river flow patterns [29][30][31][32][33].This study analyzed rainfall data from 2010 to 2021, while the river discharge data covered 2016 to 2020.

Consistency test
The consistency test was conducted by juxtaposing the cumulative annual rainfall against the average rainfall accumulation from neighboring rain stations, serving as reference stations for comparison [34].The outcomes of this test are considered consistent if the number of points deviating from the trend line does not exceed five.The consistent test findings for the rain station data within the Cipunagara Watershed area are illustrated in Figure 3. Based on the graph in Figure 3(a), the Pusakanegara station exhibits three years of observations that deviate from its linear trend line.In Figure 3 it is evident that all data remain consistent.This conclusion arises from the absence of more than five observation years that deviate or stray from the linear trend line.

Homogeneity test
The data employed for this homogeneous test encompass rainfall records spanning 12 years (from 2010 to 2021) across five rain stations.The TR values, denoting Total Runoff, will be calculated to represent the rain data, and subsequently, these values will be charted on a curve, illustrated in

Regional rainfall and climate type
Rainfall distribution for this study was computed using Thiessen's polygon method, as the area of the Cipunagara Watershed falls within the range of 500 km² to 5000 km², as stipulated in the criteria outlined in Table 1.The outcomes of the regional rainfall calculations within the Cipunagara Watershed are depicted in Figure 5.
The climate type prevailing in the Cipunagara Watershed is discernible through the outcomes of regional rainfall calculations.The calculations and corresponding graphs substantiate that the climate in the Cipunagara Watershed aligns with the monsoon type, characterized by a single peak of rainfall occurring during the wet months spanning the end and start of the year.Contrarily, the middle of the year experiences dry conditions with precipitation below 100 mm/month, as illustrated in Figure 6.

Degradation of flow regimes
The degradation of the flow regime in the Cipunagara Watershed area was assessed through a linear regression equation approach.Based on calculations, the runoff coefficient value in the Cipunagara Watershed demonstrated an increase from 2016 to 2021, as depicted in Figure 7. Concurrently, a decline in the baseflow amount is evident in Figure 8.These shifts in the runoff coefficient and baseflow within the Cipunagara Watershed can be attributed to land-use alterations.Subsequent land-use modifications can impact the nature of river flow, resulting in heightened water discharge during the rainy season and diminished baseflow in the dry season, consequently heightening the risk of drought.Deforestation, population growth, intensified agriculture, and the clearing of ponds contribute to accelerated land use changes in the upstream Cipunagara Watershed, consequently affecting the expansion of flood-prone areas downstream of the Cipunagara Watershed [23,25,35].

Discharge extremities
Discharge extremities in this study were determined using the moving average method.This approach computes maximum and minimum discharge data within the Cipunagara Watershed, utilizing data from 2016 to 2021.The resultant calculations reveal an upward trend or escalation in discharge.Such trends can be attributed to anthropogenic climate change, impacting both average and extreme river flows and amplifying the potential for flooding [36].The result of this calculation is depicted on the graph in Figure 9 below.

Dependable flow
The dependable flow is calculated to determine the quantitative discharge value available throughout the year, encompassing the dry and rainy seasons [37].dependable flow employed in irrigation water supply planning constitutes 80% of the dependable flow, signifying an 80% likelihood of discharge fulfillment or a 20% probability of river discharge falling below the threshold [27].Derived from the calculation results, the 80% dependable flow amounts to 16.25 m³/second, and the corresponding dependable flow calculations within the Cipunagara Watershed are illustrated in Figure 10.

Conclusion
An analysis of hydrological data, specifically rainfall and discharge within the Cipunagara Watershed, was conducted utilizing various test methods, including consistency and homogeneous tests.Based on these applied to rainfall data collected from five rain stations, it is concluded that the rainfall data remains both consistent and homogeneous.The rainfall data played a pivotal role in determining the climate type of the watershed area, with the analysis confirming the presence of a monsoon climate type in the Cipunagara Watershed.Furthermore, examining discharge data facilitated the analysis of flow regime alterations, focusing on changes in the runoff coefficient and baseflow values.The outcomes of
Figure 2 depicts the classification of monsoon types across Indonesian territory [21].

Figure 3 .
Figure 3.The outcomes of the consistency test are presented through the rainfall discharge period curves for the following stations: (a) Pusakanegara Station, (b) Karangasem Station, (c) Kroya Station, (d) Bentrahuni Station, and (e) Ciberes Station.
(b), the Karangasem Rain Station and Figure 3(d), the Bentrahuni Rain Station display observation years that lie outside their respective linear trends.Conversely, the Kroya Rain Station in Figure 3(c) and the Ciberes Rain Station in Figure 3(e) demonstrate all observation years conforming to a linear pattern.Drawing from the outcomes of the double mass curve consistency test across the five rain stations within the Cipunagara Watershed area,

Figure 4 .
The computed TR values are as follows: Pusakanegara Station at 3.30, Karangasem Station at 3.59, Kroya Station at 3.03, Bentrahuni Station at 3.21, and Ciberes Station at 3.27.These TR results are then plotted onto a homogeneity test graph.The plotted points for all five stations within the Cipunagara Watershed, as depicted in Figure4(a) through Figure4(e), remain within the confines of a homogenous graph basin.It substantiates the conclusion that the rainfall data collected from the five rain stations exhibit homogeneity.

Figure 6 .
Figure 6.Rainfall at each rain station can serve as an indicator of the climate type within the Cipunagara Watershed.

Figure 7 .
Figure 7. Changes in the runoff coefficient of the Cipunagara Watershed.

Figure 8 .
Figure 8. Changes in the basic flow of the Cipunagara Watershed.