The impact of watershed characteristics on flood behaviour in the Noelmina River region

Timor Island has two river regions, one of which is the Noelmina River Region. There are 91 watersheds in the Noelmina River Region, covering a total area of 5,418.85 km2. Each watershed possesses unique characteristics that influence the amount of flood discharge. The key watershed features that can be analysed to assess their impact on floods include shape, area, river length, slope of the watershed, slope of the river, and runoff coefficient. The aim of this study was to identify the specific characteristics of watersheds within the Noelmina River Region that contribute to significant flood discharge. The study focused on examining 20 watersheds within the Noelmina River Region, utilising spatial data such as the Timor Island DEM, land use, and hydrogeological map. The rainfall analysis was performed using the Log Pearson III distribution method with a return period of 500 years. Analysis of flood discharge using the SCS-CN method. The results obtained are that the shape of the watershed, the land use, and the curve number values have a greater impact on the amount of flood discharge produced. The time required to reach peak flood discharge in the Noelmina River Region typically ranges from 7-12 hours.

Each watershed has unique and distinct characteristics, which means that they respond differently to rainfall entering the watershed [3].The watershed characteristics have an impact on the amount of flood discharge from one watershed to another.The watershed features that can be utilised to analyse their impact on floods include watershed pattern, the large area, river length, slope, river slope, rock lithology, drainage density, and permeability coefficient.
The integration of Geographic Information System (GIS) in hydrological modelling has been conducted to incorporate several parameters such as land cover type, terrain slope, and soil type.Hydrological models based on GIS or remote sensing have become essential for accurate runoff simulations, as demonstrated by previous researchers [4], [5], [6], [7].The characteristics of the The shape of a watershed has impacts on the concentration of rainfall within the watershed and the flow of water toward the outlet.The following are four drainage patterns in the watershed: 1.Long storage pattern: In this form, the main river extends along with its tributaries, causing the river water to flow directly towards the main river.The water streams from the tributaries do not converge at the same point.This type of watershed has a relatively smaller flood discharge and a longer flood duration since the arrival time of floods from the tributaries is spread out on the left and right sides of the main river.2. Radial or circle pattern: This form of watershed appears to be centred around a single point, creating a radial pattern.The tributaries are concentrated at one point, leading to flood occurrences at the convergence points of these tributaries.3. Parallel pattern: This type of watershed is formed by two parallel river flows that join in the downstream portion of the watershed.The potential for flooding is high in this case, as the water flows converge at a single point.Floods in the downstream area generally occur after passing the meeting point.4. Complex pattern: This form represents a combination of the three aforementioned watershed forms, resulting in a complex pattern.

Soil Conservation Service-Curve Number (SCS-CN)
The Soil Conservation Service-Curve Number (SCS-CN) proposed by the United States Soil Conservation Service is used to determine peak discharge for uniform rainfall in a watershed [11].The surface runoff volume (Q) is influenced by the amount of rainfall (P) and the storage capacity available to retain water (S).The SCS-CN model assumes the following relationship between rainfall and surface runoff.
The variables in the equation are defined as follows: Q represents the volume of surface runoff, S corresponds to the maximum potential retention, P represents the amount of rainfall, and Ia denotes the initial abstraction.In order to estimate the value of initial abstraction (Ia) as follows equation: By substituting eq. 1 and eq. 2, the surface runoff volume can be calculated as follows: The maximum potential retention (S) value can be calculated using the equation [12] : Where Curve Number (CN) represents the potential runoff from the characteristics of the land coversoil complex [13].

Watershed Modeling Using HEC-GeoHMS
The use of HEC-GeoHMS is a method for watershed modelling and determining the characteristics of the watershed quickly and automatically [14].The steps for modelling the watershed using HEC-GeoHMS are: 1.Initial processing stage 2.
Formation of watershed project stage 3.
The basin processing stage 4.

Filling of characteristics of watershed and river flow data
The Soil Conservation Service (SCS) Curve Number (CN) method predicts rainfall that produces runoff as a function of cumulative rainfall, type of land use, soil type, and soil moisture.Calculation of the runoff value using the Curve Number (CN) method of this model can be done with the support of software based on the Geographical Information System (GIS).Analysis of the runoff value using the Curve Number (CN) model using ArcGIS consists of three steps: 1. Classifying the type of land use The type of land use is a factor that has a considerable influence on the watershed.Runoff with this CN value model is not only based on the type of land use but also seen on the vegetation, soil moisture, and soil conditions in the land use.The value of the Curve Number (CN) in a watershed with different soil types and land cover can be calculated using the following equation: The value of the Curve Number (CN) ranges from 0 -100.If the CN value is small and approaches 0, the soil in the watershed has the potential to have low runoff and a high infiltration rate.Meanwhile, if the CN value in a watershed is large and close to 100, the land has the potential to have high runoff and a very slow infiltration rate.

Flood Discharge Modeling Using the HEC-HMS Model
The HEC-HMS model can simulate a single storm event, as well as continuous input of precipitation in minutes, hours, or daily time steps [6], [17].The steps analysis of flood discharge in the Noelmina River Region using HEC-HMS are as follows : 1) Input basin model parameter There are four hydrological procedure processes in the HEC-HMS: the losses with the SCS Curve Number, the transform with the SCS, the baseflow with the recession, and the routing with Muskingum-Cunge.

2) Input time-series data
Hydrological modeling using HEC-HMS requires rainfall time-series data.The rainfall time-series data using the Huff-1 Cumulative Rainfall Distribution pattern for a duration of 24 hours is shown in Table 2 [18].

Results and Discussion
The rainfall for a 500-year return period with the Pearson Type III Distribution can be seen in Table 3.
The topographical data on the Digital Elevation Model (DEM) of Timor Island has been digitised using the HEC-GeoHMS feature.The form of the watershed of the Noelmina River Region and its river flow pattern can be obtained.The results of the watershed forms in the Noelmina River Region can be seen in Figure 2. Based on the results of digitisation, the Noelmina River Region has three watershed forms (drainage pattern): parallel, long storage, and radial form.The results of the analysis of watershed forms in the watershed of Timor Island can be seen in Table 4.The amount of parallel watersheds is nine, the long storage pattern is seven, and the radial pattern is four.The parallel pattern is the most dominant form in the watershed in the Noelmina River Region.Based on the topographical data of the Digital Elevation Model (DEM) for Timor Island that has been digitised using HEC-GeoHMS, the watershed area, the watershed slope, the river length, and the river slope on the watershed in the Noelmina River Region can be calculated.This calculation is automatically calculated using HEC-GeoHMS.The results of the calculation of watershed area, watershed slope, river length, and river slope on the watershed in the Noelmina River Region can be seen in Table 5.
According to Table 6, the results show that the Noelmina watershed is the largest watershed.The Noelmina watershed has a flat watershed slope.
The calculation of the Curve Number (CN) value is carried out by combining data on land use types and data from the Hydrologic Soil Group (HSG) in the watershed in the Noelmina River Region.The results of the calculation of the Curve Number (CN) value in the watershed in the Noelmina River Region can be seen in Table 7.
The curve number (CN) in the Noelmina River Region is within the range of CN value 70 -76.The CN value is quite large, so it can cause a large volume of runoff in the watershed in the Noelmina River Region.The largest CN value is found in the Oebobo Liliba watershed.Hydrographs of flood discharge in the watershed in the Noelmina River Region show that the time to reach the largest flood discharge for parallel watersheds is 7 -12 hours, long storage watersheds are 8 -10 hours, while for radial watersheds, it is 7 -1 hours.The factor that influences the time to reach the largest flood discharge is the shape of the watershed, the area of the watershed (the larger the watershed area, the longer the time to reach the largest flood discharge), and the length of the river (the longer the river is, the longer the time to reach the largest flood discharge).

Conclusions
The watershed in the Noelmina River Basin generates a considerable flood discharge.The form of the watershed, the area of the watershed (the larger the watershed area, the greater the flood discharge), and the larger the value of the Curve Number (CN), the greater the impact on the amount of flood discharge produced.The time to reach the largest flood discharge in the watershed in the Noelmina River Region takes between 7 -12 hours.The length of time to reach the peak flood discharge is influenced by the watershed forms, the area of the watershed (the larger the watershed area, the longer the time to reach the peak flood discharge), and the length of the river (the longer the river is linear with, the longer time to reach the peak flood discharge).

2. 1 Figure 1 .
Figure 1.Location map of rainfall stations on Timor Island

Table 1 .
[15]lassifying the hydrologic soil group (HSG)The Hydrologic Soil Group (HSG) watershed grouping of Timor Island is based on the Hydrogeological Map of Timor Island.The Hydrology Soil Group class based on the hydrological aspect is divided into four types: namely, group A: consists of soils with low potential for runoff and high infiltration rates; group B: consists of soils with slightly low runoff potential, moderate infiltration rate; group C: consists of soils with a rather high potential for runoff to slow infiltration rates if the soil is fully wet, and group D: consists of soils with high potential for runoff, and a very slow infiltration rate[15].CN classification based on the Hydrologic Soil Group (HSG) [16]alculation of curve number (CN) valueThe calculation of the curve number (CN) value is carried out by combining data on land use types and hydrologic soil group (HSG) data in a watershed.The combination is done by utilising the feature in the ArcGIS software.The feature serves to combine data in the attribute table for land use type and hydrologic soil group (HSG) into one value, as in Table 1[16].
Where CN comp = Composite CN, CN 1 = Curve Number for Soil Type and Land Cover 1, and A 1 = Area of Combination of Soil Types and Land Cover 1 (Km 2 ). 5

Table 2 .
The Huff-1 cumulative rainfall distribution pattern

Table 3 .
Results of rainfall data analysis

Table 4 .
Watershed forms in The Noelmina River Region

Table 5 .
Recapitulations of the characteristic watershed in the Noelmina River Region

Table 6 .
Curve number (CN) values in the Noelmina River Region