Flood peak estimation for the keureuto watershed using the hydrological modeling system HEC-HMS

The Keureuto watershed has been the water resource used mainly for irrigation and municipal use. Extreme rainfall along with changes in the system may trigger rainfall excess causing flood at downstream. The occurrences may impact the usability of the system that supports the water resources supply. Investigating the impact of the floods requires initial research on the rainfall-runoff process of the watershed system. Rainfall-runoff modeling is an approach widely used for estimating the discharge hydrograph of the river as the result of the processes. The HEC-HMS rainfall-runoff model tool is applied in this study to estimate the peak flood discharge for the Keureto watershed located in the North Aceh region. The data used are satellite Global Precipitation Measurement (GPM) data and Automatic Water Level Recorder (AWLR) data at the watershed outlet. The 302 km2 Keureuto Watershed is divided into 23 sub-basins for the HEC-HMS model. The SCS Curve Number is applied to the model. The simulated freturncompared to the observed flow using the Nash Sutcliffe Efficiency criteria which returns the NSE value of 0.38 for seven years simulation, based on data of year 2015 to 2021. At an event based on October 2020 to March 2021, the NSE value returns 0.52, satisfactory for the simulation data range. The highest peak of the event is captured by the model simulation in the timing and the magnitude which value lower 20% from the observed flow.


Introduction
The Keureuto Watershed is located in the northern part of Sumatera Island, Aceh Province.The upper reaches of the watershed are located at Mount Tungkuh Tige, Bener Meriah District.The Keureuto River runs from the southwest to the north and discharges into the Malacca Strait.The watershed provides the community in the region with water supply, particularly for irrigation and municipal purposes.It also has an important role in storing or receiving rainfall excess and draining it into the river [1].The downstream area of the watershed often experiences flooding.The last major flood occurred in early December 2020 which caused 23 sub-districts affected and thousands of agricultural lands submerged causing crop failure [2].Good watershed management is important to sustain the functionality of a watershed.Watershed management can be obtained from a good understanding of the system and correct planning of any actions toward the system.Hydrological modeling is an approach to understanding the hydrological cycle process from rainfall to runoff component and the system behavior toward the process.The HEC-HMS (Hydrologic Engineering Center's Hydrologic Modeling System) which was developed by the US Army Corps of Engineers-Institute for Water Resources [3] is the hypothesis in this study assuming that the model is suitable to the Keureuto watershed.The model simplifies the watershed system into the concept of integrating subbasin processes through junctions and summed out at the system outlet [4].Therefore, this study aims to implement the model into the system and to analyze the peak flow and peak time at the Keureuto outlet.

Study Area
The Keureuto watershed is located between 4°48' 15,59"-5°2'11,93" N latitude and 97°1'5,75"-97°16'19,58" E longitude, presented in Figure 1.The research site was located in the Keureuto River which is mostly situated in the North Aceh district and partially in the Bener Meriah District.In this study, the Keureuto watershed outlet is located in the village of Mee, Lhoksukon District, North Aceh Regency.The research study has an area of 302.23 km 2 , a river length of 58 km, and a river slope of 0.015.The river is a dendritic type.

SCS -Curve Number Method
Two parameters that define the concept of the SCS-CN are initial abstraction and actual retention [5].Twenty percent of the watershed storage is assumed as the initial abstraction,   = 0,2.In the concept, the ratio of actual runoff to the maximum potential runoff is proportional to the ratio of actual infiltration to the potential maximum retention.This excess rainfall is calculated as runoff volume in the HEC-HMS program [6].The equation for the SCS CN runoff volume model is as follows: Where: Pe = precipitation (mm); S = potential maximum retention (mm); and Ia = initial abstraction (mm).The Curve Number CN is used to estimate the S value, as shown in the equation: The Curve Number is calculated as CN composite, using the following equation.
Where Ai = areas subbasin in km square.
The direct runoff model in the HEC-HMS software is calculated using the unit hydrograph [7].The unit hydrograph model applied in this study is the SCS Unit Hydrograph.The SCS-UH method of a single peak hydrograph was obtained with the following equation: Where, C = conversion constant; A = watershed area in square km; Tp = peak time, to find the peak time of maximum rainfall can be calculated with the following equation.
Where, Δt = the time required to obtain effective rain; Tlag = the time difference between the peak of the rainfall event and the peak of the hydrograph; Tc = concentration time; L = river length.

Model performance criteria
Assuming the model result is acceptable or not acceptable, the closeness of the simulated result to the observed is evaluated using the performance criteria [8].The criteria are used to evaluate the goodness of fit of the simulated result toward the observed.In hydrological modeling, the Nash-Sutcliffe Efficiency (NSE) applied here is the most common criterion implemented for this purpose [9].The efficiency of hydrological models [10] can be concluded from the NSE value which criteria are presented in Table 1.

Results and Discussions
The calibrated simulation of seven years of data ranges from the year 2015 to 2021, returns NSE 0.38, the plot is presented in Figure 3.The validation process is not applied considering the NSE is below 0.5 Having the result of varied NSE values at a short range of simulation confirms that the calibrated model only applied for that particular range of simulated data in representing the complexities of the system.Nevertheless, the range period where the model is returning a closer result to the observed can be used for further investigation such as using ground measurement rainfall dataset.
The model is potential for the Keureuto watershed, considering that HEC-HMS is suitable for the dendritic type of the river system [11].The simplicity of the SCS-CN method and the SCS-UH with less data requirement [12], [13] applies to watershed with less data availability [14], [15].Analyzing the simulated and observed flow using the Flow Duration Curve (FDC) as shown in Figure 6, the overall simulation underestimates the observed flow.The peak flow within 30% of the exceedance probability is significantly underestimated by the model.However, having the model as satisfactory a particular range means that the model can still be considered for any other type of data for estimating peak flow.

Conclusions
Applying the HEC-HMS model for the peak flow estimation shows that the calibrated model is capable of capturing the seasonal characteristics of Keureuto stream flow satisfactorily at a particular range.This study demonstrates the potential application of HEC-HMS in flow estimation from the Keuereuto watershed.The developed model is a useful tool for water management in this watershed for peak discharge analysis.

Figure 1 .
Figure 1.The Keureuto watershed2.2.Data collectionDigital Elevation Model (DEM) data was retrieved from the DEMNAS website, the agency that provides elevation and bathymetry data for Indonesia.This data was used to identify stream paths and to delineate the watershed area.Daily rainfall of the GPM satellite dataset, used for the model input, was obtained from the https://giovanni.gsfc.nasa.gov/giovanni/.Land use data used here is the product of the Satellite Landsat 8 image year 2021 unsupervised classification in ArcGIS.The Landsat image was derived from the https://earthexplorer.usgs.gov/.A major portion of the land use of the area was identified as forest.Soil-type data was obtained from the Food and Agriculture Organization of the United Nations (FAO) clipped for the Keureuto watershed boundary.The soil types identified in the system are Orthic Acrisols (AO), Dystric Fluvisols (JD), and Chromic Fluvisols (LC) which are grouped into Hydrologic Soil Group (HSG) of HSG-B and HSG-C.These land use and soil data were used to estimate the CN values of the watershed.

Figure 2
Figure 2 -(a) Land use year 2021 of the Keureuto Watershed, (b) Soil type distribution of the Keureuto Watershed

Figure 3 -Figure 4 -
Figure 3 -Observed and simulated discharges at the AWLR Keureuto for the year 2015 to 2021