Synthetic rainfall is needed as an input in the analysis of design flood. This research intends to conduct the verification of a synthetic rainfall model based on the ArRR and satellite result. The rainfall data is as the main component in obtaining the design, detail design, operation and maintenance of effective water resource infrastructure. Rainfall data is often used for the analysis of flood balance, flood frequency distribution, estimation of river discharge, and design of water structure. However, the main constraint in designing the water resource infrastructure is less available rainfall data temporally and spatially. The limitation of available rainfall station number causes inaccuracy in analyzing the area mean rainfall. Therefore, sometimes the correlation between recorded data from the rainfall station and water estimation station does not show a positive and strong correlation. This is due to less rainfall data that are spatially representative in the watershed upstream. The verification is carried out in the Ciliwung Hulu watershed. The methodology consists of ArRR analysis of the two extreme rainfall events, then the ArRR is analyzed using the synthetic rainfall model equation that is applied for watershed areas < 750 km2; in the end, we spatially compared the satellite and observed rainfall. The analysis of the ArRR value is based on the formulation as follows: ArRR_i=(0,026FB-0,307*Re_i+1,184 Q_(i+lag)/A+0,693)/(C_ArRR.K_ArRR ). The lag time that is used is 3 h based on the analysis of time to peak by using the Nakaysu hydrograph by assuming that the run-off coefficient is 0.35. The spatial comparison between satellite and observed daily rainfall on February 19, 2021, and October 28, 2021, shows that the rainfall distribution pattern is more similar with adding the ArRR synthetic rainfall station than with only using the ARR station.

In calculating the design-flood discharge, engineers often use the frequency-distribution analysis of rainfall data as the basis for obtaining the magnitude of the design flood. The distribution of rain stations is an important factor in determining the distribution of regional rainfall; however, not all catchments have sufficient rain stations to represent the distribution of precipitation in a watershed. This results in a flood hydrograph—obtained using a rainfall–runoff model that is based on the calculation of the estimated rainfall—with a low level of correlation with the observed hydrograph. This research aims to set up a synthetic rain station that corrects the difference between the simulated hydrograph and observed hydrograph to achieve a better correlation level. Usually, researchers use the available rain stations for analyzing the design rainfall and design flood; however, in this research, a synthetic rain station has been built. Rainfall–runoff modeling methods for ungauged catchments (UCs), i.e., watersheds without observation stations, are widely available. Each method has different parameters to determine the unit hydrograph in a watershed. During the development of rainfall–runoff models at UCs, the precipitation in the upstream of a watershed is often assumed to be uniform, but it is not the case. The methodology used in this study collects hourly rainfall data from the ARR and hourly flood-discharge data from the AWLR in one catchment, then calculates the Thiessen coefficient before and after the set up of the synthetic rain station, and finally, calculates the rainfall at the synthetic rain station by trial and error. From the results of the synthetic rainfall modeling, the equation for the synthetic rainfall is obtained:ArRR=(-0.127A-0.260FB-0.307Re+1.227Q+0.51)/(C.K_Thiessen ) with a correlation coefficient of R = 0.818 in a watershed area below 300 km2.

Data hujan adalah salah satu komponen penting dalam kegiatan penelitian, perencanaan maupun pengelolaan sumber daya air. Pengaruh variabilitas curah hujan secara spasial dan temporal pada permodelan limpasan hujan telah lama menjadi perhatian dari ahli hidrologi dan menjadi sumber kesalahan utama pada analisis debit (Emmanuel et al 2015). Salah satu cara untuk mengakomodir sebaran secara spasial hujan adalah melakukan analisis hujan rerata wilayah. Metode yang sering dimanfaatkan dalam analisis hujan wilayah adalah Metode Rerata Aritmatik, Isohyet dan Thiessen. Tujuan dari penelitian ini adalah malakukan analisis pengaruh sebaran spasial hujan pada penentuan metode hujan wilayah Thiessen dan Isohyet di Daerah Aliran Sungai (DAS) yang masuk dalam DKI Jakarta. Analisis sebaran spasial dilakukan pada beberapa periode hujan yang terjadi pada tanggal 2 Februari 2007, 23 Februari 2014, 1 Januari 2020, dan 25 Februari 2020 di DAS tersebut. Analisis hujan wilayah Metode Thiessen dan Isohyet dipilih penelitian ini. Visualisasi dan analisis hujan wilayah dilakukan dengan bantuan Arc GIS. Pengaruh penggunaan dua metode hujan wilayah selanjutnya dianalisis dengan menggunakan analisis banjir dengan bantuan model hujan aliran WinTR 20. Berdasarkan hasil analisis diketahui bahwa pemilihan metode hujan wilayah harus dilakukan berdasarkan visualisasi distribusi hujan, jika hujan terkonsentrasi pada satu wilayah maka Metode Isohyet menghasilkan analisis yang lebih merepresentasikan kondisi aktual dengan perbedaan debit banjir yang dihasilkan dengan Metode Thiessen hingga 29%, sedangkan jika hujan tersebar merata di seluruh wilayah, maka Thiesen dan Isohyet menghasilkan perbedaan debit rencana  ≤ 5%.