Pengaruh Kondisi Awal Kelengasan Tanah Terhadap Debit Puncak Hidrograf Satuan

Abstract

Unit hydrograph is a popular method for predicting flood caused by rainfall in a catchment. This method has little flexibility to different storm, consequently the unit hydrographs produced differ considerably from storm to storm. This is partially due to the unit hydrograph method ignores the influence of antecedent soil moisture condition (AMC) on runoff generating processes. This paper presents the research result on the influence of AMC to peak discharge of observed unit hydrograph (q p-obs ). In this research the AMC is approached by parameter of soil moisture deficit (SMD) which has opposite meaning with the AMC. The SMD was represented by a variable of S c (storage capacity) that is the space volume remaining the soil moisture evaporated. The research was conducted using hydrologic data (rainfall, discharge, and evaporation) assembled from 4 upper basin surrounding Mount Merapi and Mount Rinjani. The research experiments were performed by deriving unit hydrographs using two methods, namely conventional method (Collins) producing observed unit hydrograph (HSobs) and simulation method using tank model of Yue and Hashino producing simulated unit hydrograph (HS sim ). The impact of Sc to the q p-obs was analyzed using peak discharge of HS obs and peak discharge of HS sim . The research result are concluded that: (1) the relationship between Sc and peak discharge of unit hydrograph showed that if the Sc increases (AMC decreases) then the peak discharge of HS obs decreases, (2) The formula of correction has exponential form of eksponensial (, with q p-obs-kor is peak discharge of corrected observed unit hydrograph (3) The average corrected observed unit hydrograph has the highest peak discharge than the peak discharge of Synthetic Unit Hydrograph of Nakayasu and the peak discharge of Synthetic Unit Hydrograph of Gama1.Keywords: AMC, observed unit hydrograph, tank model, corrected observed unit hydrograph Abstrak: Hidrograf satuan adalah salah satu cara untuk memperkirakan besarnya banjir di sungai akibat hujan pada suatu DAS. Hidrograf satuan sangat populer dan dipakai secara luas di dunia. Metode ini mempunyai fleksibilitas yang rendah terhadap hujan yang berbeda, sehingga hidrograf yang dihasilkan berbeda-beda pada setiap hujan. Hal ini diduga disebabkan antara lain karena teori hidrograf satuan mengabaikan pengaruh kondisi awal kelengasan tanah (antecedent soil moisture condition, AMC) pada proses penurunan hidrograf satuan. Tulisan ini menyajikan hasil penelitian tentang pengaruh AMC terhadap debit puncak hidrograf satuan observasi (q p-obs ). Dalam penelitian ini variabel AMC didekati dengan besaran defisit kelengasan tanah (soil moisture deficit, SMD) yang mempunyai makna berlawanan dengan AMC. SMD direpresentasikan dengan variabel Sc (storage capacity) yakni volume udara di dalam pori tanah yang ditinggalkan air karena terevapotranspirasi keluar. Penelitian dilaksanakan dengan menggunakan data hidrologi (hujan, debit aliran, dan evaporasi) yang dikumpulkan dari 3 DAS hulu (upper catchment) di k...

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