PENENTUAN KEHILANGAN AIR DARI HUJAN PADA SUB-DAS NGRANCAH HULU KABUPATEN KULONPROGO YOGYAKARTA MENGGUNAKAN METODE KOEFISIEN ALIRAN SESAAT

Abstract

Hydrologic response of a watershed or sub-watershed to the presence of rain isan interesting study in order to know the character of a watershed hydrology. Besides itcan be used to predict soil and vegetation response to rainfall in a watershed, the resultsof this study can also be used to calculate the potential of rain water into surfacestreams. This will be very useful to see how big the potential occurrence of floods froma river stream in case of rain. There are several methods to test the hydrologic responseof a watershed or sub-watershed. One of widely used model is the moment flowcoefficient (MFC). This study aims to predict some parameters of hydrologicalresponse in the Ngrancah Hulu sub-watershed in Yogyakarta Province using MFCmodel. The parameters examined, i.e the flow coefficient for a moment, lose waterfrom the rain (watershed response to rain), and the potential of rain water come intostreams. The study begins by defining the characteristics of rainfall and flow ratesthrough the primary and secondary data analysis, followed by the analyzes of waterlosses using the MFC model, and analyzes the potential of rain water into streams. Thestudy found that the moment flow coefficient in the study is the low level, which meansthat in the sub-watershed, only a little rain water flows into the rivers. The amount ofrain water lost is influenced by variables including heavy rain, I-max, long rains and theAPI-7 days respectively. From the calculations, it is obtained the equation for waterloss Y = 0.898 P + 0.03308. Flow coefficient value is not in the normal classification,i.e with an average value of 0.095. The amount of rain in the area of research becomingthe flow are 10.07% of the total of each rainfall event.

Keywords: moment flow coefficient model, hydrologic response, Ngrancah Hulu sub-watershed