ANALISIS KONDISI RESAPAN AIR PADA SUB DAS BRANTAS KABUPATEN SIDOARJO MENGGUNAKAN SISTEM INFORMASI GEOGRAFI

Hydrology is the study of water on earth. Watershed or watershed is a unit of space that consists of several abiotic and biotic elements and human activities that interact with each other so that it becomes a unity of ecosystems. The problem in the research area is flooding because Banjarsari Village, Banjarpanji Village, and Kedungbanteng Village in Tanggulangin District have been inundated for more 20 days. This study aims to find out 1) the value of coefesien C based on the conditions of land use change in Sub watershed Brantas Sidoarjo Regency in 2015 and 2020. 2) The level of criticalness of water catchment in Sub watershed Brantas Sidoarjo Regency in 2020. This research is a quantitative descriptive type of research. The data collection technique in this study used secondary data processing that included land use data, soil type, rainfall, slope slope, and vegetation density. Data analysis techniques to determine land use changes using differences in the value of coefficient run off (C) in land use 2015 and 2020, while to find out the critical condition of water catchment reviewed the magnitude of natural infiltration that occurred on some parameters processed using overlay techniques in geographic information systems or GIS. The results showed that 1) there was a difference in the value of the coefficient run off (C) from 0.267 in 2015 to 0.305 in 2020. 2) As for the results of water catchment conditions, good water infiltration conditions 22624.2 ha (29.7%), Natural normal 588.59 ha (0.7%), Critical point 4017.8 ha (5.2%) and Light critics 48921.1 ha (64.2%). Keywords: Watershed, Coefficient run off (C), Land use, Water Catchmen