Analysis of the Effect of Land Cover Changes on the Runoff Dynamics Using the HEC-HMS Model in Gajahwong Watershed, Yogyakarta

Abstract The Gajahwong watershed is part of the Opak watershed. The development of urban areas in the Opak watershed is very massive. The development of the urban regions in the Gajahwong watershed is the occurrence of land conversion from non-built land to built-up land. The land cover change will result in changes in the hydrological response of the watershed. The results showed that in the Gajahwong watershed, there was a decrease in agricultural land in the Gajahwong watershed, which was initially 30,209 km 2 in 2016, down to 20,204 km 2 in 2020. Agricultural land declines due to the conversion of functions to built-up land. Built-up land in the Gajahwong watershed increased by 4,483 km 2 . This condition resulted in an increase in the CN value in the Gajahwong watershed from the original in 2016 having a CN II of 73 and then increasing to 74. The results show that in 2016, the Gajahwong watershed had a peak discharge of 14.9 m 3 /s; in 2020, the peak discharge in the Gajahwong watershed decreased to 11.8 m 3 /s. The volume runoff in the Gajahwong watershed also reduced from 2880.21 mm in 2016 and then in 2020 to 2011,39 mm.