The effect of land use changes on flood characteristics of Pacal watershed, Bojonegoro Regency, East Java

High rainfall is one of the causes of floods that cause damage to infrastructure, cause losses and often cause casualties. Another cause is in the watershed response changes to rain due to the land use development which increases the rate and volume of runoff. This condition occurs in the upstream part of the Pacal river basin, which has decreased forest area and increased open land so that flooding in the downstream watershed has increased. Therefore, scientific based research on the effect of land use changes on flood characteristics in the Pacal watershed needs to be carried out to provide useful information in preparing an appropriate flood management plan. In this research, the effect of land use changes is analyzed by determining the curve number (CN) of the Pacal watershed for the conditions of 2011 and 2020. The CN was calculated by using ArcGIS 10.3 software, the results are used for the calculation of effective rainfall as input for simulating flood hydrograph with the synthetic unit hydrograph model of Nakayasu method using HEC-HMS software. The flood hydrograph simulation was carried out by rainfall-runoff modeling in three sub-watersheds, each of which determined the CN-composite value and the unit hydrograph. The results showed that apart from being caused by very high rainfall, the occurrence of major floods was also influenced by changes in land use. In the Pacal watershed for ten years period from 2011 to 2020, there was a change in land use, namely a 59% reduction in forest area, 12% reduction in agriculture, 56% increase in estate, 3% increase in open land, and 13% increase in rice fields. The increase in the CN-composite value is about 15% which has an impact on increasing the peak flood discharge by about 18%. The most significant increase in peak flood discharges was in the upper left watershed with 8%, 7%, and 7% for 100, 500, and 1000 years return period of flood, respectively.