Impact of El Nino and La Nina Climate Anomalies on Precipitation and Water Availability in Upper Bogowonto River Basin 2003–2022

Over the past two decades, increased greenhouse gas concentrations and global warming have caused El Nino and La Niña to occur more frequently and consistently in Indonesia, affecting agriculture and crops. The purpose of this study is to analyze the impact of El Nino and La Niña on rainfall and water availability in the Upper Bogowonto Watershed between 2003 and 2022. This study was conducted by identifying Pacific sea surface temperature (SST) anomalies, classifying them into different magnitudes of El Nino and La Niña, and comparing them with precipitation and water availability. El Nino events typically reduce rainfall and the number of rainy days, allowing for a long dry period, while La Niña extends the rainy season, increases precipitation, and increases the number of rainy days. Based on sea surface temperature (SST) anomalies, it is known that there was a strong El Nino in 2015 and a moderate La Niña in 2007–2008, 2010–2011 and 2020–2021. Under normal conditions, such as 2003 and 2013, rainfall in the Upper Bogowonto Watershed is usually wet, with the November–April rainy season followed by dry seasons in May–October, with annual rainfall varying between 2900 and 4000 mm and average water availability varies from 2.10 to 6.40 m3/s in the dry season and 17.90–23.40 m3/s in the rainy season. In 2015, El Nino was strong, four rainfall stations in the Upper Bogowonto Watershed experienced fewer precipitation and rainy days. El Nino impacts 160–180% expansion of the continuous dry days (CDD) compared to normal conditions (2003 and 2013).