Analisis Curah Hujan Ekstrem di DAS Bengawan Solo

Abstract

Beberapa dampak curah hujan ekstrem telah terjadi di DAS Bengawan Solo, antara lain banjir besar, korban jiwa dan harta benda, kerusakan infrastruktur, terganggunya pertanian, degradasi lingkungan, serta dampak sosial dan ekonomi. Penelitian ini mengkaji curah hujan ekstrim di DAS Bengawan Solo dengan menganalisis data curah hujan harian periode tahun 1991 sampai 2020 di tiga pos hujan observasi, yaitu Musuk, Tinap, dan Lowayu yang masing-masing terletak di bagian hulu, tengah, dan hilir DAS Bengawan Solo. Teori Nilai Ekstrim (Extreme Value Theoru/EVT) dengan pendekatan Block Maxima yang berdistribusi Generalized Extreme Value (GEV) digunakan untuk menentukan estimasi nilai pengembalian curah hujan maksimum, sedangkan metode Uji Mann-Kendall digunakan untuk mengetahui tren tahunan indeks curah hujan ekstrem sesuai rekomendasi Expert Team on Climate Change Detection and Indices (ETCCDI). Penelitian ini menemukan bahwa perkiraan nilai pengembalian tertinggi curah hujan maksimum tahunan untuk periode 5, 10, 20, 30, dan 50 tahun terdapat di stasiun Musuk dengan nilai yang meningkat seiring dengan lamanya periode pengembalian. Tren signifikan terlihat di wilayah tengah dan hilir dalam hal intensitas curah hujan harian, jumlah hari hujan tahunan dengan curah hujan ≥ 1 mm per hari, jumlah maksimum tahunan lima hari hujan berturut-turut, dan jumlah hari basah berturut-turut setiap tahun dengan curah hujan ≥ 1 mm per hari. Terjadi pula peningkatan jumlah maksimum curah hujan tahunan selama satu hari di stasiun Lowayu yang mengindikasikan semakin tinggi risiko bencana akibat tingginya curah hujan, serta adanya tren peningkatan jumlah curah hujan tahunan di Stasiun Musuk, Tinap, dan Lowayu yang berdampak pada peningkatan potensi ketersediaan air untuk memenuhi kebutuhan air di wilayah tersebut.

Several impacts of extreme rainfall have already occurred in the Bengawan Solo watershed, including devastating floods, loss of lives and property, infrastructure damage, disruption of agriculture, environmental degradation, and social and economic impacts. This study examines extreme rainfall in the Bengawan Solo watershed by analyzing daily rainfall data collected from 1991 to 2020 at three surface observation stations namely Musuk, Tinap, and Lowayu, which are respectively located in the upstream, middle, and downstream of the Bengawan Solo watershed. The Extreme Value Theory (EVT) using the Block Maxima approach with a Generalized Extreme Value (GEV) method was used to determine the estimated maximum rainfall return value, while the Mann-Kendall Test method was employed to assess annual trends in extreme rainfall indices recommended by the Expert Team on Climate Change Detection and Indices (ETCCDI). The study found that the highest estimated return value of annual maximum rainfall for 5, 10, 20, 30, and 50-year periods was in Musuk station, which increased with the length of the return period. Significant trends were observed in the middle and downstream areas for daily precipitation intensity, the number of annual rainy days with precipitation ≥ 1 mm per day, the annually maximum amount of five consecutive precipitation days, and the annually number of consecutive wet days with precipitation ≥ 1 mm per day. There is also an increase in the maximum amount of annual rainfall for one day (Rx1day) at Lowayu station, which indicates a higher risk of disaster due to high rainfall, as well as an increase in the trend of the amount of annual rainfall (PRCPTOT ) at Musuk, Tinap, and Lowayu stations, which affects increasing the potential for water storage to meet water needs in these areas.