S.T. Rachma, D.A.D. Nusantara, E. Rahmadyanti, Dlauissama, N.W. Hidajati
Urban coastal cities face evolving hydro-climatic risks under warming, yet the temperature-rainfall frequency link remains under-quantified in Surabaya. This study examines 2005-2024 daily records from two BMKG gauges to assess dry-season temperature trends and rainy-day frequency, addressing a regional shift toward warmer and more temporally concentrated precipitation regimes. We use a Generalized Additive Model (GAM) for monthly rainy-day counts, including an offset for days per month and smooths for temperature, seasonality (cyclic), and time, which flexibly separate seasonal and long-term components; results are also read alongside RX1day/RX5day and IDF diagnostics derived from daily maxima. The dry season shows clear background warming, with estimated trends of ∼+0.359 °C/decade and ∼+0.284 °C/decade across the two series, while rainy-day frequency falls from ∼3-4 days/month in the mid-2000s to ∼2 days/month by 2024, with recognizable interannual modulation (e.g., 2015-2017 dryness; 2019-2021 partial recovery). Extreme rainfall is episodic, RX1day/RX5day show isolated spikes, and IDF curves are well ordered by return period with strongest curvature at short durations, underscoring sensitivity of minor drainage to brief, intense storms. Taken together, the record indicates warmer, drier dry seasons punctuated by high-yield events, directly relevant to screening-level drainage checks, pre-monsoon maintenance, and first-flush water-quality management. Because inputs are daily, sub-daily IDF values are treated as provisional; expanding hourly/pluviograph monitoring and adopting routine QC with periodic updates will strengthen non-stationary design guidance and operations. © 2026 Published under licence by IOP Publishing Ltd.
Universitas Negeri Surabaya, Surabaya, Indonesia
