Hydrologic frequency analysis of river discharge at Agusan River, Philippines in a changing climate

Extreme events, such as extreme precipitation, are manifestations of a changing climate which have been a global and local issue nowadays including in Agusan del Norte Province. The area along the lower Agusan River, which is within the Agusan del Norte boundary, experiences flooding due to heavy rains. Oftentimes, duration of heavy rains do not just last for a day but could even be consistently intense for a couple of days. This study considered a 30-year record of daily precipitation data (1981-2010) of Agusan del Norte to analyze the existing climatic condition of the province. Future climate scenarios 2020, 2050 and 2080 were generated using Statisitcal Downscaling Model (SDCM). Annual 24-hour and 48-hour maximum precipitation and climatic conditions for present and future scenarios were then evaluated and compared. Estimation of runoff depth and peak discharge magnitude of Agusan River were performed using the SCS graphical peak discharge estimation method given the morphological characteristics of the river and the precipitation data across different climate scenarios. Two frequency analysis approaches were used to estimate the magnitude of hydrologic variables for a given return period namely: the plotting position approach and the analytic curve approach. Based on the results, amount of rainfall is projected to increase and extreme events are expected to be more frequent for different storm durations in future climate scenarios. This has a direct impact on the magnitude of channel discharge, thereby expecting frequent occurrence of flooding in nearby areas of the river. The results of the study can be used as a basis of the updated comprehensive management planning of Agusan River Basin and a reference for stakeholders in formulating policies, and adaptation and mitigation measures against adverse impacts of flooding in the area.