Development of Flow-duration-frequency Curves for Episodic Low Streamflow

In water resources management and practices, the time-connected duration and frequency of episodic low streamflow events (and droughts) are often needed for a range of purposes such as water supply planning, low flow management, and drought planning. However, the existing graphical tools commonly used in hydrology such as flow duration curves (FDC), discharge-duration-frequency (QdF), and severity-duration-frequency (SDF) curves, often do not provide this information. In this study, we contribute an approach to develop a flow-duration-frequency (FDF) curve by fitting the exponential function to graphically show the time-connected duration and frequency of episodic low streamflow events. The FDF fitting is comprehensively tested with a bootstrap resampling experiment using long-term continuous streamflow records for 92 USGS gages located in the state of Illinois, U.S.A. The experiment results reveal that the proposed methodology can reasonably develop FDF curves for episodic low streamflow events. The results demonstrate that the FDF fitting is improved by using longer streamflow record though the improvement appears to level off with 30 years or longer streamflow records. The results also confirm that the FDF fitting is more reliable for higher streamflow threshold to define episodic low streamflow events.