Defining an estuary using the Hilbert-Huang transform

Researchers have used various physical, chemical, or topographic features to define estuaries, based on the needs of their particular subject. The principal features of estuaries are the tides that influence their water stages; thus, the boundaries of an estuary can be determined based on whether the water stage is subject to tidal influence. However, the water stage is also influenced by the upstream river discharge. A hydrograph of water stage will therefore include both non-stationary and nonlinear features. Here, we use the Hilbert-Huang Transform (HHT), which allows us to process such non-stationary and nonlinear signals, to decompose the water-stage hydrographs recorded at different gauging stations in an estuary into their intrinsic mode function (IMF) components and residuals. We then analyse the relationships between the frequencies of IMFs and known tidal components. A frequency correlation indicates that the water stage of the station is subject to tidal influences and is located within the estuary. The spatial distribution of the stations that are subject to tidal influences can then be used to define the estuary boundaries. We used data from gauging stations in the estuary region of Taiwan's Tanshui River to assess the feasibility of using the HHT to define an estuary. The results show that the HHT is a dependable and easy method for determining the boundaries of an estuary.