Analysis of the dynamic characteristics of seawater intrusion using partial wavelet coherence: a case study at Nakaura Watergate, Japan

Abstract

Lake Shinji and Lake Nakaumi are coupled brackish lakes on the Japan Sea coast of Japan. Seawater from the Japan Sea frequently intrudes into Lake Nakaumi through the bottom. Salinity flux at Nakaura Watergate was analysed. Effects of river discharge, tide level, and meteorological forcing variables makes the seawater intrusion non-linear. To account for the non-linear characteristics, partial wavelet coherence (PWC) was used to analyse the impact of river discharge, tide level, and meteorological forcing variables on salinity flux after controlling the effect of all other forcing variables. PWC results show that tide level has impact on salinity flux over short cycles (limited to less than 4 days) throughout the analysis period with an average time lag of about 2 days. This suggests that astronomical tides are the dominant factor in the salinity intrusion processes. The river discharge largely influences salinity flux over medium cycles (5-16 days) during the months of June and July with an average time lag of 0.3 days. Atmospheric pressure occasional affect tide level, indirectly driving salinity transport. The study findings provide guidance for factors affecting salinity intrusion processes and further research.

Use of Wavelet Techniques in the Study of Seawater Flux Dynamics in Coastal Lakes

“…The red arrow indicates the third eastern-side floodgate, the location of sampling, and measurement equipment. Also shown are meteorological and hydrological monitoring stations (source: [16]).…”

“…Ideião et al studied the variability of the total monthly rainfall with the aid of CWT [4,14]. The cross-wavelet transform (XWT) and wavelet coherence (WTC) techniques were used to analyze geophysical time series, for example, the effects of tidal range and river discharge on the salinity intrusion [15,16], the impacts of Arctic Oscillation index and ENSO on the Baltic sea ice [17,18], the relative humidity, and the shortwave radiation dataset [19]. Partial wavelet coherence (PWC) and multiple wavelet coherence (MWC) were used to study the impact of ENSO on the variability of tropical cyclones [20].…”

“…It also quantified the relationships between river discharge, tide and meteorological forcing variables, and salinity intrusion, using WTC, XWT, and PWC. Several studies examined the effects of river discharge, tidal range, and meteorological forcing on salinity intrusion [15,16]. Meteorologically induced sea surface variation (MISSV) and large periodic river discharge are considered to be effective water exchange mechanism between Lakes Nakaumi and Shinji, and the Japan Sea [22].…”

“…Meteorologically induced sea surface variation (MISSV) and large periodic river discharge are considered to be effective water exchange mechanism between Lakes Nakaumi and Shinji, and the Japan Sea [22]. Though tidal amplitude on the Japan Sea is small, astronomical tides appear to be an effective water exchange mechanism [16]. The study of the influence of external forces on salinity intrusion is difficult; the same applies to the development and application of seawater intrusion countermeasures.…”

“…The previous wavelet analysis of salinity intrusion in the Sakai Channel was studied for one summer season [16]. Since salinity intrusion varies with season and time, the assessment of its dynamics requires a long-term analysis.…”