Numerical Assessment of Total Nitrogen (Tn) Load Discharged from Rivers into Harima-Nada, the Seto Inland Sea, Using A Numerical Coupled Hydrological-Water Quality Model

Abstract

Harima-Nada region, located in the east of Seto Inland Sea, is a semi-enclosed sea-area next to Osaka Bay that has been suffering a sustained oligotrophication process during the past 25 years. A hydrogeological model, coupled with a water quality model, was applied to assess the numerical discharge of freshwater and total nitrogen (TN) load from rivers into the sea. The model was validated for the main rivers from the northern and southern regions (the Kako River and the Yoshino River, respectively). The obtained results showed that the main contribution of freshwater and TN load to the sea comes from Hyogo Prefecture Rivers. The type of activities developed on each watershed showed to be substantially more relevant on TN load contribution than the size itself of the considered watershed. For the eight years considered in this study (2009 - 2016), the annual average of TN discharged into Harima-Nada was almost constant, while the volume of discharged freshwater showed more variations. Additionally, the sustained decrement in nitrogen concentration in Harima-Nada seemed not to be directly bonded with riverine TN load, suggesting that nitrogen availability is closely related to sea nutrients mechanisms more than to TN inland contribution.

Seasonal Study of the Kako River Discharge Dynamics into Harima Nada Using a Coupled Atmospheric–Marine Model

“…Ten rivers directly discharge their waters in the study area (five on the northern coast, three in the southern one, and two in the center region), plus the biggest river of Shikoku Island discharging next to the Naruto Strait, one of Harima Nada's limits (Figure 1b). Only the influence of the Kako River, which is responsible for the larger discharge of freshwater in the study region [13], was considered in this work. In Japan, the seasonal occurrence of heavy rain events is common, and their effects are very significant on Japanese rivers' nutrient loads and discharged volumes.…”

“…The most important river in Harima Nada is the Kako River, located in the center of the northern coast. The Kako River accounts for around 40% of the total freshwater discharged in Harima Nada, and it is the higher contributor of major land-derived nutrients like total nitrogen (TN) in the area [12][13][14].…”

“…Much research has been performed in the entire Seto Inland Sea [7][8][9][15][16][17][18][19][20], Harima Nada [10][11][12][13][14][21][22][23][24][25][26], and its surrounding areas like Osaka Bay [14,27,28], and different numerical and simulation models and monitoring observations have been proposed, applied, and conducted to describe and study the characteristics of these regions. Until this date, all the reported ocean dynamic models for the area used offline atmospherical forcing, which means that the input of atmospheric variables was either previously calculated using an atmospheric model, belonged to reanalysis datasets, or consisted of a time series of observed data.…”