Abstract
Abstract. Lake Kojima, an artificial lake located in the coastal area of western Japan, is categorized as a hyper eutrophic lake due to the nutrient inputs from the Sasagase River, Kurashiki River and Kamo River. The input nutrient loads from the rivers have never been assessed because there are no observation stations for runoff rate. The objective of this study is to confirm the total nitrogen (T-N) and total phosphorus (T-P) loads into Lake Kojima using the SWAT model for 60 years and considering changes in land use and the amount of domestic wastewater in the watersheds. Estimation results show that more than 90% of the total nutrient load comes from the Sasagase and Kurashiki rivers. The estimated T-N and T-P loads indicated two different trends in the rivers; an increasing trend is found during the period from 1950 to 1980, while a decreasing trend is found during the period from 1980 to 2009. It was suggested that the increasing trend was commonly caused by the combined effects of increase of the amount of domestic wastewater and agricultural wastewater in the watersheds, while the decreasing trend was caused by the expansion of domestic wastewater treatment and decrease of agricultural land use. The contribution ratio of domestic wastewater to the total amount of nutrient load was estimated to be 60% in Sasagase River and 15% in Kurashiki River because the treatment ratio of domestic wastewater in Kurashiki River was higher than in Sasagase River.
Highlights
Lakes and reservoirs which are built near cities have suffered from water quality problems, such as eutrophication and harmful algal bloom, due to human activities such as wastewater discharge (Heisler et al, 2008)
The results indicate that Kurashiki River has the highest nutrient effluent from domestic wastewater and agricultural wastewater
In Sasagase River watershed, nutrients of domestic wastewater accounted for 60% of the annual total nutrient yield of the stream; nutrients of agricultural wastewater accounted for 40% of the annual total nutrients yield
Summary
Lakes and reservoirs which are built near cities have suffered from water quality problems, such as eutrophication and harmful algal bloom, due to human activities such as wastewater discharge (Heisler et al, 2008). Hydrological models have been used to estimate nutrient loads from inflowing rivers for confirmation of lake dynamics or as support to decision makers for water management. The few studies that have covered more than 30 years have space to improve regarding the effect of human activities such as changes in land use and population (e.g. Yu et al, 2007). The objective of this study is to confirm total nitrogen (T-N) and total phosphorus (T-P) loads into a hyper-eutrophic lake in western Japan using the SWAT model for 60 years and considering both changes in land use and the amount of domestic wastewater in the watersheds
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