Land Cover Change And Fluctuation Of Lacustrine Water Quality In Okutama Lake
4 Discussion
4.1 Land Cover Change
As a result of land cover change, forest and urban area increased, but agricultural fields decreased. The period of satellite data used here corresponds with that of the 8th water supply forest management plan by Tokyo Metropolitan Government from 1986 to 1996. By this plan, the introduction of broadleaf trees into forest led to the change of forest constitution. On the other hand, by the reduction of paddy fields their cultivation area decreased. Moreover, urbanization brought sewer drainage networks to generate sedimentation of soils.
4.2 Water Quality Fluctuation
Long-term fluctuation for water quality in the lake showed two different trends: the decrease of turbidity (increase of transparency) and increase of chlorophyll-a. Turbidity performs transparency reversely, which represents runoff reduction in the watershed physically. On the other hand, the increase of chlorophyll-a means microbial breeding corresponding to the increase of total nitrogen. Moreover, urbanization seemed to make sewer drainage into the lake, which is proved by negative correlation between total nitrogen and rainfall. Rainfall also correlates with total phosphorus, which is inferred from forest runoff.
4.3 Water Quality Correlation
Turbidity correlated with total phosphorus positively, but with transparency, chlorophyll-a, total nitrogen negatively. If turbidity was brought from soils in the watershed, it should depend on rainfall and be affected by the reduction of runoff ratio. In fact, rainfall and turbidity correlated, and the runoff ratio showed decrease trend. On the other hand, total nitrogen showed negative correlation with rainfall. As sewer drainage discharges constantly, it shows negative correlation with rainfall. Therefore, total nitrogen behavior should be derived from sewer drainage. As chlorophyll-a correlates with total phosphorus negatively, its breeding should be controlled by total phosphorus critically. Moreover, chlorophyll-a has positive correlation with total nitrogen annually, but shows negative correlation with it seasonally. It is inferred that chlorophyll-a breeding requires enough total nitrogen supplies and consumes total nitrogen seasonally.
Water quality fluctuation in 1984 and 1997 was controlled not only by land cover change but also other factors. For example, vegetation constitution may contribute water quality, which should be the next theme. Sedimentation behavior may depend on sewer networks development, which should be also the next theme.
5 Conclusion
From water quality and satellite data for the Ogouchi Dame for 39 years, relationship between long-term water quality fluctuation and land cover change in the water supply forest watershed was analyzed for the next conclusions.
- Land cover classification indicated the increase of forest and urban area, and the decrease of agricultural fields.
- Long-term water quality fluctuation showed two trends: the decrease of turbidity and the increase of chlorophyll-a and total nitrogen. The former might be affected by the reduction of runoff rate accompanied with the increase of forest and the decrease of agricultural fields. The latter might be caused by sewer drainage pollution into the Tama River accompanied with urbanization.
- From the correlation of water quality items, it was inferred that turbidity was derived from soils with rainfall and chlorophyll-a was controlled by total phosphorus critically to consume total nitrogen.
Water quality fluctuation might be affected by not only land cover change but also other factors: forest constitution change and sewer networks development, which will be the next theme.
Acknowledgement
The authors thank Waterworks Bureau in Tokyo Metropolitan Government for their cooperation with their data. We also appreciate to Dr. Genya Saito, Mr. Naoki Ishituka, and Dr. Takuhiko Murakami at National Institute of Agro-Environmental Sciences, and Ms. Ikuyo Makino at Tokyo Institute of Technology for their cooperation of remote sensing analysis.
References
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Waterworks Bureau, Tokyo Metropolitan Government, Annual Report of Ogouchi Reservoir Management, 1959-1997.
