Estimation of Annual Total Nitrogen Load to lake from Basin with Remote Sensing
3.2 Basic Unit of Total Nitrogen Load from Each Land Cover Category
In order to estimate a basic unit of total nitrogen load from each cover category, influent annual total nitrogen load from each river I to the lake is defined by
Where j (1-4) represent the suffix for each land cover category, urban area, paddy fields, forest area and field, respectively. U[Kg/Km
2 * y] is named runoff load factor of total nitrogen in this study, which is a basic unit of total nitrogen load including run-off ratio from point or nonpoint sources to river mouth. Also, we assumed as follows in the Eq.(1) ;
Assumption 1: The pollutant load from point sources such as residual and industrial wastewater are included in the load from the urban area in the Eq.(1).
Assumption 2: The pollutant load from point source such as wastewater of livestock farming are included in the load from the paddy field and field (farm area) in the Eq.(1).
The Eq.(1) can describe the pollutant load from point and nonpoint sources by introducing runoff load factor of land cover categories for the large area such as lake basin area.
To estimate the unknown parameters, runoff load factor of total nitrogen Uj each land cover category, the following loss function E was minimized.
Figure 4 shows the estimated runoff load factor of total nitrogen. From fig.4, it was found that the runoff load factor of total nitrogen of the urban area and field are larger than those of forest area and paddy field. As a result, the extension of urban area and field give influence to the eutrophication of Lake Kasumigaura through the increase of nitrogen load to the lake area.
Fig.4 Runoff load factor (basic load unit) of total nitrogen
3.3 Estimation of Influent Annual Total Nitrogen Load to Lake Kasumigaura
In order to estimate influent annual total anitrogen load from Lake Kasumigaura basin to the lake, we calculated the load L as follows,
where A is Lake Kasumigaura basin area. U and r are estimated runoff load factor of each land cover category and the each land cover ratio in the Lake Kasumigaura basin, repectively.
Figure 5 shows the calculated influent annual total nitrogen load of each year from Lake Kasumigaura basin. And also, influent annual total nitrogen load calculated by Ibaraki Prefecture is shown in Fig.5 to compare with our method. The calculation of Ibaraki Prefecture is most popular method based on sum of a basic unit of total nitrogen load of each land cover category. As shown in Fig.5, it is found that result estimated by proposed method agree approximately with that of Ibaraki Prefecture method. However, the load estimated by proposed method decrease when the precipitation decreased in 1984, but the load estimated by Ibaraki Prefecture were always constant. It is because the effect of annual precipitation is considered in our proposed method. Therefore, this proposed method is effective in estimating influent annual nitrogen load to lake.
Fig.5 Calculated influent annual total nitrogen load of each year.
4. Conclusions
In this study, a new remote sensing method was proposed to valuate total nitrogen load to the Lake Kasumigaura. In the method, following two parameters were evaluated from land cover distribution produced by LANDSAT MSS data and the nitrogen concentration measured at each river mouth to the lake;
(1) A basic unit of total nitrogen load from each land cover category
(2) Influent annual total nitrogen load to the lake.
A nitrogen load model which relates total nitrogen load with land cover condition was investigated and two parameters were estimated with optimization method. The estimated result shows good agreement with the ground observation data.
Nutrient load to river and lakes is one of the essential factor polluting water quality, therefore, quantitative estimation of the load to water areas is indispensable for the improvement of aquatic environment. A remote sensing method investigated here may provide an efficient tool for the management of regional environment including land and inland water areas.
References
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Ibaraki Prefecture [1983]: the report of Eutrophication at the Lake Kasumigaura. Oki, K and Yasuoka, Y [1996]: Quantitative analysis of the relation between water quality of rivers and land cover distribution at the lake basin, Proceedings of the 20th JCRS, pp.65-66.
