A total of 414 agricultural reservoirs ranging from 180 thousand m³ to 253 million m³ in storage volume were investigated from 1990 to 2000 with respect to COD, TP, TN, Chlorophyll-a, and their annual mean values were used for the analysis. Koran agricultural reservoirs have generally small storage ...
A total of 414 agricultural reservoirs ranging from 180 thousand m³ to 253 million m³ in storage volume were investigated from 1990 to 2000 with respect to COD, TP, TN, Chlorophyll-a, and their annual mean values were used for the analysis. Koran agricultural reservoirs have generally small storage volume, shallow depth, small drainage and surface area, short residence time, and small ratio of total area to surface area when compared with the natural lakes and reservoirs in the U.S.A. contained in the NES(national eutrophication survey) data sets. The retention time of reservoirs(td) ranged 10 to 157 days, and the ratios of drainage area (DA) to reservoir surface area (SA) were 10 to 168. The residence time(Td) correlated with TA/ST was 462.8×(TA/ST)^-1.03 (R²=0.87). Korean agricultural reservoirs were contaminated by organic matter (BOD, COD) and nutrients, and easily developed algal blooms. Phosphorus is a nutrient limiting factor in 83% of total reservoirs. Trophic state analyses were conducted by using Vollenweider method, U.S. EPA method, and OECD method. Vollenweider method analysis uses only one parameter (nutrients), so it may be deficient of rationality. In U.S. EPA method, the results of trophic state by TP and Chlorophyll-a differ because TP and Chlorophyll-a relationship differs temporally and spatially, thus it may not be proper, either. The analysis by the use of OECD method using many water quality parameters such as nutrients, trophic state by Chlorophyll-a, secchi disk depth, and TP had similar results. It is recommended that OECD method be applied in analysing trophic state of Korean agricultural reservoirs. As a result of trophic state by OECD method, ultra-oligotrophic is 0~1%, oligo-mesotrophic 1~4%, mesotrophic 34%, meso-eutrophic 42~48%, eutrophic 14~22%. Chlorophyll-a concentration was positively related with the mean depth of reservoir, implying higher Chlorophyll-a concentration with deeper mean depth in spite of similar pollutant loadings. The effect of theoretical retention time on chlorophyll-a yields per TP was not noticeable in Korean reservoirs. The effect of the ratio drainage area (DA) to surface area (SA) on TP and chlorophyll-a relationship was prominent. The smaller was the ratio of DA/SA, the stronger was the TP and chlorophyll-a relationship. This seems to be from the fact that there is density overflow in deep reservoirs so actual retention time which effects algal growth is determined not by storage volume but by surface area. The empirical regression equation of TP concentration using physical parameters in the reservoirs is C=6.03W^0.68V^-0.53SA^-0.21(DA/SA)^-0.50. The relationships among the water quality parameters, TP, and COD have strong correlation with Chiorophyll-a but that of TN has weak relationship with Chiorophyll-a. It indicates that the limiting factor of Korean agricultural reservoirs is phosphorus. The relationship between nutrients and Chlorophyll-a has the most strong correlation during summer.
A total of 414 agricultural reservoirs ranging from 180 thousand m³ to 253 million m³ in storage volume were investigated from 1990 to 2000 with respect to COD, TP, TN, Chlorophyll-a, and their annual mean values were used for the analysis. Koran agricultural reservoirs have generally small storage volume, shallow depth, small drainage and surface area, short residence time, and small ratio of total area to surface area when compared with the natural lakes and reservoirs in the U.S.A. contained in the NES(national eutrophication survey) data sets. The retention time of reservoirs(td) ranged 10 to 157 days, and the ratios of drainage area (DA) to reservoir surface area (SA) were 10 to 168. The residence time(Td) correlated with TA/ST was 462.8×(TA/ST)^-1.03 (R²=0.87). Korean agricultural reservoirs were contaminated by organic matter (BOD, COD) and nutrients, and easily developed algal blooms. Phosphorus is a nutrient limiting factor in 83% of total reservoirs. Trophic state analyses were conducted by using Vollenweider method, U.S. EPA method, and OECD method. Vollenweider method analysis uses only one parameter (nutrients), so it may be deficient of rationality. In U.S. EPA method, the results of trophic state by TP and Chlorophyll-a differ because TP and Chlorophyll-a relationship differs temporally and spatially, thus it may not be proper, either. The analysis by the use of OECD method using many water quality parameters such as nutrients, trophic state by Chlorophyll-a, secchi disk depth, and TP had similar results. It is recommended that OECD method be applied in analysing trophic state of Korean agricultural reservoirs. As a result of trophic state by OECD method, ultra-oligotrophic is 0~1%, oligo-mesotrophic 1~4%, mesotrophic 34%, meso-eutrophic 42~48%, eutrophic 14~22%. Chlorophyll-a concentration was positively related with the mean depth of reservoir, implying higher Chlorophyll-a concentration with deeper mean depth in spite of similar pollutant loadings. The effect of theoretical retention time on chlorophyll-a yields per TP was not noticeable in Korean reservoirs. The effect of the ratio drainage area (DA) to surface area (SA) on TP and chlorophyll-a relationship was prominent. The smaller was the ratio of DA/SA, the stronger was the TP and chlorophyll-a relationship. This seems to be from the fact that there is density overflow in deep reservoirs so actual retention time which effects algal growth is determined not by storage volume but by surface area. The empirical regression equation of TP concentration using physical parameters in the reservoirs is C=6.03W^0.68V^-0.53SA^-0.21(DA/SA)^-0.50. The relationships among the water quality parameters, TP, and COD have strong correlation with Chiorophyll-a but that of TN has weak relationship with Chiorophyll-a. It indicates that the limiting factor of Korean agricultural reservoirs is phosphorus. The relationship between nutrients and Chlorophyll-a has the most strong correlation during summer.
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