A novel index for assessing the water quality of urban landscape lakes based on water transparency

Assessment of the aesthetic and recreational value of urban landscape lakes (ULLs) is often required but there has not been a water quality index specifically applicable for such a purpose. Under a consideration that water transparency in terms of Secchi Depth (SD), to a large extent, determines the landscape effect, a study was conducted to identify the major parameters that strongly influence SD and to develop a novel water quality index. By theoretical analyses, it was found that SD is mainly influenced by the contents of chlorophyll a, inorganic suspended solids and organic detritus in water, which collectively relate to eight independent water quality, hydraulic, and environmental parameters, including SS, DO, COD, NH4+-N, NO3--N, TP, HRT, and water temperature T. A composite index was then proposed in the form of WQI(ULL) =Pi (i)=1(n)q(i)(wi) (n = 8). Using the data of field survey of 166 ULLs in China, the cumulative probability distribution curve of each sub-index qi was characterized. Sensitive analysis was conducted for the determination of the sub-index weight (w(i)) for each qi under the consideration of two typical scenarios of ULL replenishment by stream water (traditional source) and reclaimed water (alternative source) regarding the variation of parameter on SD. With all w(i) (i = 1 to 8) thus determined, WQI(ULL) was calculated for each of the ULLs surveyed. All the calculated values of WQI(ULL) showed a good correlative relationship with the SD values practically measured (R-2 = 0.8948), indicating that the novel water quality index developed could effectively indicate the satisfactory degree of the lake water quality in terms of water landscape. Further by comparing the dimensionless WQI(ULL) (ranging between 0 and 100) with the practically acceptable SD based on experiences in China, the method for classification of ULLs by WQI(ULL) calculation was formulated.