Abstract:

The backwater of the the Pengxi River was a typical region to study the effect of water hydrology on the chlorophylla (Chla) content. The computed area was 535 km of the Pengxi River reach range from the river estuary to Qukou section in Yunyang city and the Yangtze River reach of 68 km in Shuangjiang city. Currently there are three methods to study the effect of water hydrology on the Chla content: laboratory research, field monitoring study and ecological dynamics model simulation. In this paper, the verified ecological dynamics model was used to simulate CODMn, TN, TP concentrations and Chla content. The average permanganate index (CODMn), total nitrogen (TN), total phosphorus (TP) concentrations and Chla content along the Pengxi River were on two design conditions: 1) 175 m high water level with low flow and low pollutant load; 2) 145 m low level with high flow and high pollutant load. The low pollutant loads of CODMn, TN, TP were 1028 t/d, 206 t/d and 039 t/d, respectively, while the high pollutant loads of CODMn, TN, TP were 7795 t/d, 3096 t/d and 578 t/d respectively. The computed results showed that the ranges of CODMn, TN, TP concentrations and Chla content in the backwater of the Pengxi River were 137-160 mg/L, 055-080 mg/L, 0035-0050 mg/L and 032-096 μg/L on the design condition 1, and 567-680 mg/L, 302-408 mg/L, 0131-0240 mg/L and 1060-3213 μg/L on the design condition 2, respectively. CODMn, TN, TP concentrations from upstream to downstream tended to increase, and the water quality at the estuary trended toward the Yangtze water quality. In the period of 145 m low water level with large flow and high pollutant load, the Chla content was high. In the curve or wide surface sections such as Gaoyang or Shuangjiang Bridge, the flow rate was slow and Chla content was high. The correlation of flow rate (V) and Chla content (S) was fitted under the two design conditions. The equations were S=0186 0·V-0353 6(related coefficient r2=0838 0) and S=10844 9·V-0293 9(related coefficient r2=0838 0) respectively. The simulated results showed that the increase of flow rate tended to keep down the Chla content. Under the conditions of low flow with 175 m high water level, the pollutant load was low, average water temperature was 80℃, total solar radiation was 517 J/(cm2〖DK〗·d), and average sunshine hours were 20 h/d. The Chla content increased quickly when the flow rate was lower than 001 m/s, and the Chla content tended to be stable when the flow rate was higher than 010 m/s. Under the conditions of high flow with 145 m low water level, the pollutant load was high, average water temperature was 200℃, total solar radiation was 1 464 J/(cm2〖DK〗·d), and average sunshine hours were 58 h/d. Chla content increased quickly when the flow rate was lower than 005 m/s, and Chla content tended to be stable when the flow rate was higher than 040 m/s. This study suggested that 005 m/s was the critical flow rate of Chla content change in the Pengxi backwater under the appropriate environmental conditions. The simulated results showed that it was feasible to control Chla content through regulating hydrodynamic conditions