Experimental Manipulation Of Nitrogen And Phosphorus Retention Capacity In An Agricultural Headwater Stream

The headwater stream is an important part of the river system, which has great function to control and reduce the nitrogen and phosphorus nutrient in the downstream water. To investigate the regulation of the impacts on ammonium and phosphorus retention efficiency, we choosed four artificial control measures:right angled-staggered spur dike, downward-wing spur dike, single-stage overflow dam and multistage rolled dam. The injection experiments were performed with conservative tracer (NaCl) and noconservative tracer (NH4Cl and KH2PO4) at a constant velocity. According to the data of tracer experiments, the OTIS model was used to simulate hydro logical parameters, to compare the changes of transient storage metrics and nutrient spiraling metrics, then to reveal the impacts on transient storage and nutrient retention efficiency by the artificial manipulation. In addition, this research also carry out the characteristics of nitrogen and phosphorus retention in the deep pools and tortuous channels. The main results were outlined as follows:(1) The value of As、Tsand As/A in deep pool is larger than that in winding ditch, but its value of hydrological parameter a is lower an order of magnitude than that of winding ditch. In the deep pool, the value of λ-NH4+ is higher two to three orders of magnitude than that of λS-NH4+. Vf-NH4+ is higher one to two orders of magnitude than that of Vf-SRP. U-NH4+ is higher two to three orders of magnitude than that of U-SRP. But the two pairs of parameters,λ-NH4+ and λs-NH4+, Vf-NH4+ and Vf-SRP, which are closer in terms of numerical size in winding ditch. What is more, U-NH4+ is just larger one to two orders of magnitude than U-SRP in winding ditch. In general, there are great differences in the effect of ammonium and phosphorus retention between deep pool and winding ditch, and marked retention efficiency could be provided for ammonium rather than phosphorus in the deep pool.(2) The experiments of spur dike regulation tracer show that:the value of f is increased and the average value of a with spur dikes are higher than that of no spur dike. Compared with the situation without the spur dike, the value of As/A for 1.5-m,2.5-m and 3.0-m right angled-staggered spur dike are increased by 27.27%、34.05% and 21.48%, and the increases of the Fmed200 are 191.10%.154.59% and 179.89%. The average value of nutrient spiraling metrics is higher one order of magnitude than that of no spur dike. The value of As/A for 2.0-m and 3.0-m downward-wing spur dike are increased by 105.41% and 75.68%, and the increase of the Fmed200 are 240.27% and 153.40%. The value variation tendency of Sw is 2 m spacing< 4 m spacing< no spur dike, and Vf and U are 2 m spacing> 4 m spacing>no spur dike. In a word, both of two dikes can improve the capacity of nitrogen and phosphorus retention.(3) The rolled dam regulation of tracer test shows that:the value of f and the As/A were significantly increased after adding rolled dam. The change of a is not obvious for single-stage overflow dam, but the multistage rolled dam were marked increased. Relative to no rolled dam, the value of Fmed200, for 30 cm and 50 cm single-stage rolled dam, increasing 1.44 times and 3.83 times, Vf-NH4 increasing 2.5 and 8.1 times respectively, and Vf-SRP increasing in 3.17 and 18.96 times respectively. For multistage rolled dam, the value of Fmed200, Vf-NH4 and Vf-SRP increased in 4.39,5.30 and 4.92 times respectively than that of no rolled dam. In general, the regulation capacity of nitrogen and phosphorus retention is 50 cm>30 cm> no dam, and that of single-stage rolled dam is multistage rolled dam> no dam.(4) The multivariate statistical analysis results show that:there are many important influences on the capability of nitrogen and phosphorus retention to flow rate Q. It shows that the transient storage closely relating to nitrogen and phosphorus retention improves the stream nitrogen and phosphorus retention effect with stream transient storage capacity strengthen by artificial measures.