| Phragmites australis as large emergent aquatic plants are widely distributed in the river and ditch system, not only can absorb and use the nitrogen, phosphorus and other nutrients directly in the water, can also be a number of ways to reduce transmission of non-point source pollutants to water downstream. Eight field tracer experiments, in which a solution of conservative (NaCl) tracers and biologically active (NH4Cl and KH2PO4) was added to the stream at a constant rate, were conducted at an agricultural headwater stream, dominated by Phragmites australis, in the Ershibu River, Lake Chaohu basin. Based on the data sets of tracer experiments, the OTIS model was used to characterize the migration and transformation of water-borne solutes, to analyze the sensitivity of hydrological parameters and effect of different functional modules on water environmental simulation. The transient storage metrics were calculated to assess the capacity of transient storage zone in the syudy reach. According to the nutrient spiraling theory, retention characteristics of ammonium and phosphorus were interpreted, and contribution of main channel and transient storage zone, hydrological and biological processes to nutrient retention were quantitative analyzed and estimated. The main results were summed up as follows:(1) For the hydrological parameters, the mean values of D, A, As and a in study reach predominated by Phragmites australis were 0.103 m2·s-1,0.325 m2,0.390 m2 and1.63×10-1 s-1, respectively. Parameter sensitivity analysis results showed that the order of modeled parameters of their sensitivity was A>a>As>D.(2) The analysis of different functional modules in OTIS model showed that, if ignoring the effect of transient storage, the relative deviations HI of the peak values of chloride ion concentration reached the maximum, implying the effects of solutes in the migration of water transportation of transient storage markedly exceeded those of lateral inflow.(3) The value of Ts was higher than Tc, and the ratio of As/A was greater than 1, the findings had certain relation to the condition of dominance of Phragmites australis. Regression analysis indicated that there was relation between Rh and discharge (Q) with a power function, but no obvious relationship was found between the transient storage metrics(i.e. Ts, Ls,As/A and Fmed) and Q.(4) The value of nutrient spiraling metrics Sw-NH4 and Sw-SRP ranged from 189 to 1087m and 412 to 3532m, respectively, the value of Vf-NH4+ and Vf-SRP between the order of magnitude of 10-6 and 10-4, and the uptake rate of U-NH4+ was higher than U-SRP.(5) The proportional NH4+ and SRP retention with the averages of 14.68% and 12.53%. Estimates of mass loss indicated that the transient storage zone and main channel accounted for 56.44% and 43.56% of the NH4+uptake, and 47.82% and 52.18% for SRP, respectively.(6) The practical contribution rates of hydrological and biological factors to NH4+ retention were 10.12% and 4.57%, respectively, and 10.12% and 2.41% for SRP, respectively. The relative contributions of hydrological and biological processes to NH4+ retention were 72.51% and 27.49%, respectively, and 81.42% and 18.58% for SRP, respectively. On the basis of correlation analysis, no significant correlations were found among retention ratio (i.e.ηNH4, ηpo4) and the following hydrodynamic parameters as Re and n. |
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