The Fate And Transport Of Nitrogen And Phosphate In The Headwater Stream Of Shiwulihe In Lake Chaohu Basin Based On OTIS Model

Solution of nutrient (NH4Cl or KH2PO4) and a conservative tracer (NaCl) were usedto conduct field experiments by slug injection in the headwater stream of ShiwuliheRiver, Lake Chaohu basin. On the basis, a mathematical simulation model (OTIS),viewed from the coupling effect of hyporheic exchange and lateral inflow, was used tocharacterize the fate and transport of water-borne solutes in streams and rivers. TheOTIS and OTIS-P model, proposed by Runkel (United States Geological Survey,USGS), were employed to estimate the hydrological parameters and first-order decaycoefficients and perform further hydrological parameters sensitivity analysis. Theeffects of different modules in the mathmatical model were analyzed to identify themain action mechanism. The capacity of transient storage zone and characteristics ofnutrient retention in the headwater stream of Shiwulihe River were described based onthe transient storage metrics and nutrient spiraling concept. Under the uncertainty ofwater environment system, we simulated solute transport and diffusion in the headwaterstream. Study results were outlined as follows.(1) The analysis of different functional modules in OTIS model showed that,without consideration of lateral inflow, solute would be delayed in arriving downstreamand modeled concentrations would be higher than measured concentrations. If ignoreingthe effect of transient storage, then not only would be simulation concentrations muchhigher than observed values, but also the shape of simulation concentrationbreakthrough curve would be changed, and it woud obviously shorten the solutetransport time arriving at the downstream reaches.(2) The characteristics of transient storage in the headstream showed thatconsiderable variations of numerical size existed in Tsand Lsin different hydrologicalconditions. More than half of the values ofF200medwere less than20%, and thedifference of hydraulic retention factor Rhamong different reaches was particularlyobvious. Mean values of qsand q-3Lwere2.49×10and3.36×10-4m3·s-1·m-1, respectively,implying the existence of lateral inflow. Moreover, no significant correlations werefound among most of the hydrological parameters and transient storage metrics inregression analysis results.(3) The average values of total uptake rate k-NH4and k-SRP were1.53×10-3and-1.11×10-4s-1, respectively. The uptake lengths of Sw-NH4reached29279m, suggesting low capacity to remove the nitrogen by physical or biological processes.Sw-SRP was-2568m, implying that the release of phosphorus in this headstreamexceeded the P uptake, and the headstream played a role of “source”. Compared tomountain headwater streams with low nutrient concentration, the study stream hadlower values of Vf-NH4, but higher in U-NH4. Estimates of mass loss in the upstreamreach indicated that88.14%of theNH4uptake occurred in the main channel,11.86%in the transient storage zone.(4) Parameter sensitivity analysis results showed that the order of modeledparameters of their sensitivity was A＞As＞α＞D.(5) The simulation results of Water environment model showed with the dischargegetting higher, the earlier Cl-1would arrive downstream. In the simulated breakthroughcurves of Cl-1for the different location under the same dicharge, from upstream todownstream, the declined range of the peak concentration became smaller, and theshape of the curve varied from the steep to steady stretch.