Nutrient Transport Modelling in the Daugava River Basin - DiVA Portal

Groundwater discharge to the river channel and percolation from groundwater zone 1 togroundwater zone 2 is calculated from recession coefficients, so that for each dayG1t= r1⋅S1t(4)G2t= r2⋅ S2t(5)G = gr ⋅ S1(6)ttwhereG1 t = flow from groundwater box 1 to the stream channel [cm/day]G2 t =flow from groundwater box 2 to the stream channel [cm/day]r1 = recession coefficient from box 1 [day ­1 ]r2 = recession coefficient from box 2 [day ­1 ]S1 t = soil moisture in groundwater box 1 [cm]S2 t = soil moisture in groundwater box 2 [cm]G t =flow from groundwater box 1 to groundwater box 2 [cm/day]gr = coefficient for groundwater flow from box 1 to box 2 [day ­1 ]In the model, there is also a possibility to include deep seepage, which is interpreted aswater leaving the system by percolation from groundwater box 2 to a deep aquifer. Thiswater will not contribute to the streamflow. However, assuming that all water enteringthe system as precipitation will leave the system either as evapotranspiration or end upin the stream channel the deep seepage has been set to zero in this study.3.2.3 NutrientsIn the GWLF model loads of dissolved and solid nitrogen and phosphorus are estimatedfor each day. Daily values are summed to provide monthly estimates of nutrient loads. Itis assumed that streamflow travel times are much less that one month. Dissolvednutrients are assumed to origin from surface runoff, point sources (as known, constantmass flows) and groundwater discharge to the stream. Thus, monthly loads of dissolvednitrogen or phosphorus in streamflow are:LD m =DP m +DR m +DG m (7)whereLD m = total dissolved nutrient load in month m [kg]DP m = point source dissolved nutrient load in month m [kg]DR m = rural runoff dissolved nutrient load in month m [kg]DG m = groundwater dissolved nutrient load in month m [kg]The sources of solid­phase nutrients in the model consist of rural soil erosion and washoff of material from urban surfaces:LS m = SR m +SU m (8)whereLS m = total solid phase nutrient load in month m [kg]SR m = solid phase rural runoff nutrient load in month m [kg]SU m = solid phase urban runoff in month m [kg]10