Supplementary Materials

Increasing Dissolved Nitrogen and Phosphorus Export by the Pearl River (Zhujiang): A Modeling Approach at the Sub-basin Scale to Assess Effective Nutrient Management

Maryna Strokal, Carolien Kroeze, Lili Li, Shengji Luan, Huanzhi Wang, Shunshun Yang, Yisheng Zhang

Methodology (extended)

Box A.1 Summary of equations to quantify dissolved inorganic nitrogen (DIN) and phosphorus (DIP) export by the Pearl River (Zhujiang in Chinese) to coastal waters (the Pearl River mouth) from each source (diffuse and point) and from each sub-basin (up-stream, middle-stream, down-stream), following Global NEWS-2 (Mayorga et al. 2010). Abbreviations of the variables in equations are described in Table A.1

Table A.1 Abbreviations of the equations in Box A.1 used to quantify dissolved inorganic N (DIN) and phosphorus (DIP) export by the Pearl River from each source (diffuse and point) and from each sub-basin (up-stream, middle-stream, down-stream)

Abbreviation / Description / Unit
MF.y.ju, MF.y.jm, MF.y.jd / Export of nutrient form (F: DIN, DIP) to the mouth of the Pearl River (M) from source y and from upstream (ju), middle-stream (jm) and down-stream (jd) sub-basins (j) / kg year-1
RSF.y.j / Nutrient inputs (F: DIN, DIP) to the river system (RS) from source y in sub-basin j / kg year-1
RSdifF.y.j / Nutrient inputs to the river system (RS) from diffuse (dif) source y in sub-basin j / kg year-1
WSdifE.y.j / Inputs of nutrient element (E: N, P) to land from diffuse source y in sub-basin j / kg year-1
WSdifN(P).fe.j / Use of synthetic N (or P) fertilizer / kg year-1
WSdifN (P).ma.j / Use of N (or P) animal manure / kg year-1
WSdifN.fix.ant.j / Biological N2-fixation by agricultural crops / kg year-1
WSdifN.fix.nat.j / Biological N2-fixation by natural vegetation / kg year-1
WSdifN.dep.ant.j / Atmospheric N-deposition on agricultural areas / kg year-1
WSdifN.dep.nat.j / Atmospheric N-deposition on non-agricultural areas / kg year-1
GF.j / The fraction of land-surface diffuse F (F: DIN, DIP) sources remaining after animal grazing and crop harvesting in sub-basin j / 0-1
WSdifE.ex.j / Export of nutrient element (E: N, P) by animal grazing and crop harvesting in sub-basin j / kg year-1
WSdifE.gross.j / Inputs of nutrient element (E: N, P) to agricultural land from all diffuse sources of sub-basin j / kg year-1
FEws.F.j / The fraction of nutrient form (F: DIN, DIP) that is exported from land to the river of sub-basin j / 0-1
fF(Rnatj) / Calculated function of mean annual runoff from land to streams of sub-basin j / -
Rnatj / Mean annual (natural) runoff from land to streams of sub-basin j / m year-1
eF / The constant that considers export of nutrient form (F: DIN, DIP) from land to the river / -
aDIN, aDIP, bDIP / The constant values used to calculate the function of mean annual runoff / -
RSdifDIP.wth.ant.j / Inputs of DIP to the river system of sub-basin j from weathering of P-contained minerals in agricultural areas / kg year-1
RSdifDIP.wth.ant.j / Inputs of DIP to the river system of sub-basin j from weathering of P-contained minerals in non-agricultural areas / kg year-1
ECDIP.j / The coefficient of P weathering, assumed to be 26 in kg km-2 year-1, calculated for each sub-basin j by multiplying with sub-basin area / kg year-1
Agfr.j / The fractions of agricultural areas in sub-basin j. Agricultural areas include grassland in pastoral systems (e.g., dominated by grazing, limited manure storage and application), grassland in mixed systems (e.g., areas close to rivers, manure storage and application can take place), wetland rice, legumes (e.g., soybean and pulses) and cropland (e.g., maize, cereals) / 0-1
(1 – Agfr.j) / The fractions of non-agricultural areas in sub-basin j / 0-1
RSpntF.y.j / Nutrient inputs to the river system (RS) from point (pnt) source y in sub-basin j / kg year-1
hwfrem.E.j / The fraction of nutrient element (E: N, P) removed during wastewater treatment in sewage facilities of sub-basin j / 0-1
Ij / The fraction of population connected to sewage facilities / 0-1
WShwE.y.j / The amount of nutrient element (E: N, P) generated in human waste (source y) and detergents (source y) in watersheds (land) of sub-basin j / kg year-1
WShwN.hum.j / The amount of N element generated in human waste in watersheds (land) of the sub-basin j / kg year-1
WShwP.hum.j / The amount of P element generated in human waste in watersheds (land) of the sub-basin j / kg year-1
WShwP.det.j / The amount of P element generated in detergents in watersheds (land) of the sub-basin j / kg year-1
FEpnt.F.j / The fraction of nutrient element in sewage effluents that is emitted to the river of sub-basin j as a form (F: DIN, DIP) / 0-1
FEriv.F.j / The fraction of nutrient (F: DIN, DIP) inputs to the river of sub-basin j that are exported at the outlet of sub-basin j / 0-1
LDIN.j / The fraction of DIN retained / lost in the river of sub-basin j (e.g. denitrification processes) / 0-1
LDIP.j / The fraction of DIP retained / lost in the river of sub-basin j (e.g. sediment-water retention processes) / 0-1
DF.j / The fraction of nutrient form (F: DIN, DIP) retained in dammed reservoirs of sub-basin j / 0-1
DDIN.i.j / The fraction of DIN retained in dammed reservoir i of sub-basin j / 0-1
DDIP.i.j / The fraction of DIP retained in dammed reservoir i of sub-basin j / 0-1
ΔτR.i.j / Water residence time of dammed reservoir i in sub-basin j / Years
Vi.j / Volume of dammed reservoir i in sub-basin j / km3
hi.j / Average depth of reservoir i in sub-basin j / meter
Qacti.j / Actual water discharge (after water is removed for consumption purposes) of dammed reservoir i in sub-basin j / km3 year-1
FQremj / The fraction of nutrients (general for DIN and DIP) removed from the river system by water consumption for different purposes (e.g. irrigation, hydropower) / 0-1
Qactj / Actual (after water consumption) water discharge at the outlet of sub-basin j / km3 year-1
Qnatj / Natural (before water consumption) water discharge at the outlet of sub-basin j / km3 year-1

Table A.2 Descriptions of deriving inputs for model variables/parameters to quantify dissolved inorganic nitrogen (DIN) and phosphorus (DIP) export by the Pearl River (Zhujiang in Chinese) to coastal waters at the sub-basin scale. Box A.1 and Table A.1 describe abbreviations.

Variable / parameter / Description (D)
Total area / D1
For diffuse sources:
Agricultural area (Agfr.j) / D2
Watershed diffuse sources (WSdifE.y.j) and watershed export (WSdifE.ex.j) / D3
Watershed export constant (eF) / D4
Diffuse export coefficient (ECDIP.j) / D5
For point sources:
Watershed point sources (WSpntE.y.j) / D6
Nutrient removal during sewage treatment (hwfrem.E.j) / D7
Total population and population with sewage connection (Ij) / D8
For hydrology and reservoir retentions:
Runoff (Rnatj), actual (Qactj) and natural (Qnatj) water discharges / D9
Data for each reservoir/dam: volume (Vi.j), depth (hi.j) and water discharge (Qacti.j) / D10
Retention in reservoirs (DF.j) / D11
Retention in the river (LF.j) / D12
Runoff shape constants (aF, bF) / D13
Description (D):
Ds 1-4, 6, 8, 9 - Inputs for these parameters/variables were aggregated from 0.5 by 0.5 degree cell to a sub-basin scale using ArcGIS functions. Gridded inputs prepared by Fekete et al. (2010) for variables associated with hydrology and reservoirs, Bouwman et al. (2009) for variables associated with diffuse sources and by Van Drecht et al. (2009) for variables associated with point sources. These inputs are available for 1970, 2000, 2050.
D1 – The total area of each sub-basin (km2) calculated using areas of cells (raster map of 0.5x0.5 degree, area of each cell in km2) covering this sub-basin.
D2 – The fraction of agricultural area for a sub-basin (Agfr.j, 0-1) is calculated: (the sum of sub-basin cells with agricultural areas (each cell has agricultural area in percentage) divided by the number of cells in this sub-basin) / 100. We used “mean function” of Zonal Statistics in ArcGIS.
D3 – N and P inputs to watersheds (land) of each sub-basin from diffuse sources (WSdifE.y.j, kg year-1 for each sub-basin) were calculated using N and P diffuse inputs on a 0.5 by 0.5 degree cell (raster map, nutrient inputs in kg ha-1 in each cell) and area land of cells (raster map of 0.5x0.5 degree cell, km2) in three steps in ArcGIS. The first step is to convert values from kg N(P) ha-1 of each grid cell to kg N(P) km-2 of each grid cell using “Spatial Analyst Tools” in ArcGIS. The second step is to calculate kg N(P) per grid cell by multiplying values of the first step by area land (km2) of each cell (raster map of 0.5x0.5) degree using “Spatial Analyst Tools”. The third step is to calculate kg N(P) per sub-basin by summing outputs of the second step with the total area of sub-basin cells (km2) (sub-basin polygon map was created based on STN-30 and used in this step) using “Zonal Statistics” in ArcGIS. Diffuse sources of N and P inputs to watersheds (WSdifE.y.j) include animal manure and synthetic fertilizers for N and P; atmospheric N-deposition on agricultural (for crop and livestock production) and non-agricultural areas, and biological N2-fixation by agricultural crops and by natural vegetation for N. N and P export from watersheds (WSdifE.ex.j, represents animal grazing and crop harvesting) is calculated similar to diffuse sources (3 steps above).
D4 – Watershed export constant (eF) was taken directly from Global NEWS-2 (Mayorga et al. 2010). This is a basin scale constant and thus it is the same for all sub-basins and it does not vary among years.
D5 – Diffuse export coefficient (ECDIP.j) was taken directly from Global NEWS-2 (Mayorga et al. 2010). This is a basin scale coefficient and thus it is the same for all sub-basins and it does not vary among years.
D6 – Production of N and P inputs in watersheds (land) of each sub-basin from point sources (WSpntE.y.j, kg year-1 for each sub-basin) were back calculated for 2000 except for the Beijiang sub-basin: WSpntE.y.j = RSpntE.y.j / [(1- hwfrem.E.j) ∙ Ij], where RSpntE.y.j - nutrient inputs to the river system as element (E: N, P, kg year-1) for each sub-basin; hwfrem.E.j - nutrient removal (fraction, 0-1) during sewage treatment for each sub-basin (see D7); Ij - the fraction of population connected to sewage systems in each sub-basin (0-1) (see D8). RSpntE.y.j for each sub-basin was calculated using N and P inputs to the river system on a gridded scale of 0.5 by 0.5 degree cell (raster map, nutrient inputs in kg km-2 in each cell). Calculations were done similarly to diffuse sources (see steps 2 and 3 of D3). Point sources of N and P include (WSpntE.y.j) human waste for N and P and detergents for P.
For the Beijiang sub-basin for 2000 we calculated WSpntE.y.j using values for the Zhujiang basin from Global NEWS-2 (Mayorga et al. 2010). This basin covers the Yujiang, Liujiang, Xijiang, Beijiang and Zhujiang delta sub-basins in our study. This Zhujaing basin was delineated in Global NEWS-2 using the same simulation topological network (STN-30) as we use in our study (Vörösmarty et al. 2000). We calculated WSpntE.y.j for the Beijiang as follows: WSpntE.y.j for Zhujiang basin from Global NEWS-2 (kg year-1) minus the sum of WSpntE.y.j values for the Yujiang, Liujiang, Xijiang, and Zhujiang delta sub-basins from our study (kg year-1).
For each sub-basin (except for the Dongjiang) for 1970 WSpntE.y.j was calculated as: the fraction of a sub-basin multiplied by the WSpntE.y.j value for the Zhujiang basin from Global NEWS-2. The fraction of a sub-basin was calculated based on WSpntE.y.j values for 2000: WSpntE.y.j for this sub-basin in 2000 (kg year-1) divided by WSpntE.y.j value for the Zhujiang basin from Global NEWS-2 in 2000 (kg year-1).
For the Dongjiang sub-basin for 1970 and 2050 we followed the approach described above for 2000 (back calculations).
D7 - Nutrient removal during sewage treatment (hwfrem.E.j, fraction, 0-1) was taken directly from Global NEWS-2 (Mayorga et al. 2010) for the Zhujinag River for 1970, 2000 and 2050. The Zujiang River covers five sub-basins in our study: the Yujiang, Liujiang, Xijiang, Beijiang and Zhujiang delta. Nutrient removal is the same for these sub-basins. For the Dongjiang sub-basin, nutrient removal was taken from Global NEWS-2 (Mayorga et al. 2010) for 1970. For 2000 and 2050 nutrient removal is 0.80 for N and P in 2000, and 0.80 for N and 0.90 for P in 2050. Assumptions are based on expert knowledge regarding the current economic development in the Dongjiang sub-basin.
D8 – Total population (inh year-1) and population with a sewage connection (Ij, inh year-1) for each sub-basin were calculated using population density (inh km-2 year-1 in each grid cell) and population density with a sewage connection (inh km-2 year-1 in each grid cell). Calculations were performed in ArcGIS similarly to diffuse sources (see steps 2 and 3 of D3). In this study we use the fraction of population connected to sewage systems, which is population with a sewage connection (inh year-1 in each sub-basin) divided by the total population (inh year-1 in each sub-basin).
D9 – Runoff (Rnatj, m year-1) and water discharges (Qactj, Qnatj, km3 year-1) per sub-basin were calculated in several steps. First, water discharges (m3 s-1) were taken from a grid cell, which represents the outlet of the sub-basin. For the middle-stream sub-basins these water discharges account for water discharges from up-stream sub-basins. For down-stream sub-basins these water discharges account for water discharges from both up-stream and middle-stream sub-basins. Therefore we subtracted up-stream water discharge from middle-stream water discharge to calculate water discharge for middle-stream sub-basins. We did the same for down-stream sub-basins. Fig. 1 in the article shows the locations of the sub-basins their outlets. Then, we converted values from m3 s-1 to km3 year-1. Finally, we quantified runoff for each sub-basin: natural water discharge (Qnatj, km3 year-1) divided by the total area of the sub-basin (km2) (and multiplied by 1000 to get m year-1).
D10 – Inputs for volume (Vi.j, km3), depth (hi.j, meters) and water discharge (Qacti.j, km3 year-1) for individual reservoirs in the sub-basins were taken directly from the Global Reservoir and Dam database (GRanD) (Lehner et al. 2011a; Lehner et al. 2011b).
D11 – Nutrient (DF.j) retentions in all reservoirs of each sub-basin were averaged over the sub-basin using actual water discharge (Qactj, km3 year-1) at the outlet of the sub-basin (see D9, Box A.1 and Table A.1). Maximum value is 0.965 for DIN and 0.85 for DIP according to Mayorga et al. (2010). If calculated values exceed the maximum value, then maximum value is assigned.
D12 – Retention in the river of a sub-basin (LF.j, fraction 0-1) is calculated as a function of sub-basin area (Box A.1) for DIN. Here maximum value is 0.65 according to Global NEWS-2 (Mayorga et al. 2010). If this value exceeds the maximum value, then maximum value is assigned. For DIP we assumed the fraction of DIP retention in rivers (e.g. via water-sediment processes) 0.50 based on Strokal and de Vries (2012).
D13 - Runoff shape constants (aF, bF) were taken directly from Global NEWS-2 (Mayorga et al. 2010). These are assumed the same for all sub-basins and years.

Table A.3. Modeled versus observed dissolved inorganic nitrogen (DIN) and phosphorus (DIP) export by the Pearl River at the sub-basin scale (kg km-2 year-1).