Carbon Nitrogen and Water Cycle

In the carbon, nitrogen and water cycle component, IMAGE 2.4 includes models for describing the global carbon cycle (Terrestrial Carbon Model) and the global nitrogen and phosphate cycle.

Terrestrial Carbon Model	Global Nitrogen and Phosphate Cycle
Model documentation, input and output	Introduction
Model description	Point sources
Land-cover conversions	Nonpoint sources, denitrification and leaching
	Nonpoint sources, transport and retention in groundwater and surface water

Global Nitrogen and Phosphate Cycle, point sources

Human N emission

The basis of the calculation of emissions from point sources is the conceptual relationship from Van Drecht et al. (2003) between per capita human N emission from human excreta and other household and industrial wastes, and per capita income:

N_em = 8 + 11 (GDP/43,639)^0.5

where N_em is the per capita daily human N emission (g per person per day) and GDP the per capita gross domestic product (1995 US$ per capita per year). The GDP for each country is divided by 43,639, the world’s highest per capita GDP in 1995 (Switzerland). Low-income countries have human per capita N emissions of about 10 g per day; for industrialized countries these emissions amount to between 15 and 18 g per day. Data for a number of industrialized countries show good agreement with the above equation (Van Drecht et al., 2003). For phosphate a similar approach is available.

Sewage effluent

The amount of N that is actually discharged to surface water is estimated as follows:

N_sw = (1-R) D N_em

where N_sw is the net N emission to surface water (g per person per day), R, the removal of N in wastewater treatment expressed as a fraction of the N influent to treatment plants, and D the fraction of the population connected to sewerage systems. From the literature D is known for most OECD countries for at least one year in the 1970 to 1995 period. For years where no data are available, D is calculated from the trend in the product of the urban population fraction and the fraction of the urban population with access to improved sanitation.

Human N Emission
Scheme showing estimation of sewage N effluent to surface water.

Sanitation

This implies an exclusion of human N emissions from the rural population in these cases (mainly developing countries). Access to improved sanitation differs between rural and urban areas. For industrialized countries access is generally 100% for both rural and urban populations, while outside Europe, sewerage systems in rural areas are rare. Particularly in the rural areas of many developing countries, human waste is commonly collected in latrines or septic tanks; we assume that this waste does not enter the surface water.

Wastewater treatment

The removal of nitrogen (R) is calculated as the weighted average of no treatment, and mechanical, biological and advanced treatment. Data on the distribution over the different types of treatment are known for most European countries (EEA, 1998; Eurostat, 2000). Data for other countries rely on various sources. For countries where no data were found to estimate R, we use regional estimates on the basis of primarily WHO/UNICEF (2000). In most developing countries the overall N removal rate is low, because advanced and biological treatment systems are not widespread. Errors in the estimated effluent from sewerage systems in developing countries are therefore small.

Population density

To obtain spatially distributed estimates for the total N emission from point sources, we use 0.5 by 0.5 degree spatial data on population density for urban and rural areas (Van Drecht et al., 2003) and projections for population and per capita GDP.