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, nonpoint sources, transport and retention in groundwater and surface water

General

The groundwater flowing into draining surface water is generally a mixture of water with varying residence times in the groundwater system. The nitrate concentration in groundwater depends on the historical year of water infiltration into the saturated zone and on the denitrification loss during its transport.

Fertilizer history

In many parts of the world the use of N fertilizers was much less 3 to 4 decades ago than it is at present (FAO, 2005). Groundwater that infiltrated in the 1960s or 1970s will therefore contribute less nitrate to the surface water than more recently infiltrated groundwater, with concentrations corresponding to current fertilization rates (Meinardi, 1994). Hence, for simulating the outflow concentration of nitrate from the groundwater system, we need to consider the distribution of residence times of water leaving the groundwater system and the historical fertilizer N inputs. We therefore applied the flowpath approach for a typical homogenous groundwater system described in detail in Van Drecht et al. (2003).

Groundwater

Two subsystems are distinguished:

Those with rapid transport of nitrate in surface runoff and flow through shallow groundwater to local water courses.
Those with slow transport through deep groundwater towards larger streams and rivers (base flow) (Figure 8.6).

GNPC figure 8.6

The rapid flow responds with a varying but, generally, short delay to variation in the precipitation surplus during the year, while the base flow shows only slight variation throughout the year. We assumed that shallow groundwater recharges the deep groundwater layer as well as discharging into local water courses. Hence, the concentration in the upper part of the deep groundwater will equal that of the outflow from the shallow groundwater.

Groundwater recharge

The partitioning of water entering each groundwater layer and surface water is estimated according to Klepper and Van Drecht (1998) from characteristics such as soil texture, thickness of aquifers, geology, slope and other factors. The nitrate concentration of the outflow for each groundwater layer is calculated by combining the effects of historical N inputs from fertilizer, manure and atmospheric N deposition, residence time and denitrification. Historical N inputs are obtained from the above-surface N balance calculations. The residence time is obtained from the effective porosity and aquifer thickness, which depend on the characteristics of the aquifer considered. Values for effective porosity and aquifer thickness were applied to the geological map of the world taken from Klepper and Van Drecht (1998).

Groundwater denitrification

The nitrate concentration of the outflow from shallow aquifers is reduced by using a half-life of two years. We assumed most of the dissolved organic carbon compounds required by denitrifiers as electron donors to be decomposed in the shallow groundwater layer; hence denitrification rates gradually decrease with depth, and are assumed to be zero in the deep groundwater layer (see references in Van Drecht et al., (2003).).

Surface water

The total N from point sources, direct atmospheric deposition and nitrate flows from shallow and deep groundwater form input to the surface water within each grid cell. Once N enters the aquatic system, in-stream N transformations will occur, which are primarily concerned with metabolic processes removing N from the stream water by transferring it to the biota, atmosphere or stream sediments. Various factors influence N loss and retention, including temperature, availability of organic matter, nitrate concentration in stream water overlying stream sediments and the stream-flow regime and residence time. Lumped catchment models generally apply export coefficients, which are useful for obtaining rough estimates of annual nutrient loads (Heathwaite, 1993). Because we lacked a globally applicable model and input data for simulating in-stream processes, a global river-export coefficient of 0.7 (implying retention and loss of 30% of the N discharged to streams and rivers) was adopted. This represents a mean of a wide variety of river basins in Europe and the U.S.A. used by Van Drecht et al. (2003) (Figure 8.8).

GNPC figure 8.8

Phosphate

While the P balances for the non-point sources are included in the current IMAGE 2.4 framework, and a model has been developed for river export of N and P in particulate matter (Beusen et al., 2005), work on P emissions from point sources is still ongoing. We also plan to work on modelling the river Si export in order to predict changing N:P:Si ratios and the risk of algal blooms in coastal seas.

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