Figure 3. Interaction of nitrogen species and idealized model solution

formations addressed include nitrification

Initial soil water

Water

Input

and nitrogen distribution

nitrogen

of ammonium to nitrate, ammonium ex-

change on soil, and denitrification. Organic

matter mineralization and immobilization

are not specifically addressed by WASTEN,

Nitrogen

Water

although control can be asserted through

submodel

the nitrogen transformation rate constants.

The relationships among these nitrogen

forms and interacting processes are shown

WASTEN includes a water transport

Infiltration fluxes

N-Transport

submodel that is linked with a nitrogen

submodel (Fig. 2). This linkage allows

WASTEN to be used for steady-state and

transient flow conditions, and it predicts

water content and water flux throughout

the soil profile and allows for upwards

Water content

flow due to evapotranspiration.

Concentation of

N species

The nitrogen submodel of WASTEN is

Plant uptake

built around the convectivedispersive

with time

and soil depth

transport equation, which couples the

equation for continuity of mass with Fick's

second law of diffusion. The resulting

Figure 2. Simplified nitrogen model showing the water and nitro-

equation accounts for transport by diffu-

gen submodels. (After Selim and Iskandar 1980.)

sion, hydrodynamic dispersion and mass

flow. For each nitrogen form treated by WASTEN,

one analogous equation is developed (Fig. 3). Ad-

ditionally, transformation processes are added onto

each equation as source or sink terms. In use,

WASTEN solves the resulting series of coupled equa-

tions by a modified, CrankNicolson, explicitim-

plicit finite-difference method. Integrated within the

model is a water transport model that provides

moisture content and flux at each node.

2C

C

=D 2 -V

+ sour4s - sinks - plant uptake

14ce244

3

t 123 1 3

2z

z

4

Dispersion + Mass Flow Microbial

1444 24444

4

3

Transport

Transformations

where D = dispersion coefficient

C = concentration

t = time

z = depth

V = average pore water velocity

Figure 3. Interaction of nitrogen species and idealized model solution.

3