Abstract:
CTSPAC is a mathematical model for coupled transport of water, solutes, and thermal
energy in the soil-plant-atmosphere continuum. The mathematical structure consists
of coupling a model for transport through soil (soil submodel) to one for xylem and phloem
transport (plant submodel). The modeling approach is based on discrete space conceptualization
or compartmentalization of the plant into local regions of similar tissue structure
and function.
The plant is represented as a generic plant with three leaves, each simulating a cluster
of geometrically similar leaves. The soil is divided into five layers, but this number can be
increased for greater resolution. Properties of root compartments can be varied to simulate
root density. Plant compartments have "xylem" and "phloem" regions, each containing
the actual physiological and anatomical structures and functions of xylem and phloem,
which must be visualized as transport units surrounded by other tissues, conceptualized as
storage regions.
Water moves from the soil to the atmosphere through the compartments representing
roots, stems, and leaves. The boundary layer and free atmosphere effects are included in
the mathematical statement of the problem. Transport in the phloem is modeled as a
pressure driven flow according to the Munch hypothesis. Water moves between xylem and
phloem compartments in roots as well as in leaves. Movement is driven by water potential
gradients. Differences in the sugar concentrations in the phloem compartments induce
gradients in osmotic potential and turgor pressure.
Soil and plant submodels are both coupled to the atmosphere. The daily cycle of soil
temperature is determined by the energy balance at the air-soil surface. The daily cycle in
transpiration is coupled to atmospheric conditions, including air temperature, and relative
humidity. Output includes: transpiration rate, phloem transport rate, water potential in
xylem and phloem, soil water content, soil temperature, rate of uptake of nutrients from
the soil, mass of nutrients stored in each plant compartment, and changes in mass of solutes
in the soil over time.
The model can be used to evaluate drying rate of soil, the daily cycle of plant-water
potentials at all nodes, impact of atmospheric conditions on food translocation, nutrient
uptake as a function of soil-water potential, water supply from the water table and other
phenomena. The model is written in ANSI standard Fortran 77 and runs without difficulty
on most micrcomputers with 640K or more RAM and an 8087 math coprocessor. The
Microsoft DOS 3.1 or more recent operating system is required.
