| Summary: | A computer simulation model of the soil water regime can be a
useful research, planning and management tool, providing that the data
required by the model are available. Finite difference solutions of the
general flow equation can be applied to complex field situations if soil
profile characteristics are reflected by appropriate retentivity (B( Ψ))
and hydraulic conductivity (K(Ψ)) functions.
The validity of a flow simulation model depends upon the degree
to which simulated flow corresponds to the flow pattern in real soils.
Macroscopic flow in apedal soils is likely to obey Darcy's law but in
structured or swe~ling soils, macro-pores and shrinkage voids lead to
non-Darcian flow. Physical composition and structural stability properties
of a wide range of South African soils were used to assess swelling
behaviour and depth-related textural changes. The applicability of a one-dimensional
Darcian flow model to various soil types was evaluated.
Core retentivity data for South African soils were used to
derive regression equations for predicting B (Ψ) from textural criteria
and bulk density. A sigmoidal, non-hysteretic two-part retentivity function
having only two constants in addition to porosity was developed for use
in water flow simulation models. Values of the constants, shapes of the
retentivity curves and soil textural properties were related by fitting
the retentivity function to retentivity data generated using regression
equations~ Hydraulically inhomogeneous soils may be modelled by varying
the values of the retentivity constants through the profile to reflect
changing soil properties. Equations for calculating K(B) or K(Ψ) from retentivity data
were derived by applying each of three capillary models to both exponential
and two-part retentivity functions. Comparison of these equations showed
that the definition and value of semi-empirical constants in the capillary
models were as important as the choice of model in determining K(B).
K(Ψ) was calculated using retentivity constants corresponding to a range
of bulk density, clay and silt content combinations. Three retentivity
constant-soil property systems were evaluated. These were derived from
retentivity data for South African soils between 1) -10 and -1500 kPa,
2) 0 and -50 kPa and 3) from published retentivity data for British
soils. Only that derived from retentivity data accurate in the 0 to -50 kPa
range led to K(Ψ) relationships in which saturated K and the slope sK/sΨ
decreased as bulk density, clay or silt content increased. Absolute values
of K were unreliable and measured values are essential for matching
purposes.
A method for evaluating the constants in a K(Ψ) or K(B) function
from the rate of outflow or inflow of water after a step change in
potential at the base of a soil core was described. Simple exponential
g (Ψ) and K(Ψ) functions were assumed to apply to each pressure potential
range. Retentivity parameters were obtained by fitting the 8(Ψ) function
to the measured retentivity curve. A value for K[s] , the remaining unknown
parameter in the K(Ψ) function, was obtained by matching measured outflow
and inflow data to a family of simulated curves. These were computed using
measured retentivity parameters, core dimensions and ceramic plate
conductivity, and a range of K[s] values. An advantage of this method is that there are no limitations on core length, plate impedance or pressure
potential range which cannot be ascertained by prior simulation.
Regression equations relating texture to retentivity, and a
conductivity model were applied in a simulation study of the water regime
in a weighing lysimeter in which gains and losses of water were measured
accurately. Active root distribution was assumed proportional to root
mass distribution. Relative K(Ψ) curves for each node were computed
using one of the conductivity equations derived earlier. Daily water
potentials for a month were simulated using three conductivity matching
factors. By matching simulated Ψ values to tensiometer potentials measured
at five depths an appropriate matching factor was chosen. The effects
of an over- or underestimate of K(Ψ) were demonstrated.
This work simplifies the prediction and use of retentivity and
conductivity relationships in soil water flow simulation models. These
models can be used for assessing the water regime in both irrigated and
dry-land crop production. Other applications include catchment modelling,
effluent disposal and nutrient and solute transport in soil. === Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1983.
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