Estimation of hydrological properties of South African soils.

Abstract

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.