Abstract
The purpose of the study was to develop pedotransfer functions for determining saturated hydraulic conductivity (Ks). Pedotransfer functions (PTFs) for predicting soil physical properties used in determining saturated hydraulic conductivity, based on moisture retention characteristics, were developed. The van Genuchten moisture retention equation was fitted to measured moisture retention properties obtained from International Soil Reference and Information Centre (ISRIC) soils data base in order to determine parameters in the equation i.e. saturated soil moisture content (θs), residual soil moisture (θr), air entry parameter (α) and the pore size distribution parameter (n). 457 samples drawn from the data base were used to be the maximum possible sample size that contained the measured soils characteristics data required. Using statistical regression, mathematical relationships were developed between moisture retention parameters (response variables) and appropriately selected transformed basic soil properties (predictor variables). The developed PTFs were evaluated for accuracy and reliability. It was found that pedotransfer functions developed for θs produced the best performance in reliability compared to the remaining parameters yielding a correlation coefficient value of coefficient of determination (R2 = 0.76), RMSE = 2.09, NSE = 0.75 and RSR = 0.5 indicating good performance. Relatively poorest performance was obtained from the pedotransfer function developed for α which yielded a correlation coefficient, R2= 0.06, RMSE = 0.85 and a NSE of 0.02 reflecting the best possible equation derived for the parameter for use in predicting hydraulic conductivity. Out of the pedotransfer functions developed for each of the moisture retention parameters, the best performing PTF was identified for each parameter. The accuracy of the pedotransfer functions assessed based on R2 were for θs (R2 = 0.80), θr (R2 = 0.42), α (R2 = 0.04) and for n (R2 = 0.30), when the variables were expressed directly in terms of the selected transformations of the basic soil properties.
Highlights
Hydraulic conductivity is an important parameter that influences hydrological processes which affect flow in rivers
Pedotransfer functions (PTFs) for predicting soil physical properties used in determining saturated hydraulic conductivity, based on moisture retention characteristics, were developed
The van Genuchten moisture retention equation was fitted to measured moisture retention properties obtained from International Soil Reference and Information Centre (ISRIC) soils data base in order to determine parameters in the equation i.e. saturated soil moisture content, residual soil moisture, air entry parameter (α) and the pore size distribution parameter (n). 457 samples drawn from the data base were used to be the maximum possible sample size that contained the measured soils characteristics data required
Summary
Hydraulic conductivity is an important parameter that influences hydrological processes which affect flow in rivers. Ground water flow is determined by saturated hydraulic conductivity (Ks). A number of formulations have been developed over the years to predict soil hydraulic conductivity from readily measurable soil properties. The need for mathematical modeling of Ks arises from the fact that insitu or laboratory measurements of hydraulic conductivity are time consuming, labour intensive and expensive as noted by [1], making it practically unlikely in reality to collect permeability data. The use of pedotransfer functions is an alternative to determining hydraulic conductivity and involves relationships that enables Ks to be predicted using measurable soil physical and hydraulic properties. Ks serves as an input parameter in many hydrological models. There is no specific method considered most accurate in determining Ks as the method used depends on soil and environmental conditions
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