Comparison of field and laboratory measured hydraulic properties of selected diagnostic soil horizons
Chimungu, Joseph Gregory
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An adequate characterization of soil hydraulic properties is a necessary solution for agriculturally and environmentally oriented problems such as irrigation, drainage, runoff and pollutants movement. The three approaches to determine hydraulic properties of soils are field measurements, laboratory measurements and mathematical models. In situ measurements, though representative, have the inherent limitation of being costly and time consuming. Laboratory and mathematical techniques are more convenient but require extensive comparison to field results as bench mark for evaluation. The objective of this study was to characterize the hydraulic properties of Bainsvlei and Tukulu form soils utilizing the above mentioned three approaches and to compare the results. The laboratory methods selected were hanging water column and pressure plate apparatus. Undisturbed soil samples were used to determine θ-h relationships at 0-100 kPa suctions and disturbed soil samples up to 1500 kPa. The water retention characteristics for both soils were generally well defined with little variability between replicates. The main variations were due to texture differences between the horizons. The θ-h relationships were used to estimate textural and structural domains using empirical pore class limits and derivative curves. The suction value separating the structural domain from the textural domain varies from horizon to horizon. The boundary between soil pore categories cannot be taken as a fixed value for all soils and all types of soil use. The measured water retention data corresponded well with the fitted curve via the van Genuchten (1980) model, indicating that the model can be successfully used to describe θ-h relationships for Bainsvlei and Tukulu soils. Soil water sensors were calibrated using undisturbed soil samples in climate controlled room for five horizons of a Bainsvlei form soil and three horizons of a Tukulu form soil. Soil water sensors and circuitry show extremely low sensitivity to temperature fluctuations. Horizon specific calibration is essential to get accurate water content estimates from the sensors if used in different soil horizons. Our study demonstrate that horizon specific calibrations of the water sensors improves the accuracy of soil water content monitoring compared with the manufacturer‟s generic calibration equation for the soils tested in this study. Hydraulic conductivity was obtained by measuring the hydraulic head and water content of the Bainsvlei soil form in situ with tensiometers and horizon specific calibrated ECH2O EC-20 probes, respectively. The profile was characterized with several relations of hydraulic conductivity and varied with depth. The reason for this was attributed to heterogeneous nature of the profiles due to variation in particle size distribution. The van Genuchten (1980) model laboratory method was used to predict K-θ relationships utilizing laboratory determined θ-h relationships. The K-θ relationships predicted from the θ-h relationships of the soil cores corresponded well with those determined by the instantaneous profile field method for water contents which they have in common. Thus it appears that this laboratory method is applicable to the soils studied, but the accuracy of the predicted values is quite sensitive to the matching factors. Thus, accurate measurement of these parameters is necessary for its successful use. The instantaneous profile field method is regarded as a reference method to measure in situ unsaturated hydraulic conductivity for both homogenous and layered soils (Hillel et al., 1972). There are, however, several site or profile characteristics that may limit this method (Bouma, 1983). Our studies show that it is not applicable on duplex soils with slow permeable C-horizons i.e. the Tukulu form profile at Paradys, because of negative hydraulic gradients within the profile due to impaired internal drainage. There is a need to adapt this method to duplex soils. Overall results indicate that from a practical perspective, the prediction of K-θ relationship from laboratory determined water retention data can be a viable alternative for determining the hydraulic properties of diagnostic horizons. The prediction of DUL using θ-h relationship has been found to be satisfactory.