Estimating water retention for major soils in the Hararghe region, Eastern Ethiopia

[摘要] English: Soil water retention IS a fundamental property controlling water storage andmovement in the solurn. To determine the water retention characteristic curve is timeconsuming and expensive. Several attempts have been made to establish relationshipsbetween easily measurable soil properties, like particle size distribution, organiccarbon content, and the water retention characteristic curve. Those relationships arereferred to as pedotransfer functions (PTFs). More conveniently, it is described byanalytical functions that are suitable in the solution of numerical flow equations aswell as in implementation of closed-form methods for predicting other hydraulicproperties, such as unsaturated hydraulic conductivity. The objectives of this studywere to describe the water retention characteristics of soils from the Hararghe Region,eastern Ethiopia, in relation to certain soil properties; to identify water retentionfunctions for describing the water retention characteristic curves of these soils and todevelop a procedure for estimating water content either at certain matric potentials orthe complete curve from readily available soil properties. Two approaches, pointestimation and parametric estimation techniques, were used for estimating the watercontent at certain matric potentials and at any matric potential, respectively.To establish relationships between water retention and relevant soil properties,regression analyses were carried out. From the regression analyses, point PTFs thatcan be used to estimate the water content at certain matric potentials were developed.This was done firstly by using the complete data set consisting of 216 retention curvesand secondly by dividing the complete data set into topsoil and subsoil samples. Dueto observed differences in water retention characteristics, the subsoil samples weredivided into two groups based on their silt (Si) to clay (C) ratio. The dividing linebetween these two groups was 0.75. The topsoil and the two subsoil groups weredivided into classes based on their silt plus clay content. This resulted in 7 classes fortopsoils and subsoils with Si:C ratios < 0.75 and 6 classes for the subsoils with Si:Cratios> 0.75.For all the point estimation PTFs, the silt plus clay content functions described thevariability in water content best. The relationship between water content and silt plus clay content was curvilinear. In order to quantify the prediction accuracy of theseequations, the mean of the mean absolute error (mMAE), the mean of the root meansquare error (mRMSE), the mean of the mean bias error (mMBE), d-index ofagreement and coefficient of determination (R2) were used. In some instances, theslopes and intercepts of the 1:1 lines, between measured and predicted values, wereused. The silt plus clay content functions for the complete data set explained 78 to 87% of the variability in water content at specific matric potentials. The mMBE rangedfrom -0.001 to -0.003 cm' cm, the mMAE 0.022 to 0.034 crrr' cm, the mRMSE0.027 to 0.042 crrr' ern. The d-values ranged from 0.838 to 0.867. The silt plus claycontent functions for the topsails explained 88 to 94 % of the variability in waterretention with the mMBE ranging from 0 to -0.001 crrr' cm, mMAE 0.018 to 0.031crrr' cm, mRMSE 0.024 to 0.036 cm3 ern and the d-values 0.765 to 0.886. The siltplus clay content functions for the subsoils with Si:C ratios < 0.75 were able toexplain 78 to 87 % of the variability in water retention with the mMBE ranging from-0.001 to -0.004 crrr' ern, mMAE 0.019 to 0.036 crrr' ern, mRMSE 0.023 to 0.045cnr' cm and d-values 0.793 to 0.884. The silt plus clay content function for thesubsoils with Si:C ratios> 0.75 explained 86 to 98 % of the variability in watercontent with mMBE ranging from -0.001 to 0.004 cm' ern, mMAE 0.013 to 0.031ern' cm, mRMSE 0.015 to 0.038 crrr' cm and d-values 0.737 to 0.99l.Of the three groups, the mean values of the classes were used to develop PTFs withhigher R2-values and lower errors compared with the PTFs developed from thecomplete data set in each respective group.From the six water retention functions tested, the Van Genuchten (1980) function,with the restriction m = 1 - lIn, gave the best description of the water retentioncurves, followed by the Smith (1992) and the ordinary power functions. Over all, theBrooks-Corey (1964) function gave the poorest description of the water retentioncurves studied. The parameters of the Smith (1992) and Hutson & Cass (1987)functions correlated better with relevant soil properties compared to the parameters ofthe Van Genuchten function. With the parametric approach the Smith (1992) functionestimated water content for topsails and subsoils with Si:C ratios> 0.75 with a higheraccuracy compared with the Van Genuchten and Hutson & Cass functions whereasthe Hutson & Cass function was better for the subsoils with Si:C ratios < 0.75.Testing the functions derived from the point estimation and parameterizationtechniques on an independent data set indicated that both approaches estimated watercontent with a reasonable degree of accuracy, although the point estimationtechniques gave slightly better results for the subsoils with Si:C ratios> 0.75.

[发布机构] University of the Free State