prediction of soil water retention curve using soil particle-size distribution

the soil water retention curve (swrc) is a basic characteristic of the determination of soil hydraulic properties, including unsaturated hydraulic conductivity. measurement of this curve is essential to research, thus scientists have focused on indirect methods such as pedotransfer functions and empirical relationships to estimate swrc easily. since there is a close relationship between swrc and particle-size distribution (psd), fractal geometry was used in this study to define a relationship between the fractal dimensions of these two curves. this helps to obtain an swrc equation based on soil particle size distribution as a readily available parameter. to achieve the above objective, 40 soil samples with seven different textures were used. two logarithmic equations were presented for determination of the fractal dimensions of psd and swrc based on their clay percentages. their correlation coefficients equaled 0.96 and 0.93, respectively. finally, a cubic polynomial equation was obtained between the fractal dimension of swrc (ds) and the fractal dimension of psd (dp) with a goodness of fit of r2=0.94. the physical properties of another five soils collected from pars province were used to evaluate this relationship. first, the dp of the soils were calculated. next, using the ds-dp relationship, the ds and the swrc equation were estimated using the xu model. to increase the accuracy of the prediction of swrc, a calibration model was used to adjust the predicted water content values by using two measured points on the swrc. comparison of the adjusted water content values with measured ones was done by statistical analysis and calculating the relative standard error (rse) and akaike's information criterion (aic).  the results showed that the rse and aic values decreased 50% to 85% and 20% to 93%, respectively, using the calibration model.