Evaluation of the Regression Pedotransfer Functions Using Physical, Mechanical, and Chemical Soil Properties in the Estimation of the Least Limiting Water Range (LLWR)

Least limiting water range (LLWR) is one of the physical and qualitative soil characteristics with difficult, costly, and time-consuming measurement. The objective of this study was to estimate the LLWR using easy to measure soil properties by regression method. In this study, 192 soil samples were taken from 24 locations of West Azarbaijan provinceand soil water retention curve, soil penetration resistance curve, and confined compression curve were measured. To estimate the four moisture criteria of LLWR (θFc ¡θPWP¡ θAFP andθSR), the soil basic properties (clay content and silt to sand ratio) were used as inputs and the basic model was developed. Then,the confined compression curve parameters only and along with the physico-chemical properties at different levels were added to the previous inputs and 7 pedotransfer functions (PTF) were developed. By entering parameters of confined compression solely and along with different physico-chemical properties in the second and third steps, respectively, for estimating the moisture criteria of the LLWR and calculating LLWR, root mean square error and Akaike’s information criteriondecreased significantly. The R values calculated for PTFs showed that the use of PTF3, PTF5 and PTF7 instead of PTF1 improved the estimation of the moisture criteria considerably (R values were equal to 0.70, 0.73, and 0.77, for PTF3, PTF5, and PTF7, respectively, in contrast to the 0.39 for PTF1). So, the moisture criteria of LLWR can be estimated with acceptable accuracy (R=0.77 and AIC= -154.31) using confined compression parameters as estimators.