Evaluation of Pedo transfer functions for predicting soil water retention in Mubuku Irrigation Scheme, Uganda

Abstract

Water use efficiency in most irrigation schemes in Uganda remains low (<50%). This is partly due to insufficient integration of soil information into irrigation planning. However, obtaining soil information especially on soil hydraulic properties remains a challenge because direct estimation of these properties is time-consuming, expensive and not feasible over large areas. Therefore, indirect estimation using pedo-transfer functions (PTFs) that use easily measurable soil properties is preferred. However, the performance of PTFs may vary from soil type to soil type depending on the sensitivity to different soil parameters. Consequently, PTFs must first be evaluated for site-specific performance. The aim of the study was to (i) identify input parameters that have the greatest influence on the performance of selected PTFs for the main soil types and (ii) assess the performance of selected PTFs in predicting soil water retention across the major soil types in the Mubuku irrigation system. Soil hydraulic properties including field capacity (FC), permanent wilting point (PWP), saturated water content (ϴs), plant available water (PAW), saturated hydraulic conductivity (Ksat) and infiltration capacity (Ic) were determined and were found to vary significantly across the major soil types. Fluvisols had significantly higher Ic and Ksat compared to Anthrosols and Cambisols, indicating higher irrigation demands and higher rates of nutrient leaching compared to Anthrosols and Cambisols. Some of the sensitivity coefficients of the input parameters of the selected PTFs were within the range of -0.1 to + 0.1 when predicting soil water content at either FC or PWP whereas others were not. Aina & Periaswamy (1985) was not sensitive to sand and clay in predicting soil water content at FC and PWP respectively and Adhikary (2008) was also not sensitive to sand when predicting soil water content at FC. Adhikary (2008) was negatively sensitive to silt in predicting soil water content at FC. Minasny & Hartemink (2011) and Adhikary (2008) were positively sensitive to clay in predicting soil water content at PWP. Minasny & Hartemink (2011) were positively sensitive to soil organic carbon in predicting soil water content at PWP and negatively sensitive to sand when predicting soil water content at FC. Botula (2013) was negatively sensitive to sand when predicting soil water content at FC and PWP and was also negatively sensitive to clay in predicting soil water content at PWP. Among the evaluated PTFs, Aina & Periaswamy (1985), Minasny & Hartemink (2011) and Botula (2013) PTF models were good predictors of volumetric water content in Anthrosols while all the PTFs were the good predictors in Cambisols. In Fluvisols, all the PTFs performed fairly in predicting volumetric water content.