A Functional Analysis of Pedotransfer Functions Developed for Sri Lankan soils: Applicability for Process-Based Crop Models

M. H. J. P. Gunarathna,Manjula Ranagalage,Kazuhito Sakai,M. K. N. Kumari

doi:10.3390/agronomy10020285

Abstract

As measurements are expensive and laborious, the estimation of soil hydraulic properties using pedotransfer functions (PTFs) has become popular worldwide. However, the estimation of soil hydraulic properties is not the final aim but an essential input value for other calculations and simulations, mostly in environmental and crop models. This modeling approach is a popular way to assess agricultural and environmental processes. However, it is rarely used in Sri Lanka because soil hydraulic data are rare. We evaluated the functionality of PTFs (developed to estimate field capacity (FC) and the permanent wilting point (PWP) of Sri Lankan soils) for process-based crop models. We used the Agricultural Production Systems sIMulator (APSIM) as the test model. Initially, we confirmed the importance of PWP (LL15) and FC (DUL) by assessing the sensitivity of the soil input parameters on the growth and yield of rice under rainfed conditions. We simulated the growth and yield of rice and the four selected outputs related to the APSIM soil module using the measured and estimated values of FC and PWP. These simulations were conducted for ten years in 16 locations of Sri Lanka, representing wet, intermediate, and dry zones. The simulated total aboveground dry matter and weight of the rough rice, using both input conditions (the measured and PTF-estimated soil hydraulic properties), showed good agreement, with no significant differences between each other. Outputs related to the soil module also showed good agreement, as no significant differences were found between the two input conditions (measured and PTF-estimated soil hydraulic properties). Although the DUL and LL15 are the most influential parameters for the selected outputs of APSIM–Oryza, the estimated FC and PWP values did not change the predictive ability of APSIM. In this way, the functionality of PTFs for APSIM crop modeling is confirmed.

Highlights

The direct measurement of some soil hydraulic properties is laborious and expensive
We selected rice to study the functionality of the developed Pedotransfer functions (PTFs) in process-based crop models, using Agricultural Production Systems sIMulator (APSIM) version 7.10
We considered the rice direct-seeding for this simulation, as it is the most common practice of field establishment for rice crops in Sri Lanka [52]

Summary

Introduction

The direct measurement of some soil hydraulic properties is laborious and expensive. information about soil hydraulic properties at an appropriate scale is limited in most parts of the Agronomy 2020, 10, 285; doi:10.3390/agronomy10020285 www.mdpi.com/journal/agronomyAgronomy 2020, 10, 285 world, especially in tropical regions [1]. Point PTFs were developed [5] for Sri Lankan soil and confirmed the statistical accuracy of these PTFs for Sri Lankan conditions. These PTFs cannot perfectly calculate soil hydraulic parameters. There are always some errors that could be over or underestimated according to soil type These errors may be different according to the application of the PTFs. the final estimation of the model quality should be defined by the model’s applicability [6] because the estimation of hydraulic properties is not the final aim. Wosten et al [7] proposed to use a functional criteria analysis, which evaluates the functional behavior of estimated parameters in different applications

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Conclusion