Use of soil penetration resistance and group method of data handling to improve soil water retention estimates

The accuracy of pedotransfer functions can be improved using more flexible equations and additional input variables. Penetration resistance as a parameter related to soil structure can be a useful additional input to pedotransfer functions. Our objectives were to see whether using penetration resistance can improve the accuracy of estimating water retention from soil composition and bulk density. To develop pedotransfer functions, we applied group method of data handling (GMDH) resulting in hierarchical polynomial regression networks or abductive networks. The advantage of GMDH is that it automates finding essential input variables to be included in pedotransfer functions and, unlike the artificial neural networks (ANN), presents an explicit form of the equations. We developed pedotransfer functions from data on texture, bulk density, penetration resistance, and water content at 0, -5, -10, -20, -100 and -1500 kPa in 180 samples of soils in New Zealand. Abductive networks were used to estimate water content at particular matrix potentials. The water content at -1500 kPa and the penetration resistance were the essential variables to include in pedotransfer functions along with bulk density and texture. The pore volume fractal dimension could be reliably estimated from the water content at -1500 kPa and penetration resistance. The variation coefficient rather than average value of penetration resistance was found to be a good predictor in some cases.