Effects of Compaction at Different Soil Depths on Water Balance As Simulated by Watermod

The effect of soil compaction on water balance was investigated using the simulator Watermod. Soil samples were compressed to obtain 10% and 20% reduction in bulk density and the hydraulic conductivity was measured on the non-compacted as well as on both levels of compaction. Bundaberg climatic data together with the hydraulic conductivity values were used to run the model with all other parameters kept constant. Outputs generated by Watermod revealed that ground evaporation and total evapotranspiration decrease with higher levels of compaction as a result of an alteration of the soil water storage capacity. Runoff also increases with compaction levels as a consequence of reduced soil porosity, pore numbers and hydraulic conductivity. It was found that soil drainage is inversely related to runoff, as compaction decreases soil drainage. Two main factors - the silty texture of the soil and the poor distribution of rainfall over the year - could explain this soil drainage reduction. The relatively high content of organic matter in the soil buffered the damaging effects of moderate levels of compaction. This study builds confidence in the accuracy of Watermod simulations as its results confirmed field experimental data. Furthermore, it highlights the need for improved soil management to avoid or limit soil compaction