TEMPORAL VARIABILITY OF SOIL HYDRAULIC PROPERTIES SUBSEQUENT TO TILLAGE (SORPTIVITY, CONDUCTIVITY, MODELING)

Computer simulation of water and solute movement provides a means of optimizing water management with less field experimentation. Reliable estimates of soil hydraulic properties, which are the input parameters in numerical simulation models, are difficult to obtain because of spatial and temporal variability. Spatial variability has received more attention than temporal variability in recent years.;Hydraulic conductivity near saturation was the property which showed the greatest decrease with wetting and drying following tillage. Sorptivity and soil water retention also decreased significantly for both the soils. The first and second wetting and drying cycles caused the most compaction.;The importance of temporal variability (from wetting and drying) relative to spatial variability was evaluated by comparing temporal changes in sorptivity measured on small plots with spatial changes measured in a large sugarcane field. Geostatistical analysis of the field sorptivity data indicated no structure in the variance with measured distances. The comparison of temporal and spatial variability showed that temporal variability may in some cases be of greater consequence than spatial variability.;The importance of temporal variability of hydraulic properties in modeling soil water movement was further illustrated with a numerical simulation model using K((THETA)) and h((THETA)) data for the Molokai and Waialua soils. The computed water content profiles for infiltration and redistribution showed considerable differences for the pre-irrigation and post-irrigation input functions. These results illustrate that modelling soil water movement for the entire cropping cycle using the parameters measured at only one stage may result in unrealistic predictions for other parts of the cycle.;Temporal variability of five soil physical properties for two soils, Molokai series (Typic Torrox) and Waialua series (Vertic Haplustolls), were measured under controlled field conditions. All external compaction components such as traffic, intercultivation and rainfall impact, that cause temporal variability, were eliminated. Thus, changes in hydraulic properties were imposed principally by internal forces, that is, the change in the pore water component of effective stress resulting from wetting and drying. A drip irrigation system provided controlled water application at desired intervals.