Sensitivity of drainage to rainfall, vegetation and soil characteristics

Abstract

Rainfall, vegetation characteristics and soil hydraulic properties influence deep drainage patterns in agricultural landscapes, but more information is required on the variability of their interactions with site conditions. Therefore, the objective of the study was to investigate the impact of the interactions of soil permeability, vegetation rooting depth and growth duration on drainage in 3 sites in northern New South Wales, Australia. Local sensitivity analysis was used on drainage estimated by two biophysical models—WaterMod 3, with a crop growth module, and HYDRUS-1D without a crop growth module. The effect of saturated hydraulic conductivity (Ks), growth duration (GD), rooting depth (RD), annual rainfall, and their interactions on deep drainage was evaluated at 3 sites. Simulations were conducted using 30 years of randomly selected climate data from 115 years historical data. Rainfall variability was similar in all 3 sites, so annual rainfall was the dominant factor dictating drainage in all 30 rainfall-years whereas GD was more important than RD after accounting for rainfall and drainage was least sensitive to Ks. The minor impact of RD was ascribed to the soil water content being at the lower extraction limit of crops due to potential evaporation being greater than rainfall in almost all months of the rainfall-year. The importance of GD varied between rainfall-years and sites, and was generally higher at high annual rainfall. We conclude that the level of precision at which model inputs are defined would vary with annual rainfall level. Therefore, GD could be defined on a rough scale in low rainfall zones, whereas more precise definitions are necessary at high rainfall. This would depend on classification of rainfall zones based on reliable rainfall data.