Fallow soil evaporation and water storage as affected by stubble in sub-humid (Argentina) and semi-arid (Australia) environments

The effect of stubble mulch on soil water storage during fallow is highly variable. In environments with low rainfall and coarse-textured soil, contribution of stubble to gains in water storage is often smaller than in wetter environments with heavier soil. While this pattern is recognised, the effect of rainfall, evaporative demand and soil remains confounded.This paper investigates the effect of stubble on soil evaporation and soil water storage during fallow in contrasting environments at Balcarce (Argentina) and Walpeup (Australia). Field and modelling experiments were combined to quantify stubble effects on soil evaporation and water storage for loam (Balcarce) and sandy loam soils (Walpeup). Simulations with the CropSyst model and long-term weather records included reference evapotranspiration from 185 to 240 mm, and rainfall from 108 to 471 mm for the winter fallow period at Balcarce, and reference evapotranspiration from 595 to 686 mm, and rainfall from 46 to 305 mm for the summer fallow period at Walpeup. The effect of size of rainfall events, quantified with power laws, was emphasised.Amount of rainfall during fallow accounted for most of the variation in soil evaporation, irrespective of reference evapotranspiration and soil type. For bare soil, evaporation was lower than expected in years with higher frequency of large rainfall events, but no relationship between soil evaporation and size of rainfall event was found for soil with stubble. The response of soil evaporation and soil water storage to rainfall comprised three phases. In the first phase, all the rainfall was evaporated irrespective of soil cover; hence, there were no benefits of stubble in terms of water storage. In the second phase, stubble reduced soil evaporation with respect to bare soil, with a subsequent gain in soil water. In the third phase, the benefit of stubble declines up to the point in which rainfall is high enough to meet evaporative and storage demands irrespectively of soil cover. It is proposed that gains in stored soil water due to stubble could be estimated as a function of rainfall using a non-linear model with locally adjusted parameters.