Environmental risk analysis of farming systems in a semi-arid environment: effect of rotations and management practices on deep drainage

Abstract

We investigated the impact of crop rotations and management practices on the water balance of farming systems in a semi-arid region of south-eastern Australia, where drainage beyond the root zone and rising water tables contribute to salinisation of soils and water streams. The CropSyst model was locally tested and used with long-term weather data to analyse the risk of deep drainage associated with alternative farming practices; water balances were estimated using both cascade and finite difference approaches. Model estimates were compared with (a) yield, biomass, phenology, soil water content and water use measured in a 5-year experiment involving two rotations, i.e. FWP (fallow-wheat–pea) and MWP (mustard–wheat–pea) and (b) yield measured in grower-managed wheat crops encompassing three seasons and broad ranges of weather, rotations, management practices and soils. In the rotation experiment, the simulations of yield were close to measured yield for wheat (observed range from ≈0 to 2.4 t ha −1; R 2 = 0.78; RMSE = 0.42 t ha −1), mustard (range from ≈0 to 0.6 t ha −1; R 2 = 0.57; RMSE = 0.20 t ha −1) and field pea (range from ≈0 to 1.4 t ha −1; R 2 = 0.72; RMSE = 0.29 t ha −1). Simulated wheat yield also compared well with on-farm measurements (observed range from ≈0 to 3.5 t ha −1; R 2 = 0.72, RMSE = 0.21 t ha −1). The model provided reasonable estimates of crop biomass, phenological development, soil water content and water use. Water use tended to be overestimated in some cases using the finite difference approach, but soil water content was well simulated in the presence of crop residues. The use of a finite-difference approach highlights the need to account for upward movement of water in the soil and its consequences on deep drainage. Median annual drainage simulated at 1.5 m was +9.1 mm for the fallow-wheat rotation, −5.4 mm for fallow-wheat–pea, −8.4 mm for mustard–wheat–pea, and +0.6 mm for wheat–wheat. The 3-year rotations (FWP and MWP) had the lowest median drainage and lowest probability of large drainage events in wet years. In the fallow-wheat rotation, median annual drainage was 24.8 mm with stubble retention and zero tillage, compared to −2.8 mm for stubble burnt and conventional tillage. Whilst the median response is indicative of comparative system performance, large episodic drainage events can be expected to be more frequent under fallow-wheat and even larger events with stubble retention and zero tillage. Stubble retention and zero tillage might be justified to increase and stabilise crop yield and reduce soil erosion but they do need to be assessed against the requirement to reduce deep drainage that is implicated in rising river and soil salinisation.