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Modelling trash management and its impacts: model performance.

Methods The model The effects of sugarcane trash on the water balance and plant growth conditions (available soil water and soil temperature), as well as changes to the trash itself, are calculated simultaneously, with a timestep of one day. Trash deposited on the simulated soil surface at harvest is divided into a bottom ‘contact’ layer with a maximum thickness of 5 cm and a top layer accounting for the remainder. Decomposition of the trash occurs mainly in the bottom layer (due to shielding against drying by the top layer), the thickness of which is maintained by attributing trash from the top layer (until it disappears) to the bottom layer. The model accounts for reduced evaporation according to Scopel et al. (2004). Stormflow is calculated according to Schmidt et al. (1998) and Lumsden et al. (2003). Equations to determine rain/irrigation interception by the trash were derived from Thompson (1965). Soil and trash temperatures are estimated pragmatically from a weighted average of recorded air temperature, trash temperature of the previous day, and a long term average temperature. A virtual canopy is simulated according to the equations used in the Canesim model (Singels and Donaldson, 2000), in order to estimate crop water use, and along with evaporation, stormflow and drainage, to calculate a soil water balance. A more detailed description of the model is provided by Jones and van den Berg (2006). Modelling experiments Simulated results are compared with yield records from the BT1 (van Antwerpen et al, 2001) trashing vs burning trial at the South African Sugarcane Research Institute, Mount Edgecombe). The BT1 treatments, ‘Trash, tops raked, with fertiliser’ and ‘Burnt with fertiliser’, were modelled for the period 1978-2005. Calculated transpiration is correlated with measured yield to evaluate the performance of the model, assuming adequate nutrition and equivalent radiation and soil characteristics. Features of the water balance of each crop are compared to assess modelled effects of the different treatments.