Water stress affects the rate of water uptake, biomass accumulation and structural growth of sugarcane differently and consequently, significantly alters the partitioning of assimilate to sucrose storage. The Canegro sugarcane model is unable to accurately simulate the subtle rate changes in the source and sink processes during the progression of water stress during dry spells, with subsequent poor prediction of sucrose yields. The aim of this study was to test different models of water uptake by comparing simulated and measured rates of water uptake (WU), carbon exchange rate (CER) and plant extension rate (PER). The relative (stressed vs unstressed) rate measured for each process reflects the stress impact on that process and could be compared to the soil water deficit factors for source (CER) and sink (PER) processes calculated in the Canegro model.

WU, CER and PER were measured over a 40-day period on five month old sugarcane receiving either adequate or no water, at Mount Edgecombe, South Africa. Two approaches to modelling WU were evaluated, namely a simple approach based primarily on whole profile soil water content and a complex approach based on layer specific soil water retention and rooting density.

Increased modelling complexity did not result in better predictions of WU for this experiment. The simulation accuracy of both approaches was acceptable. The commencement of simulated stress in all three processes was too late and its progression too abrupt. Furthermore the simulated distinction in stress development between the source and the sink processes was less than what measurements suggested. The calculation of the Canegro soil water deficit factors were adjusted to fit measurements more accurately. The consequence of this refinement on the simulation of sucrose accumulation is discussed.