Crop coefficients and water-use estimates for sugarcane based on long-term Bowen ratio energy balance measurements

Abstract

Sugar industries in Australia and Swaziland rely on irrigation to a large extent (60 and 100%, respectively) to produce a viable crop. Some large irrigation schemes are vulnerable to degradation of ground water quality and it is important to have reliable estimates of water use in order to better calculate runoff and drainage losses from sugarcane fields. It is also important to improve estimates of crop water use in order to improve irrigation design parameters and scheduling. Matching water supply and demand on a daily or weekly basis is essential for productivity and sustainability in any irrigation scheme. The United Nations Food and Agriculture Organisation has recently produced clear guidelines (‘FAO 56’) on determination of water requirements for a number of crops including sugarcane. Two process-level models, APSIM and CANEGRO are being used to support various decisions regarding the use of irrigation in sugarcane. The aims of this research were to confirm or otherwise refine FAO 56 crop coefficients for sugarcane, to refine simulation of water use in the APSIM-Sugarcane model and to assess the reference evapotranspiration (ET) procedure from the CANEGRO model now used in Swaziland for irrigation scheduling and system design. Bowen ratio energy balance (BREB) systems were installed in the Burdekin district, Australia and in Swaziland to determine daily ET from well-irrigated sugarcane crops. Automatic weather stations were installed within 1 km of the BREB systems in order to determine ET 0 according to FAO 56. Radiation interception and biomass accumulation were measured in the Australian experiment. The results from the two countries provided a sound basis for confirmation of the current FAO 56 crop coefficients for sugarcane during ‘initial’ (0.4) and ‘mid’ (1.25) growth phases. The FAO 56 crop coefficient for the ‘final’ stage (0.7) was not supported. Instead a value of 1.25 is suggested for working out the water balance. However, 0.7 may be desirable for irrigation scheduling in order to impose some stress on the crop and so enhance sucrose content. Transpiration use efficiency for APSIM-Sugarcane was increased from 8.0 to 8.7 g kPa kg −1 on the basis of the Australian results. The reference ET procedure was supported equally by the new BREB measurements in Swaziland and by the original lysimeter data but the new data indicated that systematic improvements to the reference procedure could be made.