EVALUATION OF IRRIGATION ADEQUACY IN SUGARCANE SYSTEMS IN ESWATINI

The adoption of objective irrigation scheduling approaches in the sugarcane industry has been very slow leading to over-irrigation, low water use efficiency and consequently, low returns. With the widespread use of subjective scheduling, there is also a perception that there are no significant benefits derived from objective scheduling. A study was, therefore, conducted in Eswatini to determine the performance of irrigation systems that were using subjective irrigation scheduling. More specifically, the objective of the study was to determine the irrigation adequacy for a sugarcane crop that was under two overhead systems viz. centre pivot and sprinkler irrigation. Adequacy was determined in terms of relative irrigation supply (RIS) for the 2015/16 season. The CROPWAT 8.0 model was used to estimate crop demand in terms of net irrigation requirement, whilst the amount of water applied during the period was the supply. Seasonal RIS results indicated that the area under the centre pivot was slightly over-irrigated (RIS= 1.13), whilst irrigation in the area under the sprinkler system was found to be adequate (RIS= 0.94). Monthly RIS values, however, revealed that the crop was over-irrigated most of the time. The crop under the centre pivot was over-irrigated for the months of January, June, July, September, November, and December as indicated by the RIS values of 1.17, 1.33, 1.20, 1.77, 3.35, and 15.09, respectively. Over-irrigation under the sprinkler system was observed in June, July, September, October, and November where the RIS was 1.36, 2.19, 2.91, 1.77, and 1.67, respectively. In both irrigation systems, over-irrigation was experienced at the germination and establishment phases of development. In contrast, all of the months with acceptable RIS values were found to be within the grand growth phase. It was, therefore, concluded that the scheduling approach used in the area was biased towards the grand growth phase. Hence, it was recommended that the irrigation schedule be modified to accommodate each of the different phases of sugarcane development.

Keywords
irrigation scheduling; adequacy; sugarcane; centre pivot; sprinkler; crop water requirement; CROPWAT