The effect of different furrow irrigation regimes on infiltration and sugarcane (Saccharum officinarum L.) yield at Ubombo Swaziland.
In surface irrigation, the soil serves as a medium for infiltration and for conveying water from the upstream to the downstream end of a field. Soil infiltration characteristics are therefore extremely important for surface irrigation design and management. In this study, the infiltration characteristics of the Sibaya (Si) soil type (Glenrosa, in the South African soil classification system) was determined by a volume balance method using a two-point approach technique. The infiltration model adopted was that of Kostiakov. The purpose of the study was to examine the effect of different irrigation scheduling on infiltration characteristics, and on irrigation performance. A trial was conducted on a field with predominately Rondspring and Sibaya soils from 1999 to 2001. The five irrigation treatments were the Ubombo method, Penman-Monteith (PM) derived irrigation scheduling factors of 1.25, 1.00 and 0.75, and alternate row irrigation using Ubombo scheduling and 1.00 x PM on plant and first ratoon cane, respectively. Treatments were arranged in a randomised complete block design with five replication. The Ubombo scheduling method had the highest number of irrigation events followed by the 1.25 x PM, whilst the 0.75 x PM had the least. The infiltration variables indicated that, for the Ubombo and 1.25 x PM treatments, irrigation often occurred when the soil water content was still less than 50% depleted plant available water (DPAW). This was in agreement with the tensiometer and neutron probe data. The tensiometer readings ranged from -55 to -75 kPa, -50 to -65 kPa, and -8 to -12 kPa at 0.15 m, 0.30 m, 0.45 m soil depth respectively. Likewise, 0.75 x PM was irrigated when the soil water content was greater than 50% (DPAW). Tensiometer readings would nearly always read above -80 kPa at both 0.15 m and 0.30 m, and above -75 kPa at 0.45 m. Further examination of the tensiometer and neutron probe data suggested that irrigation scheduling determined the preferential depth of water uptake by the crop. Frequent irrigation resulted in the crop depleting soil water predominately at the 0.15-0.30 m soil depth and hardly any at 0.45 m and below, particularly when the crop was young. There were no significant differences in yield among any of the treatments in the plant or ratoon crops. The plant crop consistently recorded higher yields than the first ratoon in all the treatments. Ubombo scheduling recorded the highest sugarcane yield in both seasons at 84 tha(-1) for the plant and 82 tha(-1) for the first ratoon cane. The 0.75 x PM had the lowest yield (78.3 tha(-1)) in the plant crop as well as in the first ratoon (74 tha(-1)). The volume balance approach provided a reliable and convenient way of assessing surface inigation systems to identify alternatives that may be effective in improving the system performance, and in assessing different irrigation schedules. Sound management which comes about by selecting the most efficient stream size, length of field, and set time, and also a suitable irrigation schedule for that soil type depends on detailed knowledge of the infiltration rate of a particular soil. Information on infiltration constitutes the basis for establishing the necessary design, evaluation criteria and operational management system in irrigation.