Technical Report No.109
Paul C. Ekern
The daily water budget of drip-irrigated sugarcane at Kunia, Oahu, was measured in large hydraulic load cell lysimeters. The 10-mo plant crop period was extrapolated to 109 cm (43 in.)/12-mo water use. Extrapolation from the full-canopied period (leaf area index 4) gave only 99 cm (39 in.)/ 12 mo of water use. The initial 12 mo of the second ratoon had 128 cm (50.5 in.) of evapotranspiration-a 2-yr cycle of water use would be 278 cm (109.5 in.) or 140 cm (55 in.)/12 mo. Drip irrigated cane used an average 79% of the 173 cm (70 in.) of annual class A surface pan evaporation or 70% of the evaporation from a pan elevated to 1.52 m (5 ft) or one kept at cane canopy height. This lower ratio between cane use and pan evaporation would make water use 15% less from drip irrigated as compared to sprinkler irrigated fully canopied cane. The average portion of sunlight energy equivalent to cane evapotranspiration was 45%, but ranged from 25% immediately after planting to as great as 60% during periods of strong positive advection with full cane canopy. Further economies in water use with drip irrigation resulted from the much greater uniformity of distribution of water with drip as opposed to sprinkler irrigation. During rainy periods, percolate was not curtailed and evapotranspiration was as little as 20% of the applied rainfall and irrigation. However, during the dry summer months, evapotranspiration was essentially equal to the combined rainfall and drip irrigation. Salt rings formed at the perimeter of the 0.61-m (2-ft) diameter surface zone wetted around each Submatic dripper. The maximum percolate rates through the 1.52-m Molokai Oxisol profile were 5 cm/day (2 in./day), even with water ponded on the surface. Essentially none of the 356.73 kg/ha (318 lb/acre) of nitrogen applied as fertilizer was removed in the deep percolate. Chloride content of the percolate which had increased from 150 mg/l to over 700 mg/l during the period of sprinkler irrigation underwent a parallel increase under drip irrigation after the initial flush from the heavy rainfall in January 1971. Silica levels of 65 mg/l in the irrigation water were reduced to 20 mg/l in the percolate.