Effect of irrigation water regimes applied via subsurface drippers on soil water distribution and on sugarcane (Saccharum officinarum L.) growth.
The effects of different water regimes on sugarcane growth were investigated for subsurface drip irrigation over three seasons. Arbitrary factors (0.5, 0.75, 1.0 and 1.2 of ETc), were applied to adjust the daily estimates of Penman Monteith evapotranspiration (Et). These drip treatments were compared to an overhead sprinkler irrigated (1.0 ETc) block of sugarcane. In the plant and first ratoon crops, the drip plots were irrigated to field capacity, by replenishing an allowable deficit of 10 mm. In the second ratoon however, the soil was irrigated to a 50% total available moisture (TAM) level. Soil water movement and root distribution adjacent and below emitters was measured. There was an increase in vertical and lateral soil water movement with increase in irrigation water regime. The rooting density increased with increase in soil water content. However, the root density decreased slightly in the 1.2 ETc treatment below the emitters in the wetter season. There were no differences in cane growth among the drip treatments in the first two seasons, mainly due to the wet weather conditions. In the second ratoon the 1.2 ETc treatment had the tallest stalks, which resulted in the highest cane and sucrose yields (t ha-1). There were no differences in cane and sucrose yields among the other treatments. There were no consistent differences in the plant canopy cover among the drip treatments. The cane growth in the sprinkler block was poor in the plant and first ratoon crops, probably due to the leaching of N, and it was comparable to the drip block in the second ratoon. The crop water use efficiency (CWUE) decreased with an increase in irrigation water regimes, and all drip CWUE treatments were better than that of the sprinkler block in all years. The study has shown that, a) the estimated daily ETc could be reduced by 50 % while still achieving the Mhlume estate average cane yield of 95 t ha-1, b) rainfall use efficiency, cane and sucrose yields could be increased by using 1.2 ETc at the 50% TAM level, c) there is a need to re-examine N applications under drip irrigation, particularly where heavy rains occur after leaf samples have been taken.