8The spatial and temporal impacts of climate change on irrigation water requirements and yield 9 for sugarcane grown in Swaziland have been assessed, by combining the outputs from a 10 general circulation model (HadCM3), a sugarcane crop growth model and a GIS. The 11 CANEGRO model (embedded with the DSSAT program) was used to simulate the baseline 12 and future cane net annual irrigation water requirements (IR net ) and yield (t ha -1 ) using a 13 reference site and selected emissions scenario (SRES A2 and B2) for the 2050s (including 14 CO 2 -fertilisation effects). The simulated baseline yields were validated against field data from 15 1980-1997. An aridity index was defined and used to correlate agroclimate variability against 16 irrigation need to estimate the baseline and future irrigation water demand (volumetric). To 17 produce a unit weight of sucrose equivalent to current optimum levels of production, future 18 irrigation needs were predicted to increase by 20-22%. With CO 2 -fertilisation, the impacts of 19 climate change are offset by higher crop yields, such that IR net is predicted to increase by 9%. 20The study showed that with climate change, the current peak capacity of existing irrigation 21 schemes could fail to meet the predicted increases in irrigation demand in nearly 50% of years 22 assuming unconstrained water availability. 23

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Over an eight-year period, harvesting methods based on simple mechanical aids (blade and shear) were evaluated against hand harvesting on mature morphologically contrasting tea clones in Southern Tanzania. The effects of shear step height (5-32 mm) and the harvest interval (1.8-4.2 phyllochrons) were also examined. Except in the year following pruning, large annual yields (5.7-7.9 t dry tea ha −1 ) were obtained by hand harvesting at intervals of two phyllochrons. For clones K35 (large shoots) and T207 (small shoots), the mean harvested shoot weights were equivalent to three unfurled leaves and a terminal bud. The proportions of broken shoots (40-48 %) and coarse material (4-6 %) were both relatively high. Using a blade resulted in similar yields to hand harvesting from K35 but larger yields from T207 (+13 %). The yield increase from clone T207 was associated with the harvest of more shoots and heavier shoots, smaller increases in canopy height, and a higher proportion (7-9 %) of coarse material compared to hand harvesting. On bushes, which had been harvested by hand for two years following pruning, using flat shears (no step) supported on the tea canopy resulted, over a three year period, in yields 8-14 % less than those obtained by hand harvesting and, for clone K35, a reduction in the leaf area index to below 5. The development of a larger leaf area index is made possible by adding a step to the shear. However, since annual yields were reduced by 40-50 kg ha −1 per mm increase in step height, the step should be the minimum necessary to maintain long-term bush productivity. As mean shoot weights following shear harvesting were about 13 % below those obtained by hand harvesting, there is scope, when using shears, to extend the harvest interval from 2 to 2.5 phyllochrons.

To assist commercial producers with optimizing the use of irrigation water, the responses to drought of mature and young tea (Camellia sinensis) crops (22 and 5 years after ®eld planting respectively) were compared using data from two adjacent long-term irrigation experiments in southern Tanzania. Providing the maximum potential soil water de®cit was below about 400±500 mm for mature, and 200±250 mm for young plants (clone 6/8), annual yields of dry tea from rainfed or partially irrigated crops were similar to those from the corresponding wellwatered crops. At de®cits greater than this, annual yields declined rapidly in young tea (up to 22 kg ha 71 mm 71 ) but relatively slowly in mature tea (up to 6.5 kg ha 71 mm 71 ). This apparent insensitivity of the mature crop to drought was principally due to compensation during the rains for yield lost in the dry season. Differences in dry matter distribution and shoot:root ratios contributed to these contrasting responses. Thus, the total above-ground dry mass of well-irrigated, mature plants was about twice that for young plants. Similarly, the total mass of structural roots (>1 mm diameter) to 3 m depth was four times greater in the mature crop than in the young crop and, for ®ne roots (<1 mm diameter), eight times greater. The corresponding shoot:root ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit area of leaf in the canopy of a mature plant had six times (by weight) more ®ne roots available to extract and supply water than did a young plant. These results show that young tea should be irrigated in preference to mature tea, especially where the maximum soil water de®cit is likely to exceed 250 mm. introdutionThe irrigation of commercial tea (Camellia sinensis) estates in the Mu®ndi district of the southern highlands of Tanzania was initiated in the 1970s. It followed the demonstration of yield and other advantages to be gained by applying water during the annual six months of dry weather (Carr, 1974). Currently about

We did this research as part of our overall approach to improving the sustainability and resilience of freshwater fish stocks in Queensland.

There are currently concerns within some sugar industries that long-term monoculture has led to soil degradation and consequent yield decline. An investigation was conducted in Swaziland to assess the effects of fallowing and green manuring practices, over a seven-month period, on sugarcane yields and the physical properties of a poorly draining clay soil. In the subsequent first sugarcane crop after planting, yields were improved from 129 t ha −1 under continuous sugarcane to 141-144 t ha −1 after fallowing and green manuring, but there were no significant responses in the first and second ratoon crops. Also, in the first crop after planting, root length index increased from 3.5 km m −2 under continuous sugarcane to 5.2-6.8 km m −2 after fallowing, and improved rooting was still evident in the first ratoon crop where there had been soil drying during the fallow period. Soil bulk density, total porosity and water-holding capacity were not affected by the fallowing practices. However, air-filled porosity increased from 11 % under continuous sugarcane to 16 % after fallowing, and steady state ponded infiltration rates were increased from 0.61 mm h −1 to 1.34 mm h −1 , but these improvements were no longer evident after a year back under sugarcane. Levels of soil organic matter were reduced in all cases, probably as a result of the tillage operations involved. In the plant crop, root length was well correlated with air-filled porosity, indicating the importance of improving belowground air supply for crop production on poorly draining clay soils.

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