Crop yield response to water - Cra

The stalk is composed of an immature, rapidly growing section at the top with a low sucrosecontent (top 8 internodes) and a mature, slower growing section with high sucrose contentat the bottom. As the stalks become longer (up to 3 m) and older (8 months and older) themature section forms a large portion of the stalk making it suitable for harvest. The maturationprocess can be enhanced by withholding irrigation to induce a mild water deficit (Smith andInman-Bamber, 2005) or by applying chemical ripeners.Flowering is initiated when minimum temperature exceeds 18 o C and water status is favourableduring a 3-week window period of declining photoperiod at about 12.5 hours (Bull andGlasziou, 1976). When a stalk has initiated a flower, it will cease initiating new leaves andinternodes and the stalk will mature and, if left long enough, produce side shoots resulting ininferior cane quality. Cultivars differ hugely in their propensity to flower.The duration of the different development stages are shown in Table 1. With its tropicalorigin, sugarcane is intolerant to cold temperature. Studies of various growth and relatedprocesses of sugarcane have pegged the base temperature in a range of 8 to 18 o C, and theoptimum temperature in the range of 30 to 35 o C (Ebrahim et al., 1998; van Dillewijn, 1952;Inman-Bamber, 1994). At the time of this publication, the T base and T upper are set at 9 o C and32 o C in the preliminary calibration of AquaCrop for sugarcane, and may be adjusted slightlyas more data become available for calibration and testing.Water use and irrigation practicesAlthough sugarcane develops a canopy relatively slowly (especially for a C 4 species),evapotranspiration of a fully canopied crop is a little higher than that from short grass(Inman-Bamber and McGlinchey, 2003). Depending on climate, peak evapotranspiration ratesranges from 6 to 15 mm/day (Thompson, 1976), and annual ET is between 800 and 2 000 mmof water. At least 850 mm of water per year is required for sustainable rainfed production.For commercial production, full irrigation is practised when annual rainfall is less than800 mm and supplemental irrigation is applied when annual rainfall is less than 1 000 mm.Irrigation is applied using furrow, sprinkler (portable and centre pivot) and drip systemsand scheduled according to soil water status as determined by (1) direct measurements ofsoil water potential (threshold -40 to -80 kPa) or soil water content (threshold 50 percentof available capacity) or by (2) profit and loss method using weather data (Carr and Knox,2011). The latter involves estimates of reference evapotranspiration determined from UnitedStates Department of Agriculture (USDA) Class A-pan evaporation, evaporation mini-pansor weather data and crop canopy cover (crop factor). Profit and loss methods include simplecalculations or complex methods using simulation models.Adequate irrigation is important during crop establishment and during the stalk growthphase. Irrigations could be reduced during the tiller and maturation phases withoutsignificant yield losses (Carr and Knox, 2011). In fact, controlled water deficit during the dryout period near harvest increases sugar content per unit dry stalk biomass and can enhancesugar yield.Sugarcane 177