Crop yield response to water - Cra

with the lower densities being used in drier environments. Row spacingranges typically from 0.15 to 0.25 m, depending on the production system.Sowing is by broadcasting in some cases. In many developing countriesthe use of reduced tillage and stubble retention systems is increasingbut in developed countries multiple pre-sowing cultivations are morecommon. Stubble retention and the degree of cultivation influence therates of infiltration, evaporation, and runoff. The impact of soil type andmanagement, particularly cultivation and irrigation, on soil compactionhas a profound effect on the depth of root exploration, and hence accessto soil water, and the frequency of anaerobic conditions resulting fromwaterlogging.The length of the total growing period (life cycle) of spring wheat (sown inspring) ranges from 100 to 170 days while winter wheat needs about 180 to300 days to mature. In some exceptional cases, season lengths of more than300 days have been recorded. Day length and temperature requirements arekey factors in cultivar selection. Cultivars can be grouped as winter or springtypes according to chilling requirements, winter hardiness and day lengthsensitivity. Some winter wheat cultivars in early stages of developmentexhibit a strong resistance to cold temperature, surviving down to -20 ºC.The resistance is lost in the active growth period in spring, and during thehead development and flowering period frost may lead to loss of spikelets,and in extreme circumstances, loss of the whole head.In areas with severe winters, cold winds and little snow, spring wheatcultivars are planted after winter. Spring wheat is also sown in the autumnto over-winter in regions with winter dominant rainfall and mild wintertemperatures such as some arid and Mediterranean regions, as well as inthe cool season of high lands in the tropics. Spring wheat requires little orno chilling to initiate head development. Winter wheat and some springwheat cultivars are also photoperiod sensitive, which delays the end of thetillering phase to long sunshine days.Crop development or phenology is dependent on temperature (Porterand Gawith, 1999). For crop growth, minimum mean daily temperature formeasurable growth is about 5ºC for winter and spring wheat. Mean dailytemperature for optimum growth is between 15 and 23 ºC. In AquaCrop, thegrowing degree day (GDD) for wheat is calculated with a base temperatureof 0°C and an upper temperature of 26 °C. This means crop developmentspeeds up as the mean daily temperature increases from 0 °C to 26 °C, andfurther increase above 26 °C does not enhance growth and development.Maximum canopy is often reached before heading at booting stage whileflowering of individual heads can last between one and 10 days. Grainfilling usually occurs into the warming part of the year when averagemaximum temperatures are between 20-30 ºC. Prolonged periods below5 ºC can cause dormancy in winter wheat. Vernalization requirements andphotoperiod sensitivity vary substantially between cultivars and alter theduration of the tillering phase. Table 1 shows the duration from sowing toScientists contributingwith experimentalDATA and tests for thecalibration of AquaCropMarie-Therese Abi Saab(formerly CIHEAM-IAMB,Bari, Italy; currently LARI,Fanar, Lebanon),Rossella Albrizio(formerly CIHEAM-IAMB,Bari, Italy; currently CNR-ISAFOM,Ercolano, Italy),John Angus(CSIRO, Canberra, Australia),Senthold Asseng(formerly CSIRO, Wembley,Australia; currently Universityof Florida, Gainesville, USA),Simona Bassu, Francesco Giunta(University of Sassari, Sassari, Italy),Adriana Bruggeman, Theib Oweis(ICARDA, Aleppo, Syria),Angelo Caliandro, Pietro Rubino(University of Bari, Bari, Italy),David B. Cobell(LLNL, Livermore, California, USA),Mariano Cossani(University of Lleida, Lleida, Spain),Steven R. Evett, Terry A. Howell(USDA-ARS, Conservation& Production ResearchLaboratory, Bushland, Texas, USA),Hamid J. Farahani(formerly ICARDA, Aleppo, Syria;currently Clemson University,Clemson, South Carolina, USA),Bram Govaerts,Ivan Ortiz-Monasterio(CMMYT, El Batán, Mexico),Gerrit Hoogenboom(University of Georgia,Athens, USA),Gabriella Izzi(formerly FAO, Land and WaterDivision, Rome, Italy; currently WB,Washington, USA),Riza Kanber and Mustapha Ünlu(University of Çukurova, Adana,Turkey),Victor O. Sadras(SARDI Waite Campus, Australia),Gustavo A. Slafer(University of Lleida, ICREA,Lleida, Spain),Mladen Todorovic(CIHEAM-IAMB, Bari, Italy),Yuming Zhang(Institute of Genetics andDevelopment Biology,Chinese Academy of Science,Shijiazhuang, China)wheat 93