the Symposium on Wheats for More Tropical Environments - cimmyt

210Climatic CODsideratloDsTable 1 is a summary of wheat climatesin <strong>the</strong> region within 23 degrees northand south of <strong>the</strong> equator and below1500 meters in altitude; Dhaka (24°Nlatitude) is included because of <strong>the</strong>importance of wheat in Bangladesh. Itis assumed that wheat is grown in <strong>the</strong>winter and that <strong>the</strong> temperature inJanuary (or July in <strong>the</strong> sou<strong>the</strong>rnhemisphere), almost always <strong>the</strong> coolestmonth. is <strong>the</strong> simplest and mostrelevant climatic feature. Thus.January mean temperatures above22.5°C are considered very hot. thosebetween 22.5 and 17.5°C hot. andthose between 17.5 and 12.5°C warm.giving three regimes with meantemperatures of about 25. 20 and 15°C.respectively. These are fur<strong>the</strong>r divided.according to January atmospherichumidity. into humid (With estimatedaverage leaf-to-air vapor pressure deficit(VPD) being below about 10 mb) anddry (VPD above 10 mb); dew is unlikelyto form in <strong>the</strong> latter case. A fourth highlatitudewarm regime (25 to 30°C), isincluded because several majortraditional low-latitude. wheat-grOWingregions are represented and arerelevant to this conference.The range between mean maximumand mean minimum temperatures isgiven in Table 1. for it may be directlyrelevant to crop growth and can beused to calculate mean minimumtemperatures, which is relevant tovernalization. and which some studiessuggest may be more closely correlatedto crop performance than meantemperature (25. C.E. Mann. personalcommunication). The temperaturerange tends to be least at humid coastallocations. Information on solar radiationis, unfortunately. not always availableor accurate; January radiationdecreases with distance from <strong>the</strong>equator. with cloudiness (In humidlocations) and with dust haze (e.g.•nor<strong>the</strong>rn Nigeria). Seasonal changes intemperature. radiation and vaporpressuredeficit for October to April(April to October in <strong>the</strong> sou<strong>the</strong>rnhemisphere) are shown forrepresentative locations in Figure 1.Winter rainfall totals in Table 1 are aninadequate deSCription of water supplyto <strong>the</strong> crop. In few places is <strong>the</strong>resufficient rainfall dUring <strong>the</strong> cropseason for viable cropping (e.g..possibly in IndoneSia. sou<strong>the</strong>rn China.Paraguay and sou<strong>the</strong>rn Brazil). Moregenerally. crops rely on soil waterstored from <strong>the</strong> summer wet season indeep retentive soils (e.g.. central India.Bangladesh. Thailand and Queensland.Australia). and/or on irrlgatlon.Irrigation is essential in places likeSudan and Nigeria and. for reasons ofsoil type. in central Brazil. Thus. watersupplyregimes dominated by irrlgatlon.stored soil water or. less commonly.rain on <strong>the</strong> crop need to bedistinguished.This brief discussion of climate has nottaken into consideration severalmeteorological factors of lesserimportance to tropical wheat. Theseinclude sun angles and mid-dayradiation intensity, potential or panevaporation rates. <strong>the</strong> sharpness of <strong>the</strong>post.January air-temperature rise and<strong>the</strong> incidence of frost and severe dryWinds. Frost occurs only in <strong>the</strong> warmregime; this can be quite significant insou<strong>the</strong>rn Brazil and Queensland. Severeatmospheric drought is apparentlysignificant at locations bordering <strong>the</strong>Sahara.