Study On Measuring Crop Evapotranspiration And Crop Coefficients Changes

Crop evapotranspiration and crop coefficients are important elements for estimatingirrigation water requirements and in water resources management. They are important foragricultural production. In this study, the bowen ratio instrument station, the large-scaleweighing lysimeter and micro-lysimeter are employed to measure the evapotranspiration andevaporation of winter wheat and summer maize in Yangling, Shaanxi. Based on theexperimental data of 1998-2003, the main results are as follows: (1)Net radiation and short-wave radiation have linear correlation; sensible heat andlatent heat have big changes daily; Heat flux of the earth's surface has same change trend withnet radiation, and there are linear correlation between them; Peak soil heat flux of 8cm depthis lagged an hour than the earth's surface'and net radiation; There exists downwards heatmovement in soil between 6:40 to 16:40, and exists upwards heat movement before 6:40 andafter 16:40 on sunshine; The proportion of soil heat flux in net radiation diminishes with leafarea index'augments; Evapotranspiration of the bowen ratio energy balance method has thesame change trend with lysimeter measurement, and there are distinct positive correlation inthem. (2)Observed evaporation descends when soil water content in 10%－20%, whileincreases when soil water content in 20%－33.26%. Observed evaporation increases with soiltemperature'heighten and net radiation'augment, and diminishes with air relative humidityand leaf area index'increases, and has positive correlation with evaporation of E601 watersurface. (3)The total evapotranspiration of summer maize is 330－410mm, the totalevapotranspiration of winter wheat is 400－460mm. Reference evapotranspiration that usingsunshine duration in a day are larger than using radiation data. The peak consumption ofevapotranspiration of winter wheat is after turning green, particularly heading to milk stage,the evapotranspiration of winter wheat increases with precipitation and irrigation'increase.The evapotranspiration of summer maize increases with precipitation and irrigation's increase,diminisheswhen precipitation and irrigation is excessive in growth stages. (4)Under the climate conditions of Yangling in Shaanxi, if variety is no changes, basalcrop coefficients of winter wheat: Kcbini=0.25, Kcbmid=1.20, Kcbend=0.20, water stress摘要 3conditions in the mid-season stage and late season stage may influence basal crop coefficients;Summer maize:Kcbini=0.25,Kcbmid=1.25,Kcbend=0.65. (5)Soil water content of 10-15cm layer has big changes, and the larger depth's changesmild. Curves of evapotranspiration and crop coefficients have same change trend with soilwater content's. There exists quadratic correlation between evapotranspiration and leaf areaindex, and linear correlation between crop coefficient and leaf area index. (6)Straw-mulching has effect on soil water content of 0－10cm layer, crop waterrequirement, evaporation and the soil evaporation coefficient. The soil evaporation coefficientis lower for high treatment straw-mulching during the initial growth stage. Straw-mulchedmaize uses less water than other non-mulched field.