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To elucidate the effect of environmental factors on ET during the entire growing period of summer maize, the correlation coefficients between ET and AT or RH were calculated in homogeneous and layered soils (Table 3). ET was positively correlated with AT and negatively with RH. These results are consistent with previous studies (Li et al. 2013; Yang et al. 2013). The highest correlation coefficient between ET and AT was 0.70 (p < 0.01) in homogeneous and layered soils at PT–V3. At the beginning of the crop growth period, soil evaporation accounted for the larger part of ET, and it was the reason for the highest correlation coefficient between ET and AT occurring at PT–V3 in our study. The transpiration then increased, accompanying the growth of the leaf area (Liu et al. 2002; Kang et al. 2003), during which the minimum correlation coefficient occurred between ET and AT. Therefore, the lowest correlation coefficient observed at V12–VT was 0.53 (p < 0.01) and 0.55 (p < 0.01), respectively. RH had a negative effect on soil evaporation and transpiration. The highest correlation coefficient between ET and RH was −0.50 (p < 0.01) in homogeneous soil and −0.51 (p < 0.01) in layered soil at V12–VT, where ET maintained a relatively high value while RH decreased (Figures 3 and 4). The lowest correlation coefficient observed at V6–V12 was at −0.11 (p < 0.05) and −0.14 (p < 0.01), respectively. The correlation coefficient between ET and RH resulted from the integrated effects of AT, leaf area, and their interactions in this experiment.

Table 3

Spearman's correlation coefficients between ET at different growth stages of summer maize and AT or RH in homogeneous and layered soils

AT
RH
CorrelationGrowth stagesHomogeneous soilLayered soilHomogeneous soilLayered soil
 PT–V3 0.70** 0.70** −0.47** −0.47** 
 V3–V6 0.66** 0.64** −0.36** −0.34** 
ET V6–V12 0.60** 0.62** −0.11* −0.14** 
 V12–VT 0.53** 0.55** −0.50** −0.51** 
 VT–R3 0.60** 0.60** −0.46** −0.46** 
 R3–R6 0.56** 0.57** −0.39** −0.40** 
AT
RH
CorrelationGrowth stagesHomogeneous soilLayered soilHomogeneous soilLayered soil
 PT–V3 0.70** 0.70** −0.47** −0.47** 
 V3–V6 0.66** 0.64** −0.36** −0.34** 
ET V6–V12 0.60** 0.62** −0.11* −0.14** 
 V12–VT 0.53** 0.55** −0.50** −0.51** 
 VT–R3 0.60** 0.60** −0.46** −0.46** 
 R3–R6 0.56** 0.57** −0.39** −0.40** 

PT–V3, planting time to third leaf stage; V3–V6, third leaf to sixth leaf stage; V6–V12, sixth leaf to 12th leaf stage; V12–VT, 12th leaf to tasseling-silking stage; VT–R3, tasseling-silking stage to milk stage; R3–R6, milk stage to physiological maturity stage.

*Significant at p < 0.05.

**Significant at p < 0.01.

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