| 英文摘要: | To the Editor —
In their statistical analysis of temperature and rainfall effects on maize yield, Lobell et al. concluded1 that excessive temperature above 30 °C during the June–August period contributed more significantly to lowering yields in the US corn belt than did the total rainfall during the same period. The authors used yield simulations from a process-based model (Agricultural Production Systems Simulator, APSIM) to verify their statistical conclusions. For reasons we outline below, we believe that these conclusions can be misleading because the major and consistent cause of rain-fed maize yield reductions in the humid and sub-humid US corn belt is the prolonged absence of significant rainfall and the resulting soil-water deficit.
First, we question the conclusion of Lobell et al. that rainfall during growing season (June–August) is less important in maize yield reduction than higher temperatures1. Their analysis of the observed data used in the study does not take into account either rainfall distribution or the rainfall not available to the crop due to surface runoff, drainage or soil evaporation. Furthermore, water stored in the soil profile at the beginning of June, should supply 150 to 180 mm of water available for transpiration — over a month's supply of water without any more rainfall. This initial soil-water supply added to the approximately 300 mm average rainfall occurring during the June–August period (Fig. 1b in ref. 1), even with a decrease of 20%, should have little influence on yield, as confirmed by their model analysis.
Secondly, use of constant transpiration efficiency (TE) in APSIM when normalized with vapour-pressure deficit (VPD) leads to biases in transpiration at high VPD. This is confirmed by the unrealistically high values of transpiration demand reported in Fig. 2c of ref. 1 (15 mm per day on apparently clear and hot days), two to three times higher than the potential evaporation calculated with commonly used and field-tested combination equations for humid and sub-humid climates like that of Iowa (Table 1).
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