Canopy Temperatures in Irrigated and Non-Irrigated Maize.

Several recent papers have suggested that high air temperatures are associated with reduced maize yields. To better understand the conditions under which these yield reductions occur, we analyzed maize yields from 1981 to 2011 for 100 U.S. counties with extensive areas planted to maize in the mid-West and Great Plains.  We found that yields in extensively irrigated counties had minimal dependency on air temperature while yields in non-irrigated maize were moderately correlated with temperature in many locales. This finding suggests that past observations of yield reductions due to high temperatures were actually driven by water stress. However, canopy temperature can be greater than or less than air temperature; the relationship between canopy and air temperature changes throughout the day and also usually differs between irrigated and non-irrigated maize. Using the Penman-Monteith model in conjunction with flux tower observations we found that the elevation of canopy temperature above air temperature in irrigated maize is greatest in the late morning, when net radiation is increasing and vapor pressure deficits are generally relatively low.  After midday, due to decreasing net radiation and increasing vapor pressure deficits, the difference in canopy temperature and air temperature declines, becoming negative later in the afternoon. We also make use of previously published empirical functions relating midday air and canopy temperature differences to assess the statistical relationship between canopy temperature and annual yield.  We use these analyses to assess whether the lack of response of irrigated maize yields to high air temperature is due to canopy temperatures being lower than the threshold necessary to diminish yields, even when ambient air temperatures are above the threshold. Conversely, we also assess whether non-irrigated maize sometimes have canopy temperatures higher than that reflected by ambient air temperatures.