Untangling irrigation effects on maize water and heat stress

Irrigation has important implications for sustaining global food production, 8 enabling crop water demand to be met even under dry conditions. Added water also 9 cools crop plants through transpiration; irrigation might thus play an important role in 10 a warmer climate by simultaneously moderating water and high temperature stresses. 11 Here we used satellite-derived evapotranspiration estimates, land surface temperature 12 (LST) measurements, and crop phenological stage information from Nebraska maize 13 to quantify how irrigation relieves both water and temperature stresses. Unlike air 14 temperature metrics, satellite-derived LST revealed a significant irrigation-induced 15 cooling effect, especially during the grain filling period (GFP) of crop growth. This 16 cooling appeared to extend the maize growing season, especially for GFP, likely due 17 to the stronger temperature sensitivity of phenological development during this stage. 18 Our analysis also revealed that irrigation not only reduced water and temperature 19 stress but also weakened the response of yield to these stresses. Specifically, 20 temperature stress was significantly weakened for reproductive processes in irrigated 21 maize. Attribution analysis further suggested that water and high temperature stress 22 alleviation were responsible for 65±10% and 35±5.3% of irrigation’s yield benefit, 23 respectively. Our study underlines the relative importance of high temperature stress 24 alleviation in yield improvement and the necessity of simulating crop surface 25 temperature to better quantify heat stress effects in crop yield models. Finally, 26 untangling irrigation’s effects on both heat and water stress mitigation has important 27 implications for designing agricultural adaptation strategies under climate change. 28

between mixing samples and retaining as many samples as possible. Our choices of 133 <10% as the threshold for rainfed maize and 60% to define irrigated maize 134 represented the best optimization in our sample, as we found that more stringent 135 threshold had a very small effect on LST differences between irrigated and rainfed 136 maize at county level but resulted in significant data omission (more details in 137 supplementary Figure 1-2).  2.3 Temperature exposure during maize growth 161 We used daily 1-km spatial resolution MODIS Aqua LST (MYD11A1) data to 162 characterize the crop surface temperature; since its overpassing times are at 1:30 and 163 13:30, it is closer to the times of daily minimum and maximum temperature than the 164 MODIS Terra LST (Wan et al., 2008)  Water stress during maize growth was characterized by the ratio of evapotranspiration 188 (ET) to potential evapotranspiration (PET), as in a previous study (Mu et al., 2013). 189 We used MODIS products (MYD16A2) for both ET and PET, based on its good

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We compared the results of our statistical analysis with four gridded crop models.   To quantify the relative contribution of water and high temperature stress alleviation 281 to yield benefit, we related the yield difference between irrigated and non-irrigated 282 maize (irrigation yield-rainfed yield, ∆ ) to a quadratic function of growing 283 season EDD and ET/PET differences between irrigated and rainfed maize:

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The yield improvement explained by heat and water stress alleviation was estimated

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As expected, irrigation improved maize yield and the yield benefit showed a distinct 300 spatial variation when we compared areas we identified as irrigated versus rainfed strongly collinear variables, with 5 being a more strict standard). As shown in Figure   374 7, we found that temperature sensitivity of yield was significantly weakened from -375 6.9%/℃ (p<0.01) to -1%/℃ (p<0.01) in rainfed vs. irrigated areas, and this yield 376 sensitivity change was mainly driven by a change in the sensitivity of the HI, which 377 was weakened from -4.2%/℃ (p<0.01) to 1%/℃ (p<0.01). In both rainfed and 378 irrigated maize, temperature sensitivity of GSL was quite close (approximately -379 2%/℃ (p<0.01) ), while BGR was only slightly influenced by temperature (Figure 7).

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We found that irrigation not only lowered water and high temperature stress, but also 382 made yield less sensitive to water and high temperature stress (Figure 8a-c water stress alleviation. We further calculated that 79±13% of that yield improvement 390 was due to water stress alleviation and 21±3.2% was due to heat stress alleviation.

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Because the distribution of ∆EDD was truncated for points with ∆EDD>0 (Figure 8e), 392 we explored an alternative model with quadratic functions of ∆LST and ∆ET/PET 393 (Eq. (9)). In this specification, 72±12% of yield improvement was explained by water 394 and high temperature stress alleviation, with 65±10% and 35±5.3% of yield 395 improvement due to water and high temperature stress alleviation, respectively. We 396 also estimated VIF in the model; this was found to be well below standard thresholds, 397 with a value of 2.2. Intuitively, our low VIF value was likely due to the use of 398 differences in LST and ET/PET between irrigated and rainfed maize, rather than 399 directly using LST and ET/PET as the explanatory variables. We also note that the 400 high temperature stress alleviation estimated here appears larger than the estimation in   methods do not account for the water stress and heat stress interaction effects, so these 494 "heat" and "water stress" channels should be interpreted carefully. We note that our All data related to this paper, along with code for interpretation of data are available 516 upon request from the corresponding author.

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Author contribution 518 All co-authors designed the overall study. Peng Zhu performed the analysis and 519 prepared the manuscript. All co-authors contributed to the interpretation of the results 520 and writing of the paper.

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Competing interests 523 The authors declare that they have no conflict of interest.