Investigation of the Intra-Seasonal Sensitivity of Maize Evapotranspiration to Water Stress, at Different Irrigation Levels

Adeqiate water use in the agricultural sector requires accurate knowledge of crop sensitivity to environmental stresses (such as water stress). The crop sensitivity to water stress may be different at different growth stages and may have a different effect on the actual amount of crop evapotranspiration compared to the standard conditions. At different levels of water stress, studying the sensitivity of crop evapotranspiration at different growth stages can be provided management strategies for optimal water consumption. In the present research, the intra-seasonal sensitivity coefficients of maize were modeled by using the Jensen model.

In this research, the effect of water stress levels and growth stage sensitivity on the amount of maize (S.C 704) evapotranspiration was investigated. The experiment was performed as factorial based on randomized complete block design. The treatments included four irrigation levels of 100 (I0), 80 (I1), 60 (I2), and 40 (I3) percent of the crop water requirement and four growth stages of initial, development, middle and final. In between two irrigations, the amount of daily soil moisture was measured in the center of each plot and the depth of the crop root zone. Therefore, the amount of evapotranspiration of crops per unit area was estimated according to the soil water balance. Analysis of variance and mean data comparison of evapotranspiration and dry biomass yield were performed by SPSS software and using Duncan's multiple tests. By actual evapotranspiration and yield data, intra-seasonal sensitivity coefficients of maize to water stress (λ1 to λ4) were determined by SPSS software.

Evapotranspiration The effect of irrigation water amount and growth stage on the maize evapotranspiration amount was significant at the probability level of 1%. Evapotranspiration amounts at the initial, developmental, middle, and final of maize growth stages were estimated equal to 79, 201.8, 123.8 and 14.6 mm (in I0 treatment), 78.3, 196, 126.6 and 14.6 mm (in I1 treatment), 72, 173.6, 99 and 11.7 mm (in I2 treatment), 62.8, 147.5, 81.5 and 8.4 mm (in I3 treatment), respectively. Reduction of evapotranspiration compared to control treatment (I0) in the initial, developmental, middle, and final growth stages were estimated equal to 0.9, 2.8, 9, and 0 (in I1 treatment), 8.8, 14, 20, and 19.8 (in I2 treatment), 20.5, 26.9, 34.2 and 42.4 (in I3 treatment) percent, respectively. The results showed that the slope of evapotranspiration reduction was not the same at different irrigation levels. Also, the relative evapotranspiration of maize (in all growth seasons) at irrigation levels of I1, I2, and I3 were estimated equal to 95.6, 85, and 71.6 percent, respectively. Therefore, when applying water stress, the optimal evapotranspiration rate can be adjusted by selecting the suitable growth stage. Yield The effect of irrigation levels on the dry biomass yield of maize was significant at the level of 1% probability. The dry yield of maize in treatments of I0, I1, I2, and I3 were equal to 17.1, 15.8, 12.6, and 8.7 (tons per hectare), respectively. The relative yield of maize at irrigation levels of I1, I2, and I3 were estimated to be 92.4, 73.7, and 50.9 percent, respectively, in the Qazvin region. The reduction of soil available water affected the water uptake by the crop and reduced the yield of maize. Modeling of intra-seasonal sensitivity coefficients of water stress At the initial, developmental, middle, and final growth stages of maize, stress sensitivity coefficients of λ1, λ2, λ3, and λ4 were estimated in water stress treatments. The mean of mentioned coefficients in stress treatments was calculated to be 0.421, 1.37, 0.274, and 0.133, respectively. The results showed that during the development stage of maize growth, the effect of water stress on yield reduction was more. The model efficiency for estimating the amount of relative yield was evaluated. Evaluation statistics of R2, EF, RMSE, ME and CRM were estimated to be 0.998, 0.986, 2.753, 0.026 and 0.021, respectively. The results showed that the Jensen model efficiency was good, and it can be used in planning the low irrigation for different growth stages of maize. Yield-Evapotranspiration Function of Maize in all of the growth stages Across different irrigation levels, a simple linear relationship of Y=69.935ET-12281 (with a correlation coefficient of 0.999) was fitted between two parameters of evapotranspiration and dry biomass yield of maize. Therefore, using the above equation in low irrigation management, the amount of maize yield can be estimated based on the evapotranspiration amount. In this research, 175 mm evapotranspiration was needed for the production of the initial unit of maize biomass. That is, the transpiration portion in the above amount was negligible, and it was mostly allocated to the soil evaporation portion.

The crop sensitivity to water stress and different needs to transpiration at different growth stages were the reasons for the different reduction of maize evapotranspiration. Reduction of soil available water reduced the water uptake and transpiration, and crop biomass. The results showed that reducing the water stress was effective in increase of maize evapotranspiration efficiency. In order to produce the maximum crop biomass, the sensitivity of the maize growth stage and the water stress level must be considered.