Comparison of Water Uptake Reduction Functions in Water Stress Conditions, by Using the Constant and Separate Coefficients in Maize Growth Stages

Background and objectives: For reduction of water deficiency effects in agricultural sector, it is needed to management of optimal water consumption. Irrigation scheduling should be based on the actual water requirement during the crop growth period. So that the crop growth stages sensitivity is considered in the water uptake process. The application of water uptake reduction functions (α(h)) in water stress conditions, is one of methods for irrigation scheduling. In past researches, the calibration and evaluation of water uptake reduction functions have been done by constant coefficients in the crop growth period. But the aim of this research, is to compare the water uptake reduction functions by application of constant and separate coefficients, in crop growth stages. Therefore, the accuracy of estimation of actual α(h) data, is investigated in crop growth period.
Materials and methods: This research was conducted on maize S. C 704, in Qazvin region. The mini-lysimeter with diameter of 40 cm and height of 70 cm was used for crop cultivation. The main factor was defined as soil water depletion at levels of 45%(I1), 55%(I2), 60%(I3), 65%(I4), 70%(I5), 75%(I6), 80%(I7) and 85%(I8), relative to total available water. Sub main factor was selected as the sensitivity to water stress in growth stages of initial (P1), development (P2), mid (P3) and late (P4). So that, the effect of soil water depletion (water stress) on water uptake amount, was investigated in maize growth stages (as separately). The experiment was performed as factorial and in a completely randomized design, with 32 treatments and three repetitions. Daily soil moisture was measured by moisture meter (HH2 model and made by ΔT company). For simulation the water uptake reduction (in water stress conditions), were applied the models of Van Genuchten (1987), Dirksen et al (1988) and Homaee et al (2002). The actual data of α(h) in I1, I3, I5 and I7 treatments, were used for models calibration. For this work, the models coefficients were estimated constantly (in whole growth period) and variably (in 4 growth stages). The models evaluation were performed for estimation of α(h) actual values in I2, I4, I6 and I8 treatments. The statistics indicators of CRM, EF, R2, RMSE and ME were used for models evaluation.
Results: The effect of soil water depletion and crop growth stages sensitivity on water uptake, was significant at the level of 1%. From I1 to I8 treatments, the water uptake amount in the whole of growth period, was decreased by slope of 7.4%. But the water uptake amounts in the initial, development, mid and late stages were decreased to 5%, 6.9%, 9% and 4.6%, respectively. As a result, the most sensitive growth stage of maize (relative to water stress) was included the mid, development, initial and late, respectively. The models calibration showed that in a specific model, the function coefficients amounts were different in growth stages (compared to whole of growth period). The best priorities for optimal model choosing, was assigned to application of separate coefficients in the crop growth stages. Among the different models, the Van Genuchten model with statistical indices of CRM=0.032, EF=0.928, R2=0.942, RMSE=0.057 and ME=0.1, was determined as the optimal model.
Conclusion: The results of this research showed that the modeling accuracy was increases, by application of variable coefficients in water uptake reduction functions. Due to different sensitivity in crop growth stages, is done a better estimation of actual water uptake amount, in water stress conditions. As a result, the actual crop water requirement is calculated by more accurately. Also, the water consumption amount is considered according to actual crop needs, which will cause to better management of water resources.