Analysing drought stress effects on yield and water use efficiency of rice and the root zone salinity

Introduction
Proper water management is the most important operation in paddy fields which plays a fundamental role in the usefulness of other inputs in rice cultivation. Mid-season and end-season drainages are two major water management practices in paddy fields so that, the former increases crop yield and the latter provides more suitable conditions for rice harvest. Providing better condition for winter cropping in the fields, subsurface drainage can also facilitate such management practices during rice growing season. Field experiments should be done to quantify the effects of different water management strategies. However, with regard to the high cost- time consuming field experiments, simulation models are suitable alternative tools for analyzing the effects of different water management options. Among various simulation models, the AquaCrop model is a newly developed model for simulating the effects of irrigation management. This model is accurate, robust and requires fewer input data compared with the other models. AquaCrop has a water-driven growth engine for field crops with a growth module that relies on the conservative behavior of biomass (B) per unit transpiration (Tr) relationship. Another distinctive feature of the model is the expression of canopy development as canopy cover rather than leaf area index (LAI). Farahani et al. (2009) investigated full and deficit irrigation regimes on cotton using AquaCrop model in Syria and Spain. They showed that, for calibration, the key parameters such as normalized water productivity, canopy cover and total biomass, must be tested under different climate, soil, cultivars, irrigation methods and field management. In Italy, sunflower was studied using AquaCrop, CropSyst, and WOFOST models. There was less variation between observed and simulated grain yields by AquaCrop model than that of the other two models.
 Materials and methods
In this research, using AquaCrop model, the effects of drought stress were investigated in surface- subsurface drained paddy fields. Required data were obtained from subsurface drainage pilot of paddy fields of Sari Agricultural Sciences and Natural Resources University during 2011 and 2012 rice growing seasons. The ET_o was accounted with the use of ET_o calculator 3.1. The inputs values for daily weather parameters were maximum air temperature (Tmax), minimum air temperature (Tmin), maximum relative humidity (RHmax), minimum relative humidity (RHmin), sunshine hours (n/N) and wind speed at a height of 2 m (u2) based on weather station data and the ETo Calculator was based on FAO website. Drainage treatments were: three conventional subsurface drainage systems including drainage system with drain depth of 0.9 m and drain spacing of 30 m (D0.9L30), drain depth of 0.65 m and drain spacing of 30 m (D0.65L30), and drain depth of 0.65 m and drain spacing of 15 m (D0.65L15); a bi-level subsurface drainage system with drain spacing of 15 m and drain depths of 0.65 and 0.9 m as alternate depths (Bilevel), and surface drainage system (Control). Calibration and validation of the model was performed using field measurements. Using the calibrated model, the effect of full irrigation (I1) and drought stress during 16-22 (I2), 23-29 (I3), 30-36 (I4), 37-43 (I5) and 44-50 (I6) days after rice planting on yield and water use efficiency of rice and root zone salinity was investigated.
 Results and discussion
Drought stress in I2, I5 and I6 periods caused considerable decrease in rice yield and biomass compared to I1. The stress at the stages close to the heading, resulted in more reduction in rice yield so that the average rice grain yield loss due to drought stress in the I2 to I4 periods was 5% while, it was 15% due to stress in I5 and I6 periods. Drought stress in I2 period increased water use efficiency of surface and subsurface drainage treatments. In Bilevel subsurface drainage system (drain spacing of 15 m and and drain depths of 0.65 and 0.9 m), the applied water and water use efficiency were, respectively, 620.4 mm and 0.65 kg m^-3 under full irrigation and 529 mm and 0.71 kg m^-3 under drought stress during I3 period. Drought stress caused further increases in the root zone salinity in surface drainage system compared with subsurface drainage systems.
 Conclusion
Based on the results, AquaCrop model could be used to determine the most appropriate water management system for rice cropping in terms of yield, water use efficiency and root zone salinity.