Effects Of Soil Water Deficit On Crop Water Consumption By Transpiration And Their Quantitative Delineations

With the aggravation of contradiction between the supply and demand of agricultural water resources,the intensification of greenhouse effect and the frequent occurrence of extreme climate such as high temperature and extreme drought,soil water deficit is common for agricultural production in China and will be more serious.Soil water deficit significantly impacts crop water consumption by transpiration,and therefore it is important to investigate the effects and their quantitative characterization for accurately evaluating the crop water stress extent,rationally designing irrigation scheduling and improving the efficiency of agricultural water resource utilization.In practice,the method evaluating plant water status based on soil water status has been popularly adopted,which often involves a nonlinear soil water stress reduction function containing some fitting parameters to be determined.The existing parameter determination approaches have intrinsic limitings.In addition,the hysteresis effect of water stress on root water uptake and/or transpiration has not been taken into account when evaluating plant water deficit.Based on two lysimetric experiments respectively conducted in greenhouse soil columns and field for winter wheat,therefore,the first objective of this study was to improve the optimization method for the fitting parameter in a nonlinear soil water stress reduction function.Our second objective was to quantify the hysteresis effect of water stress and incorporate it into the macroscopic root water uptake model for an improvemenmt.The final objective was to make a comparison of different estimation approaches for plant water deficit index(PWDI)and to investigate their irrigation effects.The following lists the major findings of this study:First,we improved the optimization method for the fitting parameter in a nonlinear soil water stress reduction function on the basis of three considerations closely related with interdependent relationships between soil and plant water status:(1)the effect of soil water content distribution and its relative position to roots;(2)differences of growth levels for plants exposed to different water stresses;and(3)the effect of the recovery processes of transpiration after re-watering.The optimized parameter was used to estimate transpiration and relative transpiration rate for winter wheat,and was employed to simulate soil water content distributions.The estimated actual and relative transpiration rates agreed well with the measurements.At the completion of each simulation,the modeled soil water content distributions matched the measured profiles well.Second,we proposed a water stress recovery coefficient to quantify the hysteresis effect of water stress,and then incorporated it into the traditional macroscopic root water uptake model.The new model was called as hysteresis model.Compared to the traditional model,the hysteresis model performed better in estimating relative transpiration rate and simulating soil water dynamics significantly,especially during the recovery periods upon irrigation.Third,three estimation approaches for PWDI were compared and analyzed.Compared to the arithmetic approach(depending on an arithmetic average of root-zone soil matric potentials),the estimation accuracy of the root-weighted approach(depending on a weighted average of root-zone soil matric potentials)was higher,while that of the integrative approach(considering the effects of root distribution as well as stress hysteresis)was highest.There is little difference between the root-weighted and integrative approaches when applied to trigger irrigation.Relative to the arithmetic approach,more precisely timed irrigation scheduling by the root-weighted approach,resulted in higher irrigation frequency and quantity,and thus higher aboveground biomass,leaf area,grain yield,and transpiration mostly without significant decrease in water use efficiency.