Validation And Improvement Of The RZWQM2 Model Under Different Scenarios Of Water Stress

Water is indispensable for the normal physiological activities and material transportation in plants,and is one of the key factors for agricultural development.The normal growth of crops depends on root system to absorb water from the soil to make up for the loss of transpiration.When crop transpiration exceeds root water uptake,water stresses occurred.Crop growth simulation models can play an important role in some aspects.Accurate simulation of crop growth and development under water stress can promote the application of crop growth model in arid and semi-arid areas.However,there were some limitations for the existing model to simulate crop growth under arid conditions.The RZWQM2(Root Zone Water Quality Model 2)model proposed two new water stress factors on the basis of the original water stress factor.In this study,these three indexes of water stress were used to simulate an experiment of soil water stress at different growth stages of winter wheat,which was conducted in 2012-2014 in Yangling,Shaanxi.The simulation effects of the three indexes were evaluated and the optimal index was selected.In addition,leaf is the place for photosynthesis and obviously influences dry matter accumulation and yield.Previous studies have pointed out that the underestimation of leaf area was probably one of the reasons for poor simulations of crop biomass and yield under water stress.In this study,based on the experimental data in glass columns and field under rainout shelters for winter wheat in four years(2012-2016)in Yangling,Shaanxi Province,we attempted to establish a dynamic simulation model for leaf area expansion of winter wheat under water stress.First,a temperature response function was established with four cardinal temperatures(base temperature,lower optimum temperature,higher optimum temperature,and maximum temperature).Then two soil water stress functions were established to describe the effects of soil moisture stress on the two processes of leaf extension and senescence,respectively.Relative effective water content was used to describe the intensity of water stress.The first order derivative of Logistic function was then modified by temperature response function and the water stress response function to describe the effects of soil water stress on the processes of leaf extension and senescence to simulate the daily leaf area expansion and aging rate of winter wheat under water stress,respectively.The parameters of the newly established model were estimated and validated with a trial-and-error method based on the data of soil column experiment in the 2014-2015 and 2015-2016 growing season,respectively.Finally,the model was further verified with the data of field experiment under rainout shelter in the 2012-2013 and 2013-2014 growth seasons.And the accuracy of leaf area index(LAI)simulation with the new model was compared with the CERES-Wheat model.Some main conclusions were drawn as follows.(1)After calibration,the RZWQM2 model could simulate well the growth and development of winter wheat under the condition of adequate water supply.But there were still some limitations for the RZWQM2 model to simulate winter wheat growth under water stresses.The simulation accuracy of the three water stress indexes on biomass and grain yield of winter wheat were lower than that of high water treatments,and the simulation precision of waterstress treatments in early stage were lower than that in late water-stress treatments.Therefore,the simulation of winter wheat growth process under early and severe water stress by the RZWQM2 model needs to be further improved.(2)The water stress index of WSI2 was better than WSI1 and WSI3,especially when water stress occurred during the wintering and greening stages.We suggest model users choose WSI2,if the RZWQM2 model is chosen to simulate the response of winter wheat to soil water stress.(3)When relative effective water content was greater than 0.7,the leaf area expansion of winter wheat was not affected;when relative effective water content was between 0.2 and 0.7,water stress inhibited the leaf area expansion of winter wheat;when relative effective water content was less than 0.2,value of water stress response function,which described the effects of soil water stress on process of leaf extension,was negative,indicating that the leaf area of winter wheat decreased or leaves withered and yellowed.The results of two years' plastic column experiments and two years field experiments showed that the new model could more accurately simulate the dynamic changes of leaf area growth and attenuation under water stress.(4)Based on the two-year experimental data,we compared the simulated results by the newly established leaf area model of winter wheat and those by the CERES-Wheat model.It was found that the RMSE(Root Mean Square Error)between simulated LAI by the new model and measured values was lower than the CERES-Wheat model.