Irrigation Forecasting In Winter Wheat Based On Monitoring Soil Moisture

Field experimental data of winter wheat in Xinxiang, Henan province was taken to establishmodels of forcasting irrigation. The variation of leaf area and crop coefficients of winter wheat wasanalyzed and relevant numical models of simulating changes of leaf area index （LAI） and cropcoefficaient were developed respectively. The sensitivities of reference evaptranspiration （ET₀） torelevant meteorological factors were analyzed and a suitable method of estimating ET₀was determined.By combining water balance equation and models of simulating crop coefficient and estimating ET₀, amodel of forcascting irrigation of winter wheat based on soil moisture monitored real-time wasdeveloped. The main results are as follows:（1） In early growing stage of winter wheat, leaf grown slowly and LAI also increased slowly.During winter, leaf growth stopped almostly and LAI kept nearly steady. After regreening stage, wheatplant began to grow rapidly and LAI also increased quickly. After reached peak at about heading stageand kept steady for a period, LAI decreased rapidly and finally reach to zero at mature. With normalizedeffective accumulative temperature and variation of LAI, growing season of winter wheat was dividedinto three periods: before over-winter, over-winter and after over-winter, and three models of estimatingLAI, based on accumulative temperature, were established for the three periods respectively. ByAnalyzing relationship between crop coefficients and LAI, and combining the model of simulating therelationship between LAI and accumulative temperature, a model of estimating crop coefficients basedon accumulative temperature was develooped, and practical testing of the model with field experimataldate of winter wheat in2010-2011shown that the accurate of the model may fit the practical demand offorcasting irrigation.（2） Simulating results shown that the sensitivities of ET₀to relevant meteorological factors werevery different and varied sometime with growing stage. The most sensitive factor was relative humidityand the least sensitive factor temperature during the whole year. The effects of wind speed on ET₀wererelatively more obvious than that of sunshime hour during low temperature, but the effects of windspeedon ET₀were stronger than that of sunsine hour while higher temperatue. ET₀values were positivelycorrelated to temperature, wind speed, sunshine hours, but negtively correlated to relative humidity.（3） By analyzing weather forecast information, calculating actual vapor pressure with minimumtemperature, determining solar radiation with maximum and minimum temperatures, and transferringwind speed grade to wind speed at height of2m, the ET₀value may be estimated with the P-M formula.Compared the estimeted values with the actual value, the statistical parameters were: RMSE=0.5860,RE=0.198, IA=0.963, which showed a well simulation result and the simulated meterologic factorsand ET₀may meet the practical requirement. With historical weather data, ET₀values were estimated atthree kinds of time step:10-day,5-day, and1day, respectively. Comparing simuated values to practicalvalues shown that the simulated values fitted very well to the practical values with time step of10day,the simulated results were obviously worse with time step of5-day, and the results were the worst withtime step of1-day and not suitable to estimate ET₀.（4） By combining the model of simulating crop coefficients and the method of estimating ET₀with water balance equation, the soil moisture at winter wheat field in near future were forecasted withforecasted weather data, and with historical meteorological data, respectively. The differences betweenpredicted values and measured values were significant before winter stage, and trended to be less andless after winter and regreening stage. The forecasted results was modified furtherly with practicallymeasured data and the modified values was used to next forecasting, with which it was shown that thepractical soil moisture can be simulated well.