| There is a serious shortage of water resources in the country,and agricultural irrigation accounts for a large proportion of the country’s water use.Because of the random variation of rainfall and evaporation between years,it is difficult to maintain the established irrigation water limit in dry years when the soil moisture content is used as an indicator to determine the irrigation time under the condition of limited water supply.In order to solve the problem,evapotranspiration method model with the cumulative evapotranspiration and cumulative effective rainfall between the adjacent irrigation as variables according to the water balance equation is proposed in this paper.The number of available irrigation times is introduced to correct the evapotranspiration.It is irrigated when the difference between the corrected cumulative evapotranspiration and the cumulative effective rainfall for the same period is greater than a certain value.And the optimal irrigation time is determined.The parameter rate timing adopts the theoretical analysis method.Through crop water model,irrigation systems were optimized for typical years with different frequencies(5%,25%,50%,75%,95%).Datas such as cumulative evapotranspiration between adjacent irrigation and cumulative rainfall were obtained.And the parameters were set at the solver rate.By reviewing the literature,five pilots were selected by latitude in the areas where spring maize was planted.The drought degree of the selected five pilots was evaluated by three evaluation indexes:rainfall,wetness and relative wetness index.And analyze the variation trend of evapotranspiration method model parameters.In this way,the influence of drought degree in different regions on the model parameters was explored.(1)This paper presents an evapotranspiration model(?).Based on the model,a method to determine the irrigation time is proposed(?).The cumulative evapotranspiration was corrected by the number of irrigation times available When the parameter was calibrated.This method could be used not only for limited water supply irrigation forecasting,but also for determining the irrigation time with full water supply irrigation forecast.Based on the optimization results of typical years irrigation schedule for spring maize in Wenyuhe,the evapotranspiration model parameters under three irrigation quotas(45mm,75mm and 105mm)were obtained through the planning solution.When the irrigation quota was 45mm,the parameters k=1.111,M=75.012,s=0.125;when the irrigation quota was 75mm,the parameters k=1.175,M=83.655,s=0.060;when the irrigation quota was 105mm,the parameters k=1.243,M=91.384,s=-0.150.The correlation coefficients R2 of the simulated value and the measured value of the cumulative modified evapotranspiration calculated by the model(the evapotranspiration obtained by the optimization of the irrigation system,which is still called the measured value for the convenience of expression)are 0.8883,0.8742 and 0.9020,respectively,which showed that the model had good applicability in different years.The parameter k(Correction of the cumulative evapotranspiration between two adjacent irrigations,the larger the k value,the greater the cumulative evapotranspiration between the adjacent two irrigations)and M(Implicitly represents the amount of water needed for irrigation)showed an increasing trend with the increase of irrigation quota,while the parameter s(Correction of cumulative evapotranspiration between adjacent two irrigations,positive values indicate an increase in cumulative evapotranspiration,negative values indicate a decrease in cumulative evapotranspiration)showed the opposite trend.(2)Based on the long series of meteorological data(1955-2018),the typical years corresponding to different experimental sites were selected according to the irrigation water requirement from low to high with the frequencies of 5%,25%,50%,75%and 95%.Using the irrigation schedule optimization results,the parameters were calibrated based on the evapotranspiration method model.The correlation coefficients between the cumulative simulated evapotranspiration and the measured evapotranspiration between adjacent irrigation in Xingtai,Wenyuhe,Harbin,Tongxin and Akesu were 0.9004,0.8742,0.9483,0.7317 and 0.4608,respectively.The goodness of fit of the models in Tongxin and Akesu was relatively low,because it is considered that the model did not take into account the interface flux under water during the calculation process.Therefore,for arid regions,the model needs to be further improved.(3)Because the index of humidity and the index of relative humidity rank the drought degree of different pilot sites in the same order,the easy-to-calculate Humidity M index is selected to rank the drought degree of five experimental sites in this paper.According to the degree of drought,the Order of light to heavy is Hebei Xingtai,Shanxi Wenyu River,Heilongjiang Harbin,Ningxia Tongxin,Xinjiang Aksu.(4)As the drought intensifies(Wenyuhe、Harbin、Tongxin、Akesu),the parameters k and M of evapotranspiration model became smaller and smaller as the pilot test became drier and drier(Shanxi,Heilongjiang,Ningxia,Xinjiang).Among them,The maximum value of parameter k was 1.175,the minimum value was 0.582,the maximum value of parameter M was 83.655,and the minimum value was 36.298.The irrigation times correction index s had no significant change with the degree of drought.(5)Taking full account of the uncertainty of meteorological factors between years,based on the material from 2009 to 2018 in Shanxi province,the increasing yield and benefit with evapotranspiration method,dynamic irrigation lower limit method and empirical method under different irrigation quotas and times were analyzed.The results showed that the evapotranspiration method had the best effect in saving water,increasing yield and income,followed by the dynamic irrigation lower limit method,and the empirical irrigation method was the worst. |
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