Study On Estimation Of Reference Crop Evapotranspiration And Dry And Wet Changes In Hilly Area Of Central Sichuan Basin

By using the daily ground climate data during1959-2010collected by Nanchong weather observation station, this paper calculate the reference crop evapotranspiration of52years by FAO Penman-Monteith formula in this area. On this basis, this paper modifies the Hargreaves methods by using linear regression method, and then verified the accuracy of the modified formula. The partial correlations analysis showed that the sunshine hours is the most crucial element. The result is satisfactory by introducing sunshine hours to the modified formula.According to the monthly reference crop evapotranspiration calculated by the FAO Penman-Monteith formula and the precipitation data of Nanchong weather station, this paper calculates the moisture index. It puts emphases on dry and wet changes of the April, May and summer drought period because this area is the traditional agricultural area affected by the spring and summer drought period. Using the Morlet wavelet translation method to process the moisture index signals of April, May and summer drought period can obtain the wavelet coefficients and the real part contour maps to analyze the periodic features of dry and wet changes in this area and predict the changes in the next few years. The results show that:1. The modified Hargreaves formula can accurately calculate the monthly reference crop evapotranspiration only needs temperature and sunshine hour data. The average error percentage is controlled in10%.2. The partial correlation analysis showed that sunshine hour is the most key climatic element to the reference crop evapotranspiration, and the partial correlation coefficient is0.833. The correlation of other climatic elements as order by:wind speed>mean temperature>actual water vapor pressure> maximum temperature> minimum temperature.3. The average annual moisture index of this area is1.19, belongs to the humid range. The average annual moisture index is increasing accompanied by obvious fluctuation in the52years. Only the winter has the similar changes with the annual moisture index. There are more complex changes in spring, summer and autumn which are different with the annual moisture index.4. From1959, the moisture index of April, summer drought period and full year exhibits upward tendency in this area, the tendency rate as order by0.03/10a,0.21/10a and0.029/10a. The moisture index of summer drought period has a maximal increase slope. It is in the trend of becoming dry only in May at the tendency rate of-0.04/10a.5. Prediction according to the periodic feature and trend line previous:In April, this area is in relatively humid period at present and in next8-9years, relatively drought period at present and in next1-3year in May, may enter humid period about in2014. In summer drought period, this area is in relatively humid period at present and in next6-9years, but it does not exclude the possibility of serious summer drought disaster in some years. According to the annual moisture index, this area is in relatively humid period since2007, and it will last for6-8years from2012.