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Owing to the improvement of farmland irrigation in the recent 30 years,the present soil moisture data cannot reflect the variation regularity of natural soil moisture.The variation(gain and loss) of soil moisture is calculated on a ten-day base by using the soil moisture balance equation and the Penman formula.Compared to the soil moisture data without irrigation from 2001 to 2003,the fitting equation and drought index are established and the drought series from 1970 to 2001 on a ten-day base is reconstructed.The results show that the effect of irrigation is well filtrated out in the calculated drought series,which is very close to the natural soil moisture.