Abstract: The serial daily potential evapotranspiration was calculated from meteorological data collected from the Yucheng Comphrensive Experimental Station,Chinese Academy of Sciences.Based on the serial data,normalized ETP series (NETP)were generated with the mean of ETP removed and the residue dimen-sionalized by standard deviation.Furtherly,a discrete AR(1)model was constructed for NETP.On the basis of the discrete AR(1)model,the corresponding continuous parameter model,namely A(1),wasest up and the coefficients invol vedestimated. A(1)is a first order stochastic differential equation.The probability density function,autocorrelation function and spectral density function associated with the A(1) were obtained analytically.