Abstract: Studying the frequency distribution of short-duration rainstorm could provide basis for design flood calculation in small watershed, which is of great significance to the prevention and control of mountain flood disaster and flood control and drainage of towns. On the basis of measured rainstorm data of 18 benchmark and basic meteorological stations in Shaanxi Province from 1984 to 2013, a total of 90 sequences of rainstorm of short duration (10 min, 30 min, 60 min, 90 min, and 120 min)were selected for the study using annual maximum value method. The parameters of Pearson III distribution (P-III), generalized extreme value distribution (GEV), Gumbel distribution (Gumbel), normal distribution (N), exponential distribution (EXP), and two-parameter gamma distribution (Gamma) of each sequence were estimated by linear moment method.The accuracy of fittings were analyzed according to certainty coefficient, sum of squared errors, and probability plot correlation coefficient. Results evinced that:(1) The mean value and linear dispersion coefficient of the rainstorm sequence of each site both displayed increasing trend with the elongation of rainstorm, whereas the linear skewness coefficient showed no uniform trend. (2) The mean value of rainstorm sequences of the same duration increased from the west to the east.The linear dispersion coefficient and linear skewness coefficient in the north of Qinling Mountain decreased from the west to the east, but in the south of Qinling Mountain, the two coefficients were larger in the middle and smaller in the east and west. (3) The optimal distribution of 81 rainstorm sequences, covering 90% of the total sequence groups, is P-Ⅲdistribution (including Gamma distribution), which can be used as the theoretical frequency distribution of short-duration rainstorm in the study area.

Key words: frequency distribution of short duration rainstorm, annual maximum value method, l-moment method, design flood for small watershed, mountain flood prevention and control, urban flood control and drainage