In this paper,the conditional CDF(PDF)and lower bounds related to different commonly used IDF models in water resources engineering are derived based on the relationship of return period and frequency.Maximum-Entropy criterion is tried to estimate IDF model parameters.By comparison of results of different goodness-of-fit criteria,it can be found that maximum entropy criterion used for estimating IDF model parameters is more accurate than likelihood,and that maximum entropy and likelihood criteria would yield identical parameter estimation.