The circular locality is used to express the date that the hydrologic characteristics occur in a year, the radial distance to express the correspondent characteristic value, and the density value in the partial ring area is calculated,the isodense is drawn, therefore the isogram of the hygrologic characteristics is got.The isogram reflects the dominating centre that the hydrologic characteristics occur in a year and the corresppondent dominating date and dominating value.The dominance is used to express the probability that they occur.The dominating date and dominating value are important features that the hydrologic characteristics allot in a year.The paper uses the representation of isogram to enunciate and analyze the hydrologic phenomenon in objective existence.