The water cycle constitutes a set of hydrological processes on various conditions of scales. Hydrological variables are greatly impacted by time and space parameters, being nonlinear processes related to scale responses. As such, some parameters established through scale conversion should be available for description of these processes, and the methods established to interlink / interconnecting the status of parameters with the different scales. Scale conversion is restricted across different scales (different hydrological processes). Based on understanding of the hydrological responses and scale conversion, current artery of hydrology is reductive methods, i.e. deducted method through small scales, over parameterization has led to a great uncertainty of prediction concerning watersheds because of lack of observation data. Therefore, scale conversion has become difficult issues focused on the research of hydrological process. Only hydrological models based on understanding of hydrological processes in relation to scales could provide a satisfactory prediction of hydrological responses. Predicting action of runoff in space and time is obtained by the dynamic process analysis of the runoff system across scales. Therefore this paper tries to change scale analysis into an analysis of score of the hydrological variables and develops the methods of scale conversion, finding from them the potential laws and orders existing at different scales and searching for their mechanism in order to achieve the universal ways for expressing regulation of hydrological processes, and developing the method of scale conversion.