Historical development of hydrology had three major milestones: descriptive, utilitarian, and scientific. New hydrologic disciplines are emerging at present. Hydrologic processes are investigated by five deterministic, stochastic, and combined deterministic-stochastic approaches. Hydrologic diversities stem from trends in water resources, complexities of processes and environments, and quantity and quality of basic data. Evolution from single-purpose, single-water source and single structure to multiple purpose, source, and structure approaches to the development of water resources highly affected research and application of utilitarian hydrology. Four classical statistical techniques of hydrology (sampling of samples, frequency distribution, correlation, and testing of hypotheses) were supplemented by spatialtemporal stochastic processes during the last three decades. Simple precipitation-runoff responses are being replaced by complex distributed rainfall-runoff models. Trends in water resources require new types of research and application of research results for the 21st century. Many basic questions of physical hydrology require answers, and a critical review of models which use a large number of parameters. Stress in research may well shift from shape to scale aspects of rainfall-runoff models. Historical extrapolation of application of unit hydrograph from floods to all flows, from small to larger catchments, and from humid to arid regions, need a careful re-evaluation. Taking into account the relationships of temporal input, state and output processes would increase the reliability of water-resources decisions. Study of tendency, intermittency, periodicity, and stochasticity of hydrologic temporal processes need physical supports for inferred properties. Spatial models should cover in detail the spatial distribution of parameters, changes in correlation in space of a point process with the other point processes, and the relationship of spatial correlation to the series time interval, distance and azymuth, with physical backing of statistically derived results.
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