Two trends are evident in the recent developments in engineering hydrology, one toward integrating surface-water and ground-water concepts, and the other toward integrating stochastic and deterministic techniques. This paper provides a review of these trends with the specific aim of clarifying the role of ground water in the current methodology. Among the interrelationships discussed are (1) the linear-reservoir representation of the instantaneous unit hydrograph and the concepts of ground-water storage; (2) the existence of autocorrelated stream-flow sequences and their dependence on baseflow recessions; (3) the conceptual prediction of flood exceedance probabilities and the mechanisms of streamflow generation; and (4) the nature of time-space tradeoffs in assessing the worth of data for rainfall-runoff models.
The process of streamflow generation is controlled in large part by the near-surface saturated-unsaturated hydrogeological regimes that exist on the hillslopes within a watershed. Much remains to be done before the lumped calibration parameters that arise in time-series analysis, frequency analysis, and unit-hydrograph analysis of streamflows can be interpreted in terms of the spatially distributed, physically based parameters that control the runoff-generating processes on hillslopes.