Conceptual methods in this category simulate, with a mathematical model, channel flow and watershed runoff processes. Movement and storage of water through the watershed are simulated at varying time and space scales, with varying degrees of complexity, omitting, including, or combining elements, depending on the model used and the requirements of the study.

Conceptual methods that TxDOT designers may use include the (loosely classified as a conceptual method here) and the .

Like conceptual methods, empirical methods also use a mathematical relation that predicts the design flow, given properties of the watershed, channels, rainfall, or streamflow. However, the relationship does not represent explicitly the physical processes. Instead, the relationships are derived with statistical analyses. (Some analysts even refer to empirical methods as black box methods because the presentation of the process is not visible and obvious.)

Empirical methods that TxDOT designers may use include of streamflow observations and . With flood frequency analysis, the empirical relationship predicts the design flow from statistical properties of the historical streamflow in the watershed. With regression equations, the design flow is predicted with an equation that

has been

developed by correlating flows observed with watershed, channel, and rainfall properties.