**3.1 Hydrologic and hydrometeorologic considerations**

Hydrology is basic to flood management and to answer basic questions which are fundamental to designing a flood management project. The questions needed for design are: (1) What will be the flood producing rainfall? (2) What will be the return period of this rainfall? (3) What will be the flood magnitude due to a given rainfall event? (4) What will be the probability or return period of a given flood magnitude? (5) What will be the risk of occurrence of a flood of given magnitude? The first three questions are answered by deterministic hydrometeorologic and hydrologic modeling, also called rainfall-runoff modeling or watershed modeling. There are many types of watershed models, such as empirical (regression type), conceptual (unit hydrograph theory), and physically-based (kinematic, diffusion wave, and dynamic wave theories). A comprehensive account of most of the popular models around the globe is given in Singh [1], Singh and Woolhiser [2], and in Singh and Frevert [3–5].

When managing floods, the questions are: (1) When will the flood occur at a given location? (2) How much area will be impacted by a given flood? (3) How long will a flood last? These questions are answered by stochastic hydrologic modeling, including univariate frequency analysis, multivariate stochastic analysis, and stochastic watershed modeling. Frequency analysis is done in different ways. The most popular method of frequency analysis in practice is the empirical method which involves fitting a frequency distribution to empirical flood data, use of an appropriate parameter estimation technique, goodness of fit, selection of a distribution, establishing confidence bands, and risk analysis. A goof account of the frequency distributions and their fitting and parameter estimation is given in Kite [6], Singh [7], and Rao and Hamed [8], Zhang and Singh [9]. Often multivariate frequency analysis may be needed for not only design but also for management. That is most appropriately done using copulas which along with their applications are comprehensively described in Zhang and Singh [9]. A treatise on risk and reliability analysis in environmental and water engineering is provided by Singh *et al.* [10].
