Water chemistry at a point in a hydrologic system varies continuously with time. In order to characterize this series it is necessary to monitor the water chemistry, either on a discontinuous basis, for example, discrete monthly sampling, or on a continuous basis. A general model of the resulting time-dependent behavior is the sum of two components. One component is described deterministically by a mathematical function of time which is the basis for the behavioral classification. The second component is unsystematic, fluctuating randomly. This 'noise' is an unresolved mixture of random variation in both chemistry of the water and sampling/analysis procedures. In practice, the unsystematic component is the residual from the systematic and describes some probability distribution. The unsystematic component is regarded as superimposed on the systematic component and to a varying degree obscures it. Combinations occur of trend, cyclical, and oscillatory behavior. Some examples of time-dependent behavior. Some examples of time-dependent behavior as exhibited by water points in Nevada and eastern California are discussed. (WRSIC abstract)