The analysis of some monthly hydrologic time series

The paper presents the analysis of the following time series: monthly average temperatures at Urbana, Illinois (1915–1965); monthly precipitations at Morrisonville, Illinois (1896–1969); and monthly streamflows in the Sangamon River at Monticello, Illinois (1915–1969).The identification of a model for these series had been discussed in a previous paper (Torelli and Chow, 1972). In the latter it was shown that a simple transformation makes the series stationary.The analysis of the series is completed in the present paper. By a joint use of spectral and regression analysis one arrives at the formulation of models in which the deterministic process and the variances are described by means of trigonometric functions. This allows a considerable economy of parameters in comparison with the models based on the 24 sample monthly averages and variances.The practical advantages of such an economy of parameters are discussed.