ABSTRACTA mathematical model for estimating daily water levels from less than daily observations was used to analyze an existing water‐level observation network on Cape Cod, Massachusetts. The model assumes that the water‐level fluctuation curve can be approximated by a simple harmonic function of the form: wl(x)=M+A cos(2π/T×+ C) + Bx where wl(x) is the water level for day number x. M, A, C and B are constants, and T is the harmonic period.The analysis indicated that for those sites which are not affected by tidal fluctuations or man‐made influences (such as pumping), six water‐level measurements per year would allow daily water levels to be interpolated with a standard error of estimate of about 0.