An estimate of barometric efficiency, often necessary for aquifer pump test interpretation, is difficult to obtain when there is a trend in water levels over time. Ordinary least squares (OLS) regression methods can be used to determine both the barometric efficiency and a trend function when the form of the trend is known. Clark's (1967) procedure, which does not require specification of the trend functional form, is shown to provide an unbiased and consistent estimate of barometric efficiency. OLS is only advantageous for a short data record providing the form of the trend is known. Clark's method may still be used with short data records with a linear trend when augmented by a recursive correction procedure. Both methods work best with constant barometric efficiency and rapid aquifer-borehole water level equilibration. ¿ American Geophysical Union 1993