The mechanisms of the time lag phenomenon induced in piezometric level observations using open standpipes are analyzed. A new simple method is proposed to estimate the lag time; and it is compared with the Hvorslev’s solution. It is found from parametric studies that the estimated lag time by the proposed method is about 1/5 to 1/10 of that by the Hvorslev’s method. When the length of the intake portion of the standpipe is long enough

the open standpipes can be used in a soil with the coefficient of permeability of 10-5 or even 10-6 cm/s

without inducing significant time lag phenomenon. The degrees of sensitivity of 28 open standpipes were examined by field rising-head tests; and the results from the proposed method are verified by the tested results.