In this paper the work by Sivapalan and Milly [1989] is extended beyond the
time of pending to estimate the errors of the time compression approximati
on (TCA) when an exact solution exists. The diffusion coefficient is set to
be constant, and gravitational effects are not considered here. The larges
t error of standard TCA in calculating cumulative infiltration is about 2.5
%, while its underestimate of the time to ponding is about 19%. A modified
TCA is also proposed, in which the exact time of pending is used directly i
nstead of the flux to improve the estimate of postponding infiltration. The
error for the modified TCA is about half of the error for the standard TCA
in predicting cumulative infiltration, and thus the modified TCA should be
preferred wherever pending time is or can be measured.