We conducted column experiments to examine the effects of rapidly changing upper

boundary flux conditions upon an unsaturated, near-zero pressure head flow region.

An entire 0.75 m long medium sand column was driven to a quasi-steady pressure head

and uniform water content with low-intensity irrigation. We then spike increased the

irrigation rate by 2–9 times the background intensity but 3–4 orders of magnitude below

the saturated hydraulic conductivity and for various durations. Thereafter, the irrigation

rate was reduced to the prespike value for 1 hour and then stopped. Results show that

during the postspike irrigation the pressure head, water content, and column storage

decreased to values that were lower than those observed during the prespike interval,

despite having the same steady irrigation rate. This observation indicates that a rapid

decrease in surface flux upon a near-zero pressure head region could lead to an enhanced

drainage of an unsaturated profile. The conceptual model explaining these observations

relies on the ‘‘release’’ of stored water from the bulk soil water flow.