Intensity-duration effects on drainage: Column experiments at near-zero pressure head

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.