Field Evaluation of Shallow Subsurface Drains to Vent Soil Air, Improve Infiltration, and Reduce Runoff
ABSTRACT The effect of soil air entrapment on surface runoff rate and quantity was evaluated in the field using shallow subsurface drainage and tillage treatments to modify the amount of air trapped in a fragipan soil. The volume of pore space filled with air was increased by tillage and subsurface drainage. The latter also vented entrapped soil air during infiltration events. When the entrapped soil air was vented through the drain, the average steady-state subsurface drainage rate increased 20.9 and 48.9% for grass vegetated and tilled fallow conditions, respectively. Corresponding decreases in steady state surface runoff rates were 10.4 and 14.9%, respectively. Venting air pressure, as low as 40 Pa (gauge) resulted in an increase in drain flow rate and a decrease in runoff rate. Generally, lower soil air pressures during infiltration occurred in the vegetated soil because cracking along soil aggregate faces induced by transpirational drying provided for some surface venting. As a result, soil air pressure declined more during an extended infiltration event in the grass plot than in the fallow plot. The immediate result of tillage followed by precipitation was greater infiltration and soil water retention, however as reconsolidation occurred, soil air entrapment increased and a greater portion of the applied water became surface runoff. After reconsolidation caused by 627.6 mm of precipitation, air entrapment pressure of 1.36 kPa resulted in a 36.0% decrease in tile flow rate and 20.5% increase in surface runoff rate. Likewise, with reconsolidation air entrapment pressure of 2.41 kPa resulted in an 8.8% decrease in tile flow and 32.9% increase in surface runoff volumes.