Transient infiltration from ephemeral streams: A field experiment at the reach scale

An infiltration experiment at the stream reach scale was performed to estimate infiltration rates beneath an ephemeral, losing stream during streamflow events. At a time when the stream was dry, a 7 m stream section was dammed upstream and downstream using metal sheets. During a 5 day period water was pumped into the isolated section of the stream, and the surface water level was maintained at three successive increasing stages. The infiltration rate at each water level was thereby equal to the pumping rate required to maintain that water level. The advantages of the method are that it samples a much greater area than traditional methods and provides information on infiltration through stream banks as well as through the streambed. Experimental results provide insight into transient infiltration and recharge processes beneath ephemeral streams. Although the experiment continued for 5 days, infiltration during the first hour accounted for 19% of the total infiltration. High transient infiltration rates were also observed following each increase in stream stage. Experimental infiltration rates were used to calibrate a two-dimensional model developed within Hydrus, which was subsequently used to estimate infiltration associated with a natural flow event in the same stream reach. During the natural flow event, the total infiltration was 33% greater than would have been estimated assuming steady state infiltration rates. Dry antecedent moisture content controls the transient infiltration rate and hence increases the total infiltrated volume during flow events, but it does not increase the aquifer recharge.