Field Study of Ephemeral Stream Infiltration and Recharge

Two ephemeral streams north of Socorro, NM, near Interstate-25, the Rio Puerco and Rio Salado, have been instrumented for the purpose of analyzing ground-water recharge due to channel seepage. Monitor wells and neutron logging wells provided information necessary to characterize the nature of stream aquifer interaction. Ground-water recharge was computed using convolution and other techniques.

The Rio Puerco has a relatively well-defined, straight channel with a small width-depth ratio. The Rio Puerco flows in response to both summer and winter precipitation, as well as spring runoff. The stream carries a large suspended sediment load which results in fine-textured sediments on the channel bottom. Although the water table is only about lm below the channel, the stream and aquifer are not fully hydraulically connected at all times, owing to the development of a low-permeable clogging layer on the channel bottom.

The Rio Salado is an ephemeral stream which has a very large width depth ratio and a braided channel filled mostly with permeable sand and gravel. The Rio Salado flows mostly in the summer in response to thunderstorms. Two sites were instrumented on the Rio Salado located approximately 5 km apart. Prior to runoff the depth to ground water below the channel is about lm at the upper site and 9m at the lower site. Monitoring the water table elevations and moisture content indicates that during runoff the stream and aquifer are fully hydraulically connected at the upper site but not at the lower site.

The Rio Salado flows much less frequently than the Rio Puerco at the instrumented sites, and the mean annual flow is 0.4 and 1.28 m ³/s, respectively. However, the annual recharge on the Rio Salado is much greater than on the Rio Puerco, 1×10 ⁶m ³/km-yr compared to 7×10 ⁴m ³/km-yr. This significant difference is attributed mostly to the greater permeability of channel bottom sediments and the greater channel width of the Rio Salado.