Subsurface storm flow as the upslope extensions of variable source areas

LIST Water Lectures Seminar

As part of the LIST WATER LECTURES Seminar Series organised  for the Winter/Spring Session 2017, the next event will take place on 2 May 2017 at LIST premises in Belvaux (LU) in room F0.11-A.

Prof Rhett Jackson from University of Georgia (USA) will be the invited speaker and will make an oral presentation entitled "Subsurface stormflow: functioning to extend variable source areas upslope"

Interflow, or subsurface stormflow, can occur in any hillslope where higher conductivity topsoils are underlain by an impeding layer. Such layers include B horizons, glacial till layers, hardpans, and bedrock with conductivities lower than the topsoil. As long as the impeding layer conductivity is non-zero, some water will percolate through the impeding layer. Impeding layers of essentially impermeable bedrock seem to be a rarity, as interflow studies on slopes underlain by apparently solid crystalline rock nevertheless find leakage into the rock.

Hydrologists tend to have a binary view of interflow: some slopes produce it, other slopes don’t. However, the importance of interflow as a hydrologic process exists as a continuum. The factors controlling this continuum are relatively easy to define and quantify. By making simplifying assumptions of normal gradients through the impeding layer, the downslope travel distance of a parcel of water moving through the topsoils can be estimated from the ratio of the hydraulic conductivities, the ratio of the downslope and normal hydraulic gradients, and the thickness of the saturated zone above the impeding layer.

For many hillslopes, downslope travel distances imply that only the slope segments adjacent to the riparian valley can be expected to deliver interflow to the valley during a storm. Interflow has characteristics of saturation excess flow: there is a soil moisture/rainfall threshold for occurrence, the area producing flow expands as storm size increases, and flow is produced from only certain landscape positions.

Over most of the hillslope, interflow acts only to redistribute recharge downslope from the point of infiltration. Therefore, continuous perching of water moving as interflow from the ridge to the valley does not imply continuous connectivity to the stream or riparian zone. In terms of stormflow contributions, only the lower slopes within the range of the downslope travel distance may be connected to stream valleys.

The FNR-funded LIST Water Lectures series was launched in January and covers various aspects of water resources research towards a sustainable river basin management. More info online.