In literature some times to account for semi-pervious sediments at bottom of stream, streambed leakance also called retardation coefficient (L) is used which is equal to ratio of conductivity of aquifer to streambed multiplied by thickness of streambed. On other hand another term seems to describe the effect of same less permeable sediments on bottom of river named as " Riverbed conductance" (L2/T) used in MODFLOW, which is equal to KA/L, where K is conductivity of riverbed , A is area of cell and L is length. Is this the only difference between these two terms?
Not exactly sure. There is a term deep seepage, we use in water balance for water that is assumed to be lost, it was not accounted for, such as if rainfall is 1000 mm, streamflow 300 mm, ET 675 mm, then the unallocated 25 mm may be allocated to deep seepage as it was lost somewhere. Streambed flow (hyporeic) flow is flow within the stream bed materials, and for research watersheds with bedrock controls, this flow is interrupted and brought to surface and measured, or forced into deep seepage. The rate of the stream bed flow can be estimated by porosity and transmissivity of substrate and head differences with distance (Darcy's Equation). The rate of deep seepage is normally not known, but it is probably higher when stream and water table is up, and lower when the stream is at low flow or dry. The KA/L in your formula is like Darcy's law equation, and perhaps the head is embedded in other area of your model. In low gradient streams, the head value is probably also low, so velocity is also low.
Streambed leakance (T-1) is hydraulic conductivity of the streambed (L/T) divided by the streambed thickness (L). Streambed conductance is as you defined it: KA/L (L2/T). To go from streambed leakance to streambed conductance you multiply by the area of the model cell. To obtain leakage through the streambed (L3/T), you multiply the streambed conductance by the hydraulic head difference between the water level in the stream and the water level in the aquifer.
That makes sense that stream leakage is probably the same as what used to be called years ago as deep seepage as it was portions of the water balance that were not included in the measured differences between the input rainfall, and the outputs streamflow, and evapotranspiration. In the watershed studies, we typically installed structures to bedrock, so streambed conductance was either forced to the surface or provided more head for deep seepage (stream leakage). The techniques to measure these has improved greatly over time. Anyway, as you suggest, bedflow whatever we may call it is different from the water seeping downward to lower aquifers. The old studies with stream gauging to bedrock in most instances probably forced the bedflow to become surface flow. Thanks for clarifying. As you know, the equations used are based on Darcy's Law.
They are ephemeral. One yields gold on the corner, the other yields gold underneath. I would suggest if I was asked that mixing the two is not conductive I mean conducive to finding gold. Splitting hairs.
Get amongst it amongst nature and with a decent invest you will find a hard core.
This is why I don't like online.
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