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How Does Water Get to a Stream?

USER QUESTION: “The discussion of interflow seems to be generally new in the presentation but welcome nonetheless.  Is the software addressing movement in all directions?  Jim mentioned the “sideways” component which is typically less than the vertical migration component in unsaturated soils.  Does the program address saturated conditions that are common in fine grain soils during the winter?”

“The discussion suggests that the upper 600mm is the limit of impact that soils have on infiltration or storage capacity.  I would assert that it depends on the nature of the material and any boundary layers at depth.  I have done hundreds of test pits in fine grained soils with saturated zones well below 600mm in native soils.  I have also done dozens of holes through turf and topsoil to consolidated clays showing little to no penetration as might be expected leading to the valid conclusion that 300mm of topsoil can have a huge impact on rainfall storage.”

JIM DUMONT’S RESPONSE: “The concept of interflow has always been a part of continuous simulation modelling although it has not always been described with this specific term. The incredibly complex processes associated with water movement has been simplified in QUALHYMO to the extent needed to provide a reasonably accurate description of the operation of the soil moisture reservoir and base flow into the stream. I have used the model to simulate stream flow and have calibrated it on several occasions. This leads me to conclude that this method provides sufficient accuracy while it may be less precise than other groundwater flow models.  A simple description or chart showing how the soil moisture reservoir is modelled is shown below.”

“I agree that all soils are different and site specific conditions need to be considered. Having said that there are some generalizations that can be made regarding the flow of water through soils. If I know the processes under which the soil formed I can tell you how the water moves. I have provided a second set of graphics that shows to very different soil types.”

“The figure on the left shows a very typical Podzol that is found in BC and that has formed under a forest cover. The red stains are iron oxides that are leached out to the humus and upper portions of the soil. This indicates a primary chemical process that includes oxidation and exposure to oxygen in the form of air from the surface. Because the rust is transported by water it is very easy to see the depth to which the water typically penetrates the soil profile. Since the underlying soils started out the same as the upper soils following the last glaciation the differences have taken some 10,000 years to develop.  I would say that the predominant soil in BC are the Podzols and that the typical depth of water penetration is not more than 600 mm. If you were to dig down through the reddish soil horizon you would find that the soil is dry and that the water table is some depth below the level where the colour changes.”

“The figure on the right shows a Gleysol which is formed under conditions that include long periods of saturation. The colors indicate considerable periods of limited oxygen and the chemical processes are primarily reduction. In this case the water would have moved up from below as well as down from the surface to  saturate the soil for extended periods. There are small pockets of Gleysolic soils found throughout BC and they are often readily identified because the surface vegetation tends to be water tolerant plants. If you were to dig down through these soil layers you may find that occasionally it may be dry but that the water table would be quite near the surface.

“The lesson in this and the study of soil formation is that water does not move continuously downward as a result of saturated conditions. Generally the hydrologically active soil layers are limited to the upper 600 mm or so with a majority of situations the groundwater levels are deeper. In these cases the water will move sideways until one of two things occurs: 1) it reaches a stream, or 2) it finds a discontinuity in the soil that would allow it to move downward.  As always, there are exceptions to these rules and that indicates a need to understand the soils that are present on a site so as to be able to describe how the water moves through the soil.”

“I hope this information can start an in-depth discussion and consideration of soil conditions particularly how the soils are hydrologically active.”