Noting that some literature classify the overland flow as a fast pathway and of 'new' (event) water, and soil and groundwater as slow pathway and of 'old' (pre-event) water, I perceive (as suggested by some studies- though not many) that returnflow could be a slow(er than overland flow) pathway, and having taking the soil characterictics (though with few minutes, hours or days) may become relatively 'old' BUT it is still part of the overland flow. Now is it right to describe it as a slow pathway? And is it right to say that it is old? How will you distinguish this 'old' return flow from soil water or groundwater?
dear adebayo, this is in fact quite an interesting question and I would as a first cut say that it is, little surprisingly, dependent on soils, bedrock geology and topographic gradients of your catchment. even overland flow, if generated as saturation overland flow will be a mixture of "old" and "new" as arguably it is generated by a combination of groundwater pushing upwards and new water saturating the relatively small unsatruated zone. in general, i would thus argue that the age of return flow water depends on how it is seeping out of the ground: from shallow, soil layers, characterized by preferential flow paths. or rather from a deeper layer and/or matrix flow. However, i think return flow should indeed be composed of water that is on average older than that of overland flow. maybe some results of Hrachowitz et al. (2013) could help you, although we do not directly use return flow there.
hope that helps,
all the best,
markus
Thank you very much! Your answer and reference help a great deal. Kind regards.
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