As we deal with fluid, the mechanics of the fluid will make it more complicated and not only surface features. Especially if the type of the flow changes as we 'scale-up' for instance from laminar to turbulent flow.

Do you mean if you can scale-up your understanding of point-scale hydrologic processes to hillslope-scale? If so, one of the most critical things you need to incorporate into the hillslope-scale simulation or understanding would be overland routing, which describes interaction between landscape elements such as pasture and tree cells (or parcels) along flow paths. Routing (especially overland routing) would be the key process to connect the point-scale processes to the hill-slope processes. If you want to expand the spatial scale to a watershed, channel routing would be one of the important processes you need to tackle in order to understand watershed-scale responses to inputs including rainfall and land use changes.

While routing is important in modelling, here I'm referring to a general concept of how finer scale interactions are 'usually' averaged out in larger scale approximations. Interesting point that laminar flow becomes turbulent as scale increases, but my question here is that averaging out finer scale variance would result in a seemingly more laminar flow at larger scales. More specifically, I’m interested in how the build-up of litter behind obstacles on hillslopes can transfer water against the general lay of the terrain, even to the degree of going across the slope. While obstacles with litter trapped behind them is a basic micro-scale property on most hillslopes, I have attempted to model the phenomenon as a ‘chain of obstacles’ set out to see if runoff can be directed across entire hillslopes. My modelling has been fully distributed (using MIKE SHE) and it suggests it is possible, yet my field sites show 'breaches' where water builds to a certain level and then bursts through the litter and I end up with a concentrated flow at that point, effectively ‘breaking the chain’. This is something that the model is not capturing, as it is somewhat stochastic process. This leads me to think that we cannot simply scale-up finer scale interactions and assume they still assert the same influence at larger scales. On the other hand, we too often ignore the finer scale effects of obstacles and litter so that hillslopes are treated as homogenous units, when they are anything but that in the real world. Still, I don't know how to capture the finer scale interactions and scale-up these smaller units to model their collective contributions to hillslope responses.