A3 Extreme and Accelerated Erosion

Topic

Extreme erosion occurs when the energy of water flow or wind overcomes the soil's resistance to that energy, removing soil particles at rates that vastly exceed those of natural pedogenesis—the process of new soil formation, which proceeds at a pace of merely one-hundredth to one-tenth of a millimeter per year. It is precisely this disparity between the rate of soil loss and the rate of soil regeneration that renders extreme erosion effectively irreversible on any human timescale.

Water erosion, in particular, involves three sequential mechanisms as water becomes concentrated: first, the direct impact of raindrops on bare soil; next, sheet flow, which washes away particles in a thin, uniform layer; and finally, concentrated flow, which carves out rills and gullies. It is this progressive concentration of water that causes each successive mechanism to remove more soil than the one preceding it.

A standard soil-loss equation quantifies annual water erosion as the product of several independent factors: rainfall erosivity, the soil's inherent erodibility, slope topography, protective vegetation cover, and applied land-management practices. Because of this multiplicative structure, an increase in even a single factor results in an increase in total soil loss.

Within the gullies carved by this concentrated flow, the process becomes self-reinforcing and forms a closed feedback loop: the incision created by water at the gully bottom lowers the local base level; this drop in base level increases both the gradient and the flow's sediment-transport capacity; this enhanced capacity accelerates headward erosion—advancing up the gully—which in turn causes further incision at the bottom, thereby closing the loop back to the initial incision that triggered the mechanism. Wind erosion, for its part, operates according to a different principle: particle transport via saltation begins only when the wind's friction velocity exceeds a threshold value; once that threshold is surpassed, the quantity of transported material does not increase proportionally but rather grows with the cube of the excess velocity above that threshold.

Thus, from the distinction between erosion and pedogenesis with which this document begins to the cubic growth of wind transport with which it concludes—encompassing the three progressive mechanisms of water erosion, the equation quantifying such soil loss, and the self-energizing feedback loop that deepens each gully—extreme erosion demonstrates that soil which took centuries to form can, under the right conditions, be lost in a matter of hours.

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