Soil Erosion

The forms of erosion

Water erosion follows a recognisable sequence. It starts with the impact of the raindrop. On bare soil, every drop is a small blow that breaks up crumbs and throws particles into the air; the fine material clogs the pores and seals the surface. Then a thin film of water begins to flow and carries off the surface evenly, so evenly that this sheet erosion often goes unnoticed for years: the stones seem to "grow" out of the field, and the roots of old trees stand exposed.

Where the flow concentrates, it cuts rills, small channels a few centimetres deep. Rills can still be smoothed over by working the soil. Gullies cannot: they are deep, they grow backwards up the slope with every storm, and they drain the land around them. Alongside this there is wind erosion on dry, loose ground, tillage erosion, in which the plough itself shifts soil downhill year after year, and mass movements, from small slips on a terrace edge to landslides on saturated slopes.

Italy and the Mediterranean

In 2015 the European Commission's Joint Research Centre published a harmonised model of water erosion for the whole EU, based on the Revised Universal Soil Loss Equation (RUSLE). It combines rainfall erosivity, soil erodibility, slope length and steepness, plant cover and protective practices. The mean for erosion-prone land in the EU came out at 2.46 tonnes per hectare per year. Italy had the highest country value, 8.46 tonnes, because heavy rainfall meets steep and long slopes there. About a quarter of the EU's land lost more than two tonnes a year, and these areas accounted for around 87 percent of all soil lost.

The study reports its values by country and does not give a separate figure for Sardinia, and ISPRA's national map leaves the island out because the regional models there are not comparable. The overall picture is nonetheless clear. The highest losses in Europe are modelled for the Mediterranean, and research on abandoned terraces in Spain and Greece, summarised in our article on terracing, has measured some of the highest rates of all.

A note of caution belongs here: these are model values on a 100 metre grid. They show the pattern, not the loss on an individual slope, and some erosion researchers have criticised the approach for relying on a model where field observation is needed as well.

Why the first autumn rain does the most damage

The Mediterranean climate sets a trap for the soil. After four or five dry months the plant cover is at its thinnest, the surface is crusted, and many soils have become water repellent: dried organic coatings on the particles make the first rain bead up and run off, as if the ground were waxed. Then come the storms of autumn and winter, often with a great deal of rain in a short time. In most of Europe the most erosive rain falls in summer, when fields are green; the Mediterranean is the part of the continent where significant erosive rain also falls in the cool season, when the ground is least protected.

The result is a feedback loop of the kind described in our article on desertification: bare soil erodes, eroded soil holds less water, and drier soil carries less vegetation. Every lever that breaks the loop works at the surface. Cover protects against the impact of the drops, living roots and humus keep the pores open so water can soak in, and every interruption of the slope, from terraces to hedges along the contour line, shortens the distance over which water can gather speed.

How Terra Humana understands it

For Terra Humana, erosion is the visible face of the problem we work on. It is the moment when years of degradation become a muddy stream. That is also why our starting point is the soil surface and not the plant list. As long as rain runs off a slope instead of entering it, every tree planted there works against the current.

The answer is old and unspectacular: keep the soil covered, slow the water, hold it where it falls. The most important tools are not machines but plants, mulch and the shape of the land itself.