Why Cut Steps Into a Hillside? Farming Where the Soil Would Wash Away

Water running down a slope accelerates, and faster water carries more soil, so a bare hillside under rain loses material rapidly and loses the finest and most fertile particles first. Cutting the slope into a series of level or gently sloping platforms breaks the descent into short sections, so water has no opportunity to gather speed, and it either soaks in where it falls or is carried away along a controlled channel. The level surface also holds water long enough for crops to use it and allows irrigation, which a slope cannot. A retaining wall or bank holds each platform in place, and material cut from the upper part of each step fills the lower part, which is why the work is enormously labour-intensive and why it is permanent once done.

Different forms suit different slopes, soils and crops:

• •Bench terraces, cut level with a vertical or steeply sloped retaining face, which suit steep ground
• •Broad-base terraces, wide and gently sloping, used on shallower gradients and workable by machinery
• •Contour bunds, low banks following the contour rather than full platforms
• •Stone-walled terraces, common on rocky ground where clearing the stones supplies the walls
• •Paddy terraces, level and bunded to hold standing water for rice
• •Terraces cut for tree crops, where the platform need only support a single row

The technique appears in many places without evident contact between them, which indicates that the problem and its solution are both obvious enough to be found repeatedly. Rice terraces in the Philippines, Indonesia, China, Japan and Nepal cover enormous areas and in several cases date back a thousand years or more. Andean terraces supported agriculture at altitudes and on gradients that would otherwise be impossible, with sophisticated water management and with evidence that the fill material was layered deliberately for drainage. Mediterranean terraces for vines and olives are found across southern Europe, North Africa and the Levant. Yemeni highland terraces are among the oldest continuously worked. The scale of some systems is difficult to grasp, with individual regions holding tens of thousands of hectares of hand-built platforms.

The most sophisticated systems manage water as carefully as they manage soil, and the hydrology is the part outsiders miss. Andean terraces were built with graded layers of coarse stone, gravel and finer material beneath the soil, which drains excess water while retaining enough for crops and prevents the saturation that would burst a wall. Rice terraces depend on a network of channels distributing water from a source at the top through every platform in sequence, which requires agreement among everyone on the slope about timing and quantity, and those arrangements are governed by institutions that in several regions are centuries old and are studied as examples of successful common resource management. The terraces also recharge groundwater, holding water in the landscape instead of shedding it, which is why abandoning them changes downstream flows as well as upstream soils.

Maintenance is the weakness of the whole system and it is failing in many regions. A terrace wall requires regular repair, since frost, roots, burrowing animals and heavy rain all work at it, and a single breach concentrates water and can destroy a series of platforms below in one event. That work is labour-intensive and cannot be mechanised, so it depends on a rural population willing to do it, and migration away from upland farming has left large areas of terracing unmaintained across the Mediterranean, in parts of Asia and elsewhere. Abandoned terraces collapse over decades, and the resulting erosion can be worse than if the slope had never been terraced, because a partial collapse channels water. Several countries now subsidise maintenance explicitly, treating the terraces as infrastructure rather than as private assets.

Steps break a slope into short sections so water cannot gather speed, and the level surface holds water for crops and allows irrigation. Bench, broad-base, stone-walled and paddy forms suit different gradients and crops. Maintenance cannot be mechanised, so abandonment is widespread, and a collapsing terrace channels water and can erode worse than an untouched slope.