The Water Cycle

Where the cycle breaks in ordinary land use

The usual way of farming in dry regions breaks the cycle at almost every station. Ploughed or herbicide-cleaned soil lies bare for months. The first heavy autumn rain hits an unprotected surface, the drops break the crumbs, the fine particles seal the pores, and water runs off instead of soaking in. It takes soil with it. By the European Commission's harmonised model, land prone to erosion in the EU loses about 2.5 tonnes of soil per hectare and year on average, Italy about 8.5 tonnes, the highest value among member states, while soil forms at around 1.4 tonnes per hectare a year at best.

Where water does get in, a bare and dark surface heats up in summer and sends it back to the air directly, without it ever passing through a plant. Machines and hooves compact the soil and close the pores that were meant to take in the next rain. Sprinklers spray water through hot, dry air, and part of it evaporates or drifts away before it reaches the ground. Fast drainage moves water off the land as quickly as possible, and from houses it leaves through the sewer, often clean enough to water a garden. Groundwater pumped up for irrigation falls in many dry regions faster than winter rain can refill it.

The result is a landscape that receives the same rain and still dries out. Less water in the soil means fewer plants, less transpiration, a hotter surface and, in the long run, less local rain. According to figures presented by Italy's research council CNR in 2015, 30 to 50 percent of Sardinia's land area counts as at risk of desertification. That is not a fate of the climate alone but also of the way water is handled on the land.

Techniques that close the water cycle

The principle behind all of them is simple, and Bill Mollison and Brad Lancaster have taught it for decades: slow water down, spread it out, let it soak in, and keep the soil covered. The cycle graphic above shows which technique works at which station.

At the rain. Rainwater harvesting is the umbrella term for everything that holds rain where it falls instead of letting it leave. It starts with reading the slope: where water gathers, where it rushes, where it could be stopped.

At the soil surface. Mulch shields the ground from sun and rain; in field trials on the North China Plain straw cut evaporation from the soil by about a fifth to two fifths. Rock mulch does the same where organic material would blow away or burn. Living mulch and a permanent cover crop keep roots in the ground all year and slow runoff. Small earthworks catch the water of a larger area for a single plant: the half-moon, the zaï pit and the microcatchment, all tried and tested in the Sahel and other drylands.

In the soil. Infiltration decides whether rain becomes soil water or runoff, and it depends above all on soil structure. Humus builds that structure. Its direct effect on storage is smaller than often claimed: a meta-analysis by Minasny and McBratney found only about 1.2 millimetres more plant-available water per 10 centimetres of soil for each additional percentage point of organic carbon. Its value lies in letting water in. Keyline design uses the shape of the land to move water from moist valleys to drier ridges, terracing turns a slope into level steps, and hügelkultur stores water in a buried core of rotting wood.

In storage. A retention pond holds winter runoff and lets it seep slowly into the ground. The water retention landscape combines ponds, swales, plant cover and soil care into a whole system that keeps rain in the area.

In plants. Mycorrhiza extends the roots with fine fungal threads into pores no root can reach. Olla irrigation gives water through the porous wall of a buried clay pot, straight to the roots, with hardly any loss. Nurse trees shade young plants and the soil beneath them. A food forest layers plants so that the ground never sees full sun, xeriscaping chooses plants that need little water, and vetiver hedges on the contour hold soil and slow water.

At the house. Every roof is a catchment: one millimetre of rain puts one litre of water on every square metre of roof. Led through a gutter and leaf screen into a cistern, it is there in summer for the garden and animals, and the overflow runs into a shallow hollow in the garden instead of the sewer. Greywater from showers and basins can water the garden a second time. A composting toilet saves flushing water altogether and returns the nutrients to the land.

In the air. Regreening brings plant cover back to land that has dried out, agroforestry puts trees back among crops, and climate-adapted trees keep that cover alive in hotter summers. Every leaf that transpires cools the land and returns water to the sky.

How it looks at Sa Medada

Sa Medada is a slope in Jerzu, in Ogliastra on the east side of Sardinia. The winters are mild; the dry season runs roughly from May to September. An old olive grove covers part of the land, and macchia grows on the steeper slopes. The LiDAR wetness map shows a comparatively dry main area and moister channels along the edges. That is the starting point: a slope that receives its rain in the cooler half of the year and must carry its plants through months with hardly any.

Project Eden therefore begins with three things: pioneer plants, ground cover and holding water. Swales are planned, none has been built yet. Soil cover and shade come first, because on this land they decide how much winter rain stays in the slope at all. There are no dry stone walls on the site, and a laboratory analysis of the soil does not exist yet, so we make no claims about how much water the soil stores. What we can say is the direction: water should stay where it falls, soak in, pass through plants and cool the slope, before anyone thinks of open water or tanks.

How the water cycle meets the nutrient cycle is the subject of the next page in this section: rain that runs off carries soil and nutrients with it, and water that soaks in carries them to the roots.