Water, Forest, and Soil Resources

Water, forests and soil are three basic renewable resources that support life and the economy: water is used for drinking, farming, industry and energy; forests provide timber, fuel, habitat and climate regulation; and soil provides the nutrients, water and support for plants and crops. In brief: all three are renewable only if used carefully. Fresh water is limited and unevenly distributed, forests cover about 31 percent of the land (about 4 billion hectares) but are being lost in many tropical regions, and soil forms very slowly (on the order of centimeters per several hundred years or more) but can be eroded away much faster. Their management is the base of food security.

These resources are linked: forests protect soil and regulate water flows, and healthy soil stores water and supports vegetation. This guide describes each resource, its uses and threats, compares them in a table, works through calculations of irrigation needs and soil loss, outlines the history of their use and corrects common mistakes.

What are water resources?

Water resources are the sources of water that are useful to people, such as rivers, lakes, reservoirs and groundwater. Only about 2.5 percent of the world’s water is fresh, and only a small part of that is easily accessible. Uses: agriculture takes about 70 percent of the withdrawals (mainly irrigation), industry about 20 percent (cooling, manufacturing and energy) and households about 10 percent (drinking, cooking, washing and sanitation). Water is also used for hydropower, transport, fishing and recreation. Pressures include growing demand, pollution, over-extraction of groundwater, and climate variability. Management involves storage (reservoirs), efficient use, pricing, pollution control and sharing between users and countries (see the articles on fresh water and water scarcity).

What are forest resources?

Forests cover about 31 percent of the land area of Earth, roughly 4.06 billion hectares, according to the Food and Agriculture Organization (FAO), and more than half of them are in five countries: Russia, Brazil, Canada, the United States and China. Types include boreal (coniferous) forest in the north, temperate forest, and tropical rainforest and dry forests. Products: timber for building, pulp for paper, firewood (the main fuel for billions of people), food, medicines and rubber. Services: carbon storage and absorption of carbon dioxide, home for most of the world’s land species, protection of soil and water supplies, regulation of local rainfall and recreation. Deforestation (the clearing of forest for farming, grazing, logging, mining and settlement) is concentrated in the tropics, such as the Amazon and Southeast Asia, and causes loss of biodiversity, soil erosion and carbon emissions. Sustainable forest management includes selective logging, replanting, certification (such as FSC) and protection of old-growth forest.

What are soil resources?

Soil is a thin layer of weathered rock particles, organic matter (humus), water, air and living organisms on the surface of the land. It is formed slowly by weathering of rock and the decay of organic matter; the typical rate is of the order of 1 cm in 100 to several hundred years or more, depending on climate, rock and vegetation. Soils have layers (horizons): the topsoil (rich in humus), the subsoil and the weathered parent rock. Soil provides nutrients and water for crops, filters and stores water, stores carbon and supports food production: about 95 percent of the world’s food is directly or indirectly produced on soils. Threats: erosion by water and wind (often after the removal of vegetation or ploughing steep slopes), loss of nutrients, compaction, salinization (salt build-up in irrigated land), desertification, pollution and the covering of soil by buildings and roads. Conservation: contour ploughing, terracing, cover crops, windbreaks, no-till farming, crop rotation, adding organic matter, and good irrigation and drainage.

The three resources compared

Worked example: irrigation and soil loss

1. Irrigation need (hypothetical crop): a crop needs 500 mm of water over its season. On 1 hectare (10,000 m²), 0.5 m × 10,000 m² = 5,000 m³.
2. With losses: if only 50 percent of the water taken from the river reaches the plants (the rest is lost by leaks and evaporation), the amount taken is 5,000 ÷ 0.5 = 10,000 m³ per hectare. With drip irrigation at 90 percent efficiency, it is 5,000 ÷ 0.9 = 5,556 m³, a saving of 4,444 m³ per hectare, or 44 percent of the previous withdrawal.
3. Soil loss rate: a field loses 15 tonnes of soil per hectare per year. With a bulk density of 1.3 tonnes per m³, the volume is 15 ÷ 1.3 = 11.5 m³ per hectare, which spread over 10,000 m² is a layer of 11.5 ÷ 10,000 = 0.00115 m = 1.15 mm per year.
4. Over 50 years: 1.15 mm × 50 = 57.5 mm, about 5.8 cm of topsoil.
5. Compared with soil formation: if new soil forms at 1 cm per 500 years, that is 0.02 mm per year, so the loss of 1.15 mm per year is 1.15 ÷ 0.02 = about 58 times faster than the formation, and the soil is effectively a nonrenewable resource at this rate.
6. Forest share: 4.06 billion hectares of forest out of about 13.0 billion hectares of ice-free land is 4.06 ÷ 13.0 = 31 percent.

How have people used these resources?

Agriculture, from about 10,000 years ago, led to the clearing of forests and the management of water by irrigation. Soil erosion and salinization are thought to have contributed to the decline of some ancient societies, such as parts of Mesopotamia. In the 1930s, the Dust Bowl in the Great Plains of the United States, where ploughed grassland was blown away in droughts, led to the creation of the Soil Conservation Service in 1935. The Green Revolution of the 1960s increased yields using irrigation, fertilizers and new crop varieties, but also increased water use and pollution. Today, global initiatives such as the Bonn Challenge aim to restore forests and degraded land.

Which mistakes are common?

Mistake 1: Treating soil as an unlimited resource

It forms extremely slowly, and eroded soil is lost on human time scales.

Mistake 2: Equating forest cover with forest quality

Plantations of a single species have much lower biodiversity than natural forests, though both count as forest area.

Mistake 3: Thinking that planting trees immediately replaces a mature forest

It takes decades to centuries for a forest to recover its carbon stock and ecosystem.

Mistake 4: Looking at water, forests and soil separately

They are linked: forest loss increases erosion and flooding, and degraded soil stores less water.