The forest-water interaction: complex but vital to billions

Aspen regenerating on the Wasatch Plateau Utah after a forest fire

Aspen regenerating on the Wasatch Plateau Utah after a forest fire

One quarter of the world’s population depends on water from forested catchments. No one knows this better than Jay Humphries. “In 1900 this land was grazed off and forested out,” the watershed manager told foresters visiting Utah’s Wasatch Plateau from the 24th Congress of the International Union of Forest Research Organisations (IUFRO). “Water quality was very poor. Government had to step in.”

Since then the state has carefully tended the mountains. Now spruce and aspen blanket the slopes, and Salt Lake City, the state capital, and surrounding communities receive year-long high quality water. But the struggle to sustain it is constant. Precipitation is only 300mm/year in America’s second driest state. “Most is snow,” says Humphries, “We get runoff for just six weeks.”

A reservoir network holds the snow melt. But the snow pack is decreasing. There are calamities too, like a fire in 2013. “We got teams out the best we could,’ says Humphries. “But humidity was 0% and winds 35 miles/hour.” Fifty thousand acres burned. Today the forest is recovering but much ground is bare: just 5 mm of rain can dislodge car-sized boulders, close valleys and send sediment into irrigation pipes.

At the IUFRO congress, the Food and Agricultural Organization (FAO) and World Agroforestry Centre (ICRAF) convened a meeting on forests and water. Calling for international action, FAO’s Thomas Hofer said that forests stabilize soil, minimize erosion, lessen drought severity (by casting shade and slowing wind), and reduce small to medium floods and shallow landslides although not deep geological ones.

FAO’s forest conservation officer and team leader for watershed management and mountains also called for brainstorming and more research; forest and water interactions are deeply complex, depending on climatic zones, time of year, geology, tree species composition, plant density, forest management and forest quality. He defined quality as a forest with trees of different ages and heights and soil covered well with leaves, among other things,.

“What critical knowledge gaps do we need to fill to make the scientific case for the importance of forests to water?” became the meeting’s theme.

Jami Nettles said that a major gap is plant water use by typology, scale and climate variability. “Measuring and understanding the water requirements of the most important species would furnish a high return on science investment,” said the Weyerhauser scientist. Such data can already be calculated for major annual crops using FAO’s CropWat software tool.

Antonio Campo of the University of Valencia said standard methodologies are needed. He described the “tipping bucket” measurement of “throughfall”, the process by which wet leaves shed excess water onto the ground. He also enumerated technologies, many of them homemade and low cost, to assess stem flow, soil moisture, transpiration, superficial and shallow flow, and deep infiltration and ground water.

Dr Catherine Muthuri addresses tree-crop interactions in agroforestry

Dr Catherine Muthuri addresses tree-crop interactions in agroforestry

ICRAF’s Catherine Muthuri is already using the heat pulse method to gauge sap flow and hydraulic redistribution by the tree species, Faidherbia albida. “Rainfed agriculture can be strikingly ineffective at making productive use of rain. In Niger, only 4-9% of available water is used by the crop,” she said, before posing the fundamental question: can agroforestry increase capture of rainfall and utilize water better than systems without trees?

In Uganda, ICRAF found that trees in fields can improve infiltration and reduce runoff to 1.8-6% compared to 18% in soil-crop systems; in the Sahel, tree pruning, an agroforestry practice, reduces transpiration from shea trees. The roots of some species access water beyond the roots of crop and “wet the soil surface”, said Muthuri. “Generally when trees are in the system, there is a reduction in unproductive water.”

Water is high on the radar of the upcoming sustainable development goals. SDG 6 states “Ensure availability and sustainable management of water and sanitation for all. SDG 15 states “by 2020 ensure conservation…of terrestrial and inland water freshwater ecosystems and their services, in particular forests, wetlands, mountains and drylands.” Sub-goal 15.1 states “sustainably manage forests”.

Rajan Kotru of the Integrated Centre for International Mountain Development said ten rivers flow from the Himalayas, but in Nepal is experiencing deforestation and more edge and patch forest. As a result, water availability is falling with increasing conflicts over its distribution. Worldwide forests are in decline.

However, trees and forests should not be planted without research; the forest-tree-water relationship is too complex for simplistic solutions. A participant at the meeting said “Everyone knows forests are good but we made a big mistake in China and created an ecological disaster by planting trees – pines – where it should be grasslands and shrubs.” Another added, “If we know the water footprints of different species, we will know the different implications of planting them.”

Dr Margaret Kroma closed the gathering by praising the discussion for including not just the biophysical but the social aspects of water as well — because “humans are the quintessential integrators”. She urged participants to prepare for the September 2015 World Forestry Congress, where a new forest and water agenda will be launched as well as new syntheses of knowledge.

“Today’s event is just one in a process to accelerate thinking and action on forests and water with a focus on science, policy and action,” said the Assistant Director General of ICRAF.

For more information, see the 2013 FAO report Forests and Water: International Momentum and Action'

Cathy Watson

Cathy Watson is chief of programme development at the World Agroforestry Centre in Nairobi. Before joining ICRAF in November 2012, she founded and ran two NGOs in Uganda -- Straight Talk Foundation and Mvule Trust. She was made a senior Ashoka fellow for social entrepreneurship in 2006. She has also been a foreign correspondent, working for The Guardian and the BBC, among others. A graduate in biology and Latin American Studies from Princeton, she has over 30 years of work experience in Africa with a focus on trees, youth, HIV, families, and communication for social change. She holds a graduate certificate in agroforestry from the University of Missouri.

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