Afforestation Increases Water Supply – But Only with These Considerations

  • 2018/12/21     New York, United States

    Image: JurgaR/iStockphoto

    By Hakyung Lee and Lulu Zhang, Soil and Land-Use Management and Karla Locher-Krause, Systems and Flux Analysis Considering Global Change Assessment

    With increasing needs for water and food security and rapid urbanisation resulting from a booming global population, forests maintenance and development face great competition with other priorities. While we are losing forest resources at a high rate globally, afforestation efforts are made in some regions to offset this adverse effect.

    Afforestation is the preferred measure for policymakers to solve a range of ecological problems, as it is generally believed that forests help to improve soil health and quality, reverse land degradation, halt land desertification, and provide clean water through an intensified water cycle. Especially within the context of SDG 15 (life on land), does afforestation always have a positive effect? The answer is ‘no’, at least not always for water provision. A study of the relationship between afforestation and water supply in several regions under different climate conditions shows afforestation resulted in a decline in water supply in all climates (see Table 1).

    Table 1. Water supply changes after afforestation
    Country (Region) Climate Impact on Annual Runoff*
    China (Loess Plateau) Semi-arid/Arid Runoff was reduced by 32% from 1956 to 1980 (Huang et al. 2003).
    Israel (Negev Desert) Arid Runoff after afforestation showed a decrease (Zaady et al. 2001).
    New Zealand (Glendhu) Temperate Annual runoff decreased by 31% from 1991 to 1996 (Fahey et al. 1998).
    Spain (Pyrenean Basin) Mediterranean Annual runoff decreased by 40% from 1965 to 2009 (Buendia et al. 2015).
    Ecuador (Páramo region)
    Peru (La Jalca)
    Tropical Annual river discharge was reduced by 23% in Ecuador from 2013 to 2016, 41% in Peru from 2001 to 2007 (Ochoa-Tocachi et al. 2016).

    * To quantify the changes in water supply, we use runoff as an indicator. Runoff, as a blue water flux, indicates how much water is available for human use.

    These outcomes contradict the common belief that afforestation increases water supply. A critical driving process of these results is evapotranspiration, which is a key hydrological process in the water cycle. When lands are converted from other land uses to forests, the partitioning of precipitation into green (part of water available for plant use) and blue (part of water available for human use) water fluxes changes.

    Evapotranspiration is a major green water flux, while river discharge and groundwater are typical blue water fluxes. Tree growth can consume more water than other shorter vegetation. According to the mass balance principle, if more water is used by trees, less water will flow into rivers and lakes or recharge the groundwater that people can directly use. The direct impact on water supply is becoming an increasingly urgent issue for water resource managers and planners as demands for water continue to rise.

    How then can a positive effect of afforestation on water supply be achieved? We need to consider three central aspects.

    First, scale matters: afforestation can accelerate the water cycle and movement at the global level. For example, more than 2 square kilometers of forest expansion can increase the possibility of rainfall. Trees transport water to the air, and water vapour moves to another place which can be geographically far from the afforested location, such as to another country or even continent through wind-driven circulation. This increases the possibility of precipitation in another location and thus contributes to the water supply in that other region. This indicates that, at a global scale, afforestation can indeed bring benefits to the water cycle.

    Second, species matters: we must choose suitable tree species for afforestation to have a positive effect on the local water supply. Depending on the species, plantation forests may have different structure and amounts of water consumption (Schwärzel et al. 2018). Tree water use also declines with age. Over time, runoff reduction trends can be reversed, and water supply can be improved. Evidence shows, for example, that water supply may start to improve after 30 years in the case of pines and 15 years in the case of eucalypts (Scott and Prinsloo 2008).

    Third, site matters: we need to establish forests in the right place. In dryland, less water is available from precipitation. As tree cover increases, the amount of water available is likely to become less and less, which is critical for areas that already suffer from water shortage and have high demand and competition for water (Zhang and Schwärzel 2017). Compared to dry-prone climates, water is not a limiting factor in tropical climate due to high precipitation. The decline in water supply resulting from afforestation is likely less critical and might even be negligible.

    At the global level, many afforestation efforts are ongoing. There are, among others, the Grain-for-Green Program (GFGP) in China, National Afforestation and Reforestation Plan (PNFR) in Ecuador, and National Afforestation Programme (NAP) in India. In Nigeria, afforestation is considered as a promising solution to achieve climate change mitigation and to solve water supply issues (Olorunfemi et al. 2016).

    Nigeria recognises the importance of keeping their forests. The Nigerian savannah has a tropical climate with temperatures ranging from 16°C to 37°C and an annual rainfall about 1,220 milimeters. Afforestation in this region could induce more precipitation, and water availability could potentially increase in the long term. However, we should also recognise that there are risks for water supply in the short term.

    Forests – when managed properly – can increase resilience of water supply to enable adaptation to global change. The International Union of Forest Research Organizations (IUFRO) recently published a global assessment report highlighting that all of the SDGs are either directly or indirectly related to water and the forest-water nexus. Scientific insights on the forest-water nexus are particularly important to manage and optimise the trade-offs. Afforestation does not always have positive effects on the water supply. The seasonal and annual water yield from forested areas largely depends on forest management, covering selection of tree species, forest structure, and density.

    Understanding the interlinkages and feedback loops – as advocated by the UN University Institute for Integrated Management of Material Fluxes and of Resources (UNU-FLORES) via the Nexus Approach – will support more effective and integrated policymaking, and represents an important contribution towards achieving the SDGs.

    A version of this article was first published on the SDG Knowledge Hub, a project by IISD. Read the original article here.


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