Upwind Forests: Managing Moisture Recycling for Nature-Based Resilience
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14 Upwind forests: managing moisture recycling for nature-based resilience D. Ellison, L. Wang-Erlandsson, R. van der Ent and M. van Noordwijk Trees and forests multiply the fficient and effective forest and however, tends to focus on river flows and oceanic supply of freshwater water-related nature-based solu- to take rainfall for granted as an unruly, through moisture recycling, tions to challenges in human devel- unmanageable input to the system (Ellison, pointing to an urgent need to halt Eopment require a holistic understanding Futter and Bishop, 2012). Thus, the poten- deforestation and offering a way to of the role of forest–water interactions tial impact of increased tree and forest increase the water-related benefits in hydrologic flows and water supply in cover on downwind rainfall and potential of forest restoration. local, regional and continental landscapes. water supply is both underestimated and Forest and water resource management, underappreciated. Afternoon clouds over the Amazon rainforest | downloaded: 13.5.2020 David Ellison is at the Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden, Adjunct Researcher, Sustainable Land Management Unit, Institute of Geography, University of Bern, Switzerland, and at Ellison Consulting, Baar, Switzerland. Lan Wang-Erlandsson is at the Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden. Ruud van der Ent is at the Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands, and the Department of Physical Geography, Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands. Meine van Noordwijk is at the World Agroforestry Centre, Bogor, Indonesia, and Plant Production Systems, Wageningen University, https://doi.org/10.7892/boris.142918 Wageningen, the Netherlands. © NASA IMAGE COURTESY JEFF SCHMALTZ, MODIS RAPID RESPONSE NASA GSFC AT source: Unasylva 251, Vol. 70, 2019/1 15 On average, about 60 percent of all 10 percent of the Earth’s land surface but al., 2014, 2010; Gebrehiwot et al., 2019). transpiration and other sources of ter- contribute 22 percent of global evapotrans- The long-distance relationships between restrial evaporation (jointly referred to as piration (Wang-Erlandsson et al., 2014), forests, moisture recycling and rainfall evapotranspiration) returns as precipita- an important share of which returns to challenge conventional forest–water tion over land through terrestrial moisture land as rainfall. Moreover, deep-rooted analyses based on catchments as the recycling, and approximately 40 percent trees are able to access soil moisture and principal unit of analysis (Ellison, Futter of all terrestrial rainfall originates from groundwater and thus continue to tran- and Bishop, 2012; Wang-Erlandsson et al., evapotranspiration (van der Ent et al., 2010; spire during dry periods when grasses are 2018). Catchment-centric studies tend to see also Figure 1). From the perspective of dormant, providing crucial moisture for ignore evapotranspiration once it has left a river, evapotranspiration may appear as rainfall when water is most scarce (Staal the confines of the basin in which it was a loss but, for the extended landscape, the et al., 2018; Teuling et al., 2010). produced, despite its key contributions recycling of atmospheric moisture (“rivers Nature-based solutions involving for- elsewhere to downwind rainfall (Ellison, in the sky”) supports downwind rainfall. est and landscape restoration, therefore, Futter and Bishop, 2012) – and the view Forests are disproportionately impor- have the potential to influence rainfall that evapotranspiration represents a loss tant for rainfall generation. On average, and consequently sometimes very dis- rather than a contribution to the hydrologic their water use is 10–30 percent closer tant, downwind rainfall systems reliant cycle has resulted in a pronounced bias to the climatically determined potential on moisture recycling for food produc- both against forests and in favour of the evapotranspiration than that of agricul- tion, water supply and landscape resilience catchment-based water balance (Bennett tural crops or pastures (Creed and van (Bagley et al., 2012; Dirmeyer et al., and Barton, 2018; Dennedy-Frank and Noordwijk, 2018). For example, tropical 2014; Dirmeyer, Brubaker and DelSole, evergreen broadleaf forests occupy about 2009; Ellison et al., 2017; van der Ent et 1 The global hydrologic landscape ATMOSPHERIC MOISTURE TRANSPORT 45 FO HUMIDITY AND PRECIPITATION BIOPRECIPITATION RECYCLING TRIGGERS DOWNWIND L LOCAL AND GLOBAL AT REGIONAL L SCALE HEATING AND CONTINENTAL AND COOLING SCALE 75 E 120 P O FOG/CLOUD O INTERCEPTION FLOOD 455 E MODERATION 410 P DOWNSTREAM INFILTRATION AND GROUNDWATER RECHARGE 5 OCEAN LAND 45 FL Notes: F represents “net” atmospheric moisture exchange between land (L) and ocean (O). Inflows of atmospheric moisture to land from the ocean are, on average, about 75 000 km3 per year, significantly larger than the “net” inflows of 45 000 km3 suggest (van der Ent et al., 2010). Likewise, the evapotranspiration contribution to rainfall over oceans is approximately 30 000 km3 per year (van der Ent et al., 2010). Sources: Adapted from Ellison et al. (2017), with quantifications of water flow (i.e. ocean evaporation, EO; evapotranspiration, EL; ocean precipitation, PO; land precipitation, PL; net ocean-to-land moisture flow, FO, rainbow arrow; and runoff, FL, black arrow) in 1 000 km3 per year from van der Ent and Tuinenburg (2017). Unasylva 251, Vol. 70, 2019/1 16 Gorelick, 2019; Filoso et al., 2017; Jackson atmospheric long-distance forest–water form of vapour and drops (i.e. evapotrans- et al., 2005; Trabucco et al., 2008). relationships, and discuss some of the key piration and precipitation); and, second, New modelling capacities and increased challenges and opportunities for using for- those that flow horizontally as atmospheric data availability, however, make it pos- ests as nature-based solutions for water. moisture (thus, rivers in the sky) (Figure 1). sible for scientists to better and more easily Our focus is on the role of forests for On average, approximately 75 000 km3 of quantify where and how much forests rainfall and water supply through mois- water per year evapotranspires from land contribute to rainfall. The last decade ture recycling. Thus, we ignore the many into the atmosphere, where it combines has seen a surge, not only in understand- other invaluable benefits of forest–water with evaporation of oceanic origin (Oki ing of the forest–rainfall relationship interactions, such as flood moderation, and Kanae, 2006; Rodell et al., 2015; through moisture recycling, but also in water purification, infiltration, groundwater Trenberth, Fasullo and Mackaro, 2011). the scientific exploration of landscape, recharge and terrestrial surface cooling (see Of the evapotranspiration from land, some forest and water management and gov- Ellison et al., 2017). falls as rain over oceans, but 60 percent – ernance opportunities (Creed and van about 45 000 km3 per year – falls as rainfall Noordwijk, 2018; Ellison et al., 2017; Keys FORESTS SUPPLY AND MULTIPLY over land (Dirmeyer et al., 2014; van der et al., 2017). FRESHWATER RESOURCES Ent et al., 2010). In total, evapotranspira- In this article we review the role of The global distribution of moisture tion contributes approximately 40 percent forests as water recycler and water-resource recycling of the 120 000 km3 of water per year that multiplier, examine the implications of The largest water flows over land are not precipitates over land. those in rivers but rather those that “invisi- Trees, forests and other vegetation Trees contribute to evapotranspiration bly” flow first in the vertical direction in the play pivotal roles in supporting both by accessing deep soil moisture and groundwater, as well as through interception © JOAKIM WANG-ERLANDSSON Unasylva 251, Vol. 70, 2019/1 17 evapotranspiration and precipitation. On Gordon, 2001), as opposed to evaporation under current atmospheric circulation con- a global average, transpiration makes from bare soil or open water evaporation ditions. In large parts of Europe, the eastern up about 60 percent of total evapotrans- (Miralles et al., 2016; Wang-Erlandsson Russian Federation, East Africa and north- piration, with a large uncertainty range et al., 2014). Climate model simulations ern South America, more than one-third of (Coenders-Gerrits et al., 2014; Schlesinger suggest that a green planet with maximum evapotranspiration is vegetation-regulated and Jasechko, 2014; Wang-Erlandsson et vegetation could supply three times as (i.e. occurs because of the presence of veg- al., 2014; Wei et al., 2017). Vegetation’s much evapotranspiration from land and etation) and falls as precipitation over land direct contribution to total evapotrans- twice as much rainfall as a desert world (Figure 3, p. 21). In parts of Eurasia, piration, however, also includes canopy, with no vegetation (Kleidon, Fraedrich North America, southern South America forest-floor and soil-surface evaporation, as and Heimann, 2000). and large parts of subtropical and dryland well as epiphyte interception. Significantly Tree-, forest- and vegetation-regulated Africa, more than one-third of precipitation more than 90 percent of total terrestrial moisture recycling is unevenly distributed. comes from vapour flows that would not evapotranspiration comes from vegetated Figure 2a shows the rainfall-generation