Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics

Source: Nature

Water scarcity contributes to the poverty of around one-third of the world’s people. Despite many benefits, tree planting in dry regions is often discouraged by concerns that trees reduce water availability. Yet relevant studies from the tropics are scarce, and the impacts of intermediate tree cover remain unexplored. We developed and tested an optimum tree cover theory in which groundwater recharge is maximized at an intermediate tree density. Below this optimal tree density the benefits from any additional trees on water percolation exceed their extra water use, leading to increased groundwater recharge, while above the optimum the opposite occurs. Our results, based on groundwater budgets calibrated with measurements of drainage and transpiration in a cultivated woodland in West Africa, demonstrate that groundwater recharge was maximised at intermediate tree densities. In contrast to the prevailing view, we therefore find that moderate tree cover can increase groundwater recharge, and that tree planting and various tree management options can improve groundwater resources. We evaluate the necessary conditions for these results to hold and suggest that they are likely to be common in the seasonally dry tropics, offering potential for widespread tree establishment and increased benefits for hundreds of millions of people.

Introduction

Two-thirds of the world’s population may live in water-limited regions by 20251. In Africa about 340 million people already lack access to adequate and hygienic sources of water, such as groundwater2. Limited water constrains food production, nutrition, and health as well as impacting opportunities for education, work, and improved livelihoods. Reliable access to clean water is essential for achieving the UN Sustainable Development Goals.

Forests have often been described as ‘sponges’ storing rain water and slowly releasing it to maintain groundwater and streams during dry periods3,4,5. Formerly, this sponge theory and related ideas motivated policies aimed at conserving and restoring forests5,6,7. In recent decades, however, these ideas have lost credibility as studies show that forest clearance generally leads to increased and afforestation to reduced water yields3,8,9. Therefore a contrasting trade-off theory–in which more trees means less water–has become the dominant paradigm. This trade-off theory predicts that as tree densities increase, water losses from transpiration and interception dominate their hydrological effects6,8,10.

In the tropics the trade-off theory rests on limited evidence. The few available studies compare extremes: open land versus closed forest, or grasslands versus dense plantations11. Despite the recognition that trees can improve soil hydraulic conductivity and reduce overland water flow4,12,13, and other findings that question the generality of the trade-off theory14,15,16, we find no available data on the relationship between tree cover and water yields at intermediate tree densities, and few data concerning the specific mechanisms that determine groundwater reserves and dry season stream flows11,12,17. The neglect of intermediate tree cover is a striking omission given the importance of such open vegetation in terms of extent and biodiversity18, and the fact that it supports many of the world’s poorest people. In Africa, there are 350 Mha of open and fragmented forests and 514 Mha of other wooded lands (including savannah, agroforests etc.)19–more than the area under closed forest and plantations (277 and 8 Mha, respectively). Such open vegetation also plays a major role in the global carbon balance; regions with 10 to 30% tree canopy cover are estimated to store 23% of the total forest biomass carbon stock (above- and belowground) in sub Saharan Africa and 15% of the total forest carbon stocks for the global tropics20. Tree planting is, or would be, a major element in many climate mitigation projects, in efforts to combat desertification, and in livelihood focused development proposals seeking to improve access to firewood and other products. But the trade-off theory has reduced the application of such planting projects due to concerns that these efforts would jeopardize scarce water resources6,8,9,10,21,22.

Here we present and test an optimum tree cover theory for groundwater recharge that can reconcile the available scientific evidence and contrasting perceptions about forests and groundwater in the seasonally dry tropics (Fig. 1). We hypothesized that under conditions typical of the seasonally dry tropics an intermediate tree cover maximizes groundwater recharge. Below this optimum cover, the hydrological benefits gained from more trees outweigh their extra water use, while at higher values of tree cover the water use from additional trees exceeds any positive effect they might have on groundwater recharge (Fig. 1). We recognize that the tree cover value where this optimum occurs depends on various factors including tree species, local soil and climatic conditions. But before considering these influences we needed to evaluate our theory. We chose a common African semi-arid landscape known as “agroforestry parkland” where water shortage is a recognised livelihood constraint23. Parklands, in which annual crops are grown under scattered trees, constitute the most extensive farming system in semi-arid West Africa23. Indeed, about 1.9 million km2 (47%) of the total agricultural land in Sub-Saharan Africa has a tree cover above 10%24. We want to know how such intermediate tree cover influences groundwater recharge.

Figure 1: Conceptual water budget of the optimum tree cover theory.
Figure 1

Optimum groundwater recharge occurs at intermediate tree cover in seasonally dry tropical areas. Without trees, surface runoff and soil evaporation are high, leading to low groundwater recharge despite low transpiration. In closed productive forests, despite low surface runoff and soil evaporation, total transpiration and interception are high, again leading to low groundwater recharge. At an intermediate canopy cover, low surface runoff and evaporation as well as intermediate transpiration optimize groundwater recharge. The pan-sharpened satellite images were created from a WorldView-2 image from 21 October 2012 using ERDAS Imagine 2013 software (http://www.hexagongeospatial.com/products/producer-suite/erdas-imagine).

Results

Our results confirm that groundwater recharge can be maximized at a specific, non-zero, tree cover. We used field measurements of subsurface drainage and tree transpiration to model groundwater recharge in a water budget model related to tree cover and its spatial distribution (see Methods section). Field data from wick lysimeters revealed that the percentage of the yearly rainfall percolating to 1.5 m soil depth reached its maximum of 16% of the annual rainfall around the edge of the tree canopy, 4.4 m from the nearest tree stem, and decreased to 1.3% in open areas, 37 m away from the nearest tree (Fig. 2). This decrease in drainage with increasing distance from the canopy edge followed a linear relationship (y = 18.1 − 0.46x(SE 0.12); where y is the drainage at 1.5 m soil depth and x the distance to the nearest tree stem; r2adj = 0.69; p = 0.013). For distances beyond 37 m we assumed that the drainage was 1.3%, which corresponds to the lowest observed value. Therefore, less water is available for groundwater recharge in locations further from any tree and resulting in negligible groundwater recharge when trees are absent.

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