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

Bargués Tobella A., Reese H., Almaw A., Bayala J., Malmer A., Laudon H., Ilstedt U.

Water scarcity constrains the livelihoods of millions of people in tropical drylands. Tree planting in these environments is generally discouraged due to the large water consumption by trees, but this view may neglect their potential positive impacts on water availability. The effect of trees on soil hydraulic properties linked to groundwater recharge is poorly understood. In this study, we performed 18 rainfall simulations and tracer experiments in an agroforestry parkland in Burkina Faso to investigate the effect of trees and associated termite mounds on soil infiltrability and preferential flow. The sampling points were distributed in transects each consisting of three positions: (i) under a single tree, (ii) in the middle of an open area, and (iii) under a tree associated with a termite mound. The degree of preferential flow was quantified through parameters based on the dye infiltration patterns, which were analyzed using image analysis of photographs. Our results show that the degree of preferential flow was highest under trees associated with termite mounds, intermediate under single trees, and minimal in the open areas. Tree density also had an influence on the degree of preferential flow, with small open areas having more preferential flow than large ones. Soil infiltrability was higher under single trees than in the open areas or under trees associated with a termite mound. The findings from this study demonstrate that trees have a positive impact on soil hydraulic properties influencing groundwater recharge, and thus such effects must be considered when evaluating the impact of trees on water resources in drylands.Trees in dryland landscapes increase soil infiltrability and preferential flow Termite mounds in association with trees further enhance preferential flow.

Ogden F.L., Crouch T.D., Stallard R.F., Hall J.S.

Ghimire C.P., Bonell M., Bruijnzeel L.A., Coles N.A., Lubczynski M.W.

[1] Severely degraded hillslopes in the Lesser Himalaya challenge local communities as a result of the frequent occurrence of overland flow and erosion during the rainy season and water shortages during the dry season. Reforestation is often perceived as an effective way of restoring predisturbance hydrological conditions but heavy usage of reforested land in the region has been shown to hamper full recovery of soil hydraulic properties. This paper investigates the effect of reforestation and forest usage on field-saturated soil hydraulic conductivities (Kfs) near Dhulikhel, Central Nepal, by comparing degraded pasture, a footpath within the pasture, a 25 year old pine reforestation, and little disturbed natural forest. The hillslope hydrological implications of changes in Kfs with land-cover change were assessed via comparisons with measured rainfall intensities over different durations. High surface and near-surface Kfs in natural forest (82–232 mm h−1) rule out overland flow occurrence and favor vertical percolation. Conversely, corresponding Kfs for degraded pasture (18–39 mm h−1) and footpath (12–26 mm h−1) were conducive to overland flow generation during medium- to high-intensity storms and thus to local flash flooding. Pertinently, surface and near-surface Kfs in the heavily used pine forest remained similar to those for degraded pasture. Estimated monsoonal overland flow totals for degraded pasture, pine forest, and natural forest were 21.3%, 15.5%, and 2.5% of incident rainfall, respectively, reflecting the relative ranking of surface Kfs. Along with high water use by the pines, this lack of recovery of soil hydraulic properties under pine reforestation is shown to be a critical factor in the regionally observed decline in base flows following large-scale planting of pines and has important implications for regional forest management.

Krishnaswamy J., Bonell M., Venkatesh B., Purandara B.K., Rakesh K.N., Lele S., Kiran M.C., Reddy V., Badiger S.

Summary The hydrologic effects of forest use and reforestation of degraded lands in the humid tropics has implications for local and regional hydrologic services but such issues have been relatively less studied when compared to the impacts of forest conversion. In particular, the “infiltration-evapotranspiration trade-off” hypothesis which predicts a net gain or loss to baseflow and dry-season flow under both, forest degradation or reforestation depending on conditions has not been tested adequately. In the Western Ghats of India, we examined the hydrologic responses and groundwater recharge and hydrologic services linked with three ecosystems, (1) remnant tropical evergreen forest (NF), (2) heavily-used former evergreen forest which now has been converted to tree savanna, known as degraded forest(DF), and (3) exotic Acacia plantations (AC, Acacia auriculiformis) on degraded former forest land. Instrumented catchments ranging from 7 to 23 ha representing these three land-covers (3 NF, 4 AC and 4 DF, in total 11 basins), were established and maintained between 2003 and 2005 at three sites in two geomorphological zones, Coastal and Up-Ghat (Malnaad). Four larger (1–2 km2) catchments downstream of the head-water catchments in the Malnaad with varying proportions of different land-cover and providing irrigation water for areca-nut and paddy rice were also measured for post-monsoon baseflow. Daily hydrological and climate data was available at all the sites. In addition, 36 min data was available at the Coastal site for 41 days as part of the opening phase of the summer monsoon, June–July 2005. Low potential and actual evapotranspiration rates during the monsoon that are similar across all land-cover ensures that the main control on the extent of groundwater recharge during the south-west monsoon is the proportion of rainfall that is converted into quick flow rather than differences in evapotranspiration between the different land cover types. The Flow duration curves demonstrated a higher frequency and longer duration of low flows under NF when compared to the other more disturbed land covers in both the Coastal and Malnaad basins. Groundwater recharge estimated using water balance during the wet-season in the Coastal basins under NF, AC and DF was estimated to be 50%, 46% and 35% respectively and in the Malnaad it was 61%, 55% and 36% respectively. Soil Water Infiltration and Movement (SWIM) based recharge estimates also support the pattern (46% in NF; 39% in AC and 14% in DF). Furey–Gupta filter based estimates associated with the Coastal basins also suggest similar groundwater recharge values and trends across the respective land-covers: 69% in NF, 49% in AC, and 42% in DF. Soil water potential profiles using zero flux plane methods suggest that during the dry-season, natural forests depend on deep soil moisture and groundwater. Catchments with higher proportion of forest cover upstream were observed to sustain flow longer into the dry-season. These hydrologic responses provide some support towards the “infiltration-evapotranspiration trade-off” hypothesis in which differences in infiltration between land-cover rather than evapotranspiration determines the differences in groundwater recharge, low flows and dry-season flow. Groundwater recharge is the most temporally stable under natural forest, although substantial recharge occurs under all three ecosystems, which helps to sustain dry-season flow downstream in higher order streams that sustain local communities and agro-ecosystems. In addition to spatial scale effects, greater attention also needs to be given to the role of hydrogeology within the context of the above hypothesis and its implications for hydrologic services.

Beck H.E., Bruijnzeel L.A., van Dijk A.I., McVicar T.R., Scatena F.N., Schellekens J.

Abstract. Although regenerating forests make up an increasingly large portion of humid tropical landscapes, little is known of their water use and effects on streamflow (Q). Since the 1950s the island of Puerto Rico has experienced widespread abandonment of pastures and agricultural lands, followed by forest regeneration. This paper examines the possible impacts of these secondary forests on several Q characteristics for 12 mesoscale catchments (23–346 km2; mean precipitation 1720–3422 mm yr−1) with long (33–51 yr) and simultaneous records for Q, precipitation (P), potential evaporation (PET), and land cover. A simple spatially-lumped, conceptual rainfall–runoff model that uses daily P and PET time series as inputs (HBV-light) was used to simulate Q for each catchment. Annual time series of observed and simulated values of four Q characteristics were calculated. A least-squares trend was fitted through annual time series of the residual difference between observed and simulated time series of each Q characteristic. From this the total cumulative change (Â) was calculated, representing the change in each Q characteristic after controlling for climate variability and water storage carry-over effects between years. Negative values of  were found for most catchments and Q characteristics, suggesting enhanced actual evaporation overall following forest regeneration. However, correlations between changes in urban or forest area and values of  were insignificant (p ≥ 0.389) for all Q characteristics. This suggests there is no convincing evidence that changes in the chosen Q characteristics in these Puerto Rican catchments can be ascribed to changes in urban or forest area. The present results are in line with previous studies of meso- and macro-scale (sub-)tropical catchments, which generally found no significant change in Q that can be attributed to changes in forest cover. Possible explanations for the lack of a clear signal may include errors in the land cover, climate, Q, and/or catchment boundary data; changes in forest area occurring mainly in the less rainy lowlands; and heterogeneity in catchment response. Different results were obtained for different catchments, and using a smaller subset of catchments could have led to very different conclusions. This highlights the importance of including multiple catchments in land-cover impact analysis at the mesoscale.

Alvarado-Barrientos M.S., Hernández-Santana V., Asbjornsen H.

Characterizing the variability of the radial profiles of sap velocity (vs) is a critical step to improve upscaling point measurements of vs to whole-tree sap flow. One promising approach is the use of a probability distribution function (pdf) to model radial profiles of vs, because shape parameters could potentially be generalized to trees of the same species based on the premise that the shape remain consistent regardless of the tree size and age, and over time, even though the magnitude of vs may vary with environmental conditions. The objective of this study was to characterize and assess the variability of the radial profile and to examine the validity of the premises underlying this approach by applying it to Pinus patula, one of the most widely planted tree species in the uplands of central-eastern Mexico. We measured vs with the Heat Ratio method at various sapwood depths in 18 P. patula trees with a dbh between 7.3 and 59.7 cm and age between of 10 and 34 years, over a period of 1.5 years. Trees were growing in two stands: a mature forest stand and a young plantation. By fitting the Beta-pdf to hourly radial profiles of vs, we derived a lumped shape parameter (ρ) to denote the radial position relative to sapwood depth with average vs and a scaling parameter (cs). The typical radial profile was unimodal, asymmetrical and with peak vs generally within the outermost 20–33% of the sapwood depth. However, tree-to-tree variability in ρ was considerable among trees within the same stand and also across stands. Long-term and day-to-day variation of ρ was marginal. The hourly dynamics of the radial profile, characterized by cs, can be explained by a linear combination of incoming shortwave radiation, vapor pressure deficit, the hour of day and their interaction (r2 = 0.74). An independent field evaluation confirmed that a radial profile of fixed shape can be effectively used to estimate whole-tree sap flow with relatively low bias (4–26% underestimation) relative to cut-tree water uptake, particularly for trees for which vs observations covered at least 60% of the sapwood depth. Our findings emphasize the importance of conducting multiple vs point measurements covering most of the sapwood depth for accurate characterization of the radial profile, and demonstrate the utility of fitting a pdf to point vs measurements in order to assess the variability of vs radial profiles as well as to compute sap flow at the whole-tree level.

Rockström J., Falkenmark M., Lannerstad M., Karlberg L.

[1] This paper analyses the potential conflict between resilience of the Earth system and global freshwater requirements for the dual task of carbon sequestration to reduce CO2in the atmosphere, and food production to feed humanity by 2050. It makes an attempt to assess the order of magnitude of the increased consumptive water use involved and analyses the implications as seen from two parallel perspectives; the global perspective of human development within a “safe operating space” with regard to the definition of the Planetary Boundary for freshwater; and the social-ecological implications at the regional river basin scale in terms of sharpening water shortages and threats to aquatic ecosystems. The paper shows that the consumptive water use involved in the dual task would both transgress the proposed planetary boundary range for global consumptive freshwater use and would further exacerbate already severe river depletion, causing societal problems related to water shortage and water allocation. Thus, strategies to rely on sequestration of CO2 as a mitigation strategy must recognize the high freshwater costs involved, implying that the key climate mitigation strategy must be to reduce emissions. The paper finally highlights the need to analyze both water and carbon tradeoffs from anticipated large scale biofuel production climate change mitigation strategy, to reveal gains and impact of this in contrast to carbon sequestration strategies.

Kume T., Otsuki K., Du S., Yamanaka N., Wang Y., Liu G.

In this study, we aimed to clarify spatial variations in xylem sap flow, and to determine the impacts of these variations on stand-scale transpiration (E) estimates. We examined circumferential and radial variations in sap flow velocity (Fd) measured at several directions and depths in tree trunks of black locust (Robinia pseudoacacia) and native oak (Quercus liaotungensis), both of which have ring-porous wood anatomy, in forest stands on the Loess Plateau, China. We evaluated the impacts of circumferential variations in Fd on stand-scale transpiration estimates using a simple scaling exercise. We found significant circumferential variations in Fd in the outermost xylem in both species (coefficients of variation = 20–45%). For both species, Fd measured at the inner xylem was smaller than that of the outermost xylem and the Fd at the depth of > 10 mm was almost zero. The simple exercises showed that omitting circumferential variations in Fd affected the E estimate by 16–21%, which was less than the effects of omitting within-tree radial and tree-to-tree variations in Fd in both species. These results suggest that circumferential variations in Fd can be a minor source of error for E estimates compared with within-tree radial and tree-to-tree variations in Fd, regardless of the significant circumferential variations. Copyright © 2011 John Wiley & Sons, Ltd.

Saatchi S.S., Harris N.L., Brown S., Lefsky M., Mitchard E.T., Salas W., Zutta B.R., Buermann W., Lewis S.L., Hagen S., Petrova S., White L., Silman M., Morel A.

Developing countries are required to produce robust estimates of forest carbon stocks for successful implementation of climate change mitigation policies related to reducing emissions from deforestation and degradation (REDD). Here we present a “benchmark” map of biomass carbon stocks over 2.5 billion ha of forests on three continents, encompassing all tropical forests, for the early 2000s, which will be invaluable for REDD assessments at both project and national scales. We mapped the total carbon stock in live biomass (above- and belowground), using a combination of data from 4,079 in situ inventory plots and satellite light detection and ranging (Lidar) samples of forest structure to estimate carbon storage, plus optical and microwave imagery (1-km resolution) to extrapolate over the landscape. The total biomass carbon stock of forests in the study region is estimated to be 247 Gt C, with 193 Gt C stored aboveground and 54 Gt C stored belowground in roots. Forests in Latin America, sub-Saharan Africa, and Southeast Asia accounted for 49%, 25%, and 26% of the total stock, respectively. By analyzing the errors propagated through the estimation process, uncertainty at the pixel level (100 ha) ranged from ±6% to ±53%, but was constrained at the typical project (10,000 ha) and national (>1,000,000 ha) scales at ca . ±5% and ca . ±1%, respectively. The benchmark map illustrates regional patterns and provides methodologically comparable estimates of carbon stocks for 75 developing countries where previous assessments were either poor or incomplete.

Cannavo P., Sansoulet J., Harmand J.-., Siles P., Dreyer E., Vaast P.

▶ Good water balance simulations in coffee monoculture and coffee agroforestry systems. ▶ No water competition risk between coffee plants and shade trees. ▶ Important water drainage (1500–2100 mm, >55% of annual rainfall) below 200 cm depth. ▶ Climatic scenarios tested over the possible water competition in agroforestry system. The shade impact by Inga densiflora on water use and drainage in a coffee agroforestry system (AFS) was compared to coffee monoculture (MC) in Costa Rica. Rainfall interception, transpiration, runoff and soil water content were monitored during 3 years. Runoff was lower in AFS than MC (5.4 and 8.4% of total rainfall, respectively) and a higher water infiltration was observed under AFS. Still, the higher combined rainfall interception + transpiration of coffee and shade trees in AFS resulted in a lower drainage than in MC. No coffee water stress was recorded either in AFS or MC as relative extractable soil water remained above 20% during the dry seasons. Time course of soil water content showed enhanced access to soil water between 100 and 200 cm depth in AFS. This suggests complementarity for soil water between coffee and shade trees. The model HYDRUS 1D predicted that drainage at 200 cm depth accounted for a large fraction of annual rainfall (68% for MC and 62% for AFS). Climatic scenario simulations showed (1) a potential competition for water between coffee and shade trees when the dry season was extended by 4–6 weeks compared to actual, and (2) a severe reduction in annual drainage, but without competition for water when rainfall was reduced down to 40% of the actual.

MALMER A., MURDIYARSO D., (SAMPURNO) BRUIJNZEEL L.A., ILSTEDT U.

Tree planting in the tropics is conducted for a number of reasons including carbon sequestration, but often competes with increasingly scarce water resources. The basics of forest and water relations are frequently said to be well understood but there is a pressing need to better understand and predict the hydrological effects of land-use and climate change in the complex and dynamic landscapes of the tropics. This will remain elusive without the empirical data required to feed hydrological process models. It is argued that the current state of knowledge is confused by too broad a use of the terms ‘forest' and ‘(af)forestation', as well as by a bias towards using data generated mostly outside the tropics and for nondegraded soil conditions. Definitions of forest, afforestation and reforestation as used in the climate change community and their application by land and water managers need to be reconciled.

Manning A.D., Gibbons P., Lindenmayer D.B.

Summary 1. Facilitating adaptive responses of organisms in modified landscape will be essential to overcome the negative effects of climate change and its interaction with land use change. Without such action, many organisms will be prevented from achieving the predicted range shifts they need to survive. 2. Scattered trees are a prominent feature of many modified landscapes, and could play an important role in facilitating climate change adaptation. They are keystone structures because of the disproportionally large ecological values and ecosystem services that they provide relative to the area they occupy in these landscapes. The provision of habitat and connectivity will be particularly relevant. 3. Scattered trees are declining in modified landscapes due to elevated tree mortality and poor recruitment often associated with intensive land use. The continuing global decline of scattered trees will undermine the capacity of many organisms to adapt to climate change. 4. Synthesis and applications. The sustainable management of scattered trees in modified landscapes could complement other strategies for facilitating climate change adaptation. They create continuous, though sparse, vegetation cover that permits multi-directional movements of biota across landscapes and ecological networks. They have the capacity to span ecosystems and climatic gradients that cannot be captured in formal reserves alone. The management of scattered trees should be an integral part of conservation objectives and agricultural activities in modified landscapes. Public investment, through mechanisms such as agri-environmental schemes, in rotational grazing, temporary set-asides, tree-planting and regulations that reduce clearing and early mortality among standing trees will improve the capacity of biota to adapt to climate change.

Zomer R.A., Trabucco A., Coe R., Place F.

Deblauwe V., Barbier N., Couteron P., Lejeune O., Bogaert J.

Aim Vegetation exhibiting landscape-scale regular spatial patterns has been reported for arid and semi-arid areas world-wide. Recent theories state that such structures are bound to low-productivity environments and result from a self-organization process. Our objective was to test this relationship between periodic pattern occurrence and environmental factors at a global scale and to parametrize a predictive distribution model. Location Arid and semi-arid areas world-wide. Methods We trained an empirical predictive model (Maxent) for the occurrence of periodic vegetation patterns, based on environmental predictors and known occurrences verified on Landsat satellite images. Results This model allowed us to discover previously unreported pattern locations, and to report the first ever examples of spotted patterns in natural systems. Relationships to the main environmental drivers are discussed. Main conclusions These results confirm that periodic patterned vegetations are ubiquitous at the interface between arid and semi-arid regions. Self-organized patterning appears therefore to be a biome-scale response to environmental conditions, including soil and topography. The set of correlations between vegetation patterns and their environmental conditions presented in this study will need to be reproduced in future modelling attempts.

Barbier N., Couteron P., Lefever R., Deblauwe V., Lejeune O.

Karunakalage A., Sharma R., Daqiq M.T., Kannaujiya S.

Henao Casas J.D., Fernández Escalante E., Richard-Cerda J.C., Ayuga F.

Chazdon R.L., Blüthgen N., Brancalion P.H., Heinrich V., Bongers F.

Rapiya M., Ramoelo A.

Water scarcity is a growing global issue, especially in arid and semi-arid rangelands, primarily due to climate change and population growth. Groundwater is a crucial resource for vegetation in these ecosystems, yet its role in supporting plant life is often not fully understood. This review explores the interactions between groundwater and vegetation dynamics in various rangeland types. Groundwater serves as a critical water source that helps sustain plants, but changes in its availability, depth, and quality can significantly impact plant health, biodiversity, and ecosystem stability. Research indicates that groundwater depth affects vegetation types and their distribution, with specific plants thriving at certain groundwater levels. For instance, in grasslands, shallow groundwater can support diverse herbaceous species, while deeper conditions may favor drought-tolerant shrubs and trees. Similarly, in forest ecosystems, extensive root systems access both groundwater and soil moisture, playing a vital role in water regulation. Savanna environments showcase complex interactions, where trees and grasses compete for water, with groundwater potentially benefiting trees during dry seasons. Climate change poses additional challenges by altering rainfall patterns and temperatures, affecting groundwater recharge and availability. As a result, it is crucial to develop effective management strategies that integrate groundwater conservation with vegetation health. Innovative monitoring techniques, including remote sensing, can provide valuable information about groundwater levels and their impact on vegetation, enhancing water resource management. This review emphasizes the importance of understanding groundwater–vegetation interactions to guide sustainable land and water management practices. By enhancing our knowledge of these connections and utilizing advanced technologies, we can promote ecosystem resilience, secure water resources, and support biodiversity in rangeland systems. Collaborative efforts among local communities, scientists, and policymakers are essential to address the pressing issues of water scarcity and to ensure the sustainability of vital ecosystems for future generations.

Nayak N.P.

Pandey K.P., Wellstein C., Bräuning A., Bhuju D.R.

Climate change has affected forest ecosystems across the world over the past century. However, its impact is particularly high in the Himalayas due to increasing temperatures, extreme precipitation events, and regional droughts. In this context, a review of the current stage of research was deemed necessary to understand the adaptation of a key conifer species to climate variability in the Central Himalayas. Hence, we conducted a systematic review of published peer-reviewed journal articles addressing the growth performance of Abies spectabilis (D. Don) Spach in the Central Himalayas. From this review, three main patterns of climate response have emerged: a positive correlation of radial tree growth with temperature of the current and previous growing seasons, tree growth limitation by winter temperature, and by temperature or moisture in the pre-monsoon season. Overall, results indicate an elevation-dependent temperature sensitivity, a crucial role of moisture availability, and seasonal shifts in climate–growth relationships, reflecting the species’ adaptability to changing climate conditions. Our review revealed that studies on elevation-dependent adaptation of wood anatomical traits by A. spectabilis are still rare. The tree-ring growth of this species shows a complex response to climate variability, with increasing as well as decreasing growth trends across its distribution range.

Ruijsch J., Taylor C.M., Hutjes R.W., Teuling A.J.

Mlambo D., Mufandaedza E.

The climate crisis poses a substantial risk to achieving global food security and sustainability. This chapter presents an analysis of the current level of knowledge regarding agroforestry's ability to promote simultaneously climate resilience, food availability, and eco-friendly land use. Agroforestry systems (AFS) are recognized as potent carbon (C) sinks, effectively reducing atmospheric carbon dioxide (CO2) levels. It is estimated that trees in AFS can sequester 0.3-24 tC ha-1 yr-1. AFS, which occupy about 1 billion ha of land worldwide, have potential to remove more than 10GtC yr-1 from the atmosphere if fully optimized. The C sequestration benefits can be monetized by farmers, providing a supplementary income source through the C credit market. In addition to C sequestration, AFS can provide numerous co-benefits such as food, biodiversity conservation, soil erosion control, and water cycling. Agroforestry is a key strategy for counteracting excessive C emissions.

Guzmán-Rojo M., Silva de Freitas L., Coritza Taquichiri E., Huysmans M.

In response to the escalating frequency and severity of wildfires, this study carried out a preliminary assessment of their impact on groundwater systems by simulating post-fire effects on groundwater recharge. The study focuses on the El Sutó spring area in Santa Cruz, Bolivia, a region that is susceptible to water scarcity and frequent wildfires. The United States Geological Survey (USGS) Soil-Water-Balance model version 2.0 was utilized, adjusting soil texture and infiltration capacity parameters to reflect the changes induced by wildfire events. The findings indicated a significant decrease in groundwater recharge following a hypothetical high-severity wildfire, with an average reduction of approximately 39.5% in the first year post-fire. A partial recovery was modeled thereafter, resulting in an estimated long-term average reduction of 10%. Based on these results, the El Sutó spring was provisionally classified as having high vulnerability shortly after a wildfire and moderate vulnerability in the extended period. Building on these model-based impacts, a preliminary Fire-Related Forest Recharge Impact Score (FRIS) was proposed. This index is grounded in soil properties and recharge dynamics and is designed to assess hydrological vulnerability after wildfires in dry tropical forests. Although these findings remain exploratory, they offer a predictive framework intended to guide future studies and inform strategies for managing wildfire impacts on groundwater resources.

Tolentino P.L., Williams R.D., Hurst M.D.

ABSTRACTNatural flood management (NFM) has gained prominence as a flood risk management approach in temperate settings but lacks extensive applied examples and evidence in tropical settings, despite significant ecosystem degradation and high flood risk exposure. Tropical river catchments often experience highly variable hydrographs (i.e., prone to flash floods) and intense rainfall from monsoon and typhoon‐dominated weather systems that can cause landslides and sediment‐transporting river flows. These conditions provide a backdrop to the prospects for NFM in tropical Southeast Asia, of which the Philippines is representative. Catchments in the country are typically small and thus associated with short hydrological response times. They are also characterized by diversity of river types, high rates of lateral mobility, extensive downstream urbanization, and complex land use mosaics at the coast. Consideration of NFM as a conceptual framework in the Philippines may enable conversations about adapting existing flood risk management approaches. To explore these NFM alternatives, we conceptualize opportunities in a typical catchment that we divide into four nested, connected parts: managing headwaters as sponges; conserving and restoring river and floodplain width; blue‐green infrastructure in urban areas; and maintaining and creating space for water in fluvial‐coastal settings. There is potential in countries such as the Philippines to adopt NFM strategies that have shown promise in temperate regions and select Southeast Asian countries, where emerging evidence supports their effectiveness. Monitoring tropical NFM interventions remains crucial to gather evidence supporting broader application of nature‐based solutions for flood risk mitigation and biodiversity loss in tropical Southeast Asia.

Oliveira J., Karlson M., Ouédraogo A.S., Bazié H.R., Ostwald M.

Malapane C., Dube T., Dalu T.

Ruiz I., Alloza J.A., Sanz M.J., Zafra-Calvo N., Lliso B., Brugnach M.

Current knowledge of the role that forests play in recycling rainwater has led to the proposal of forest management practices to mitigate desertification in the western Mediterranean basin. Building upon this hypothesis, we present the Mijares watershed in eastern Spain as a detailed case study. We then perform a scientific literature review in the context of the western Mediterranean basin to identify the physical and biophysical uncertainties associated with forest management practices that aim to strengthen the atmospheric and terrestrial components of the water cycle. The review identified eleven interconnected uncertainties that result from complex vegetation, climate, soil, and water relationships. We discuss the need to deepen our understanding of the atmospheric dynamics of the Mijares by accounting for the role of forests in recycling rainfall and by adopting an adaptive management approach to apply the lessons learned in the territory. Based on these considerations, we propose forest management practices to reinforce the water cycle of the watershed. Our work contributes to understanding the uncertainties arising from forest management practices aimed at mitigating desertification, thereby supporting decision-making. These insights apply to other western Mediterranean coastal watersheds.

François M., Junior T.R., Mielke M.S., Rousseau A.N., Faria D., Mariano-Neto E.

The role of trees in watershed hydrology is governed by many environmental factors along with their inherent characteristics and not surprisingly has generated diverse debates in the literature. Herein, this conceptual meta-analysis provides an opportunity to propose a conceptual model for understanding the role of trees in watershed hydrology and examine the conditions under which they can be an element that increases or decreases water supply in a watershed. To achieve this goal, this conceptual meta-analysis addressed the interaction of forest cover with climatic conditions, soil types, infiltration, siltation and erosion, water availability, and the diversity of ecological features. The novelty of the proposed conceptual model highlights that tree species and densities, climate, precipitation, type of aquifer, and topography are important factors affecting the relationships between trees and water availability. This suggests that forests can be used as a nature-based solution for conserving and managing natural resources, including water, soil, and air. To sum up, forests can reduce people’s footprint, thanks to their role in improving water and air quality, conserving soil, and other ecosystem services. The outcomes of this study should be valuable for decision-makers in understanding the types of forests that can be used in an area, following an approach of environmental sustainability and conservation aiming at restoring hydrological services, mitigating the costs of environmental services, promoting sustainable land use, managing water resources, and preserving and restoring soil water availability (SWA) when investing in reforestation for watershed hydrology, which is important for the human population and other activities.

Kebede M.M., Kumar M., Mekonnen M.M., Clement T.P.

Nature-based solutions (NbSs) for water involve using or mimicking natural processes to contribute to the improved management of water. Although NbSs are gaining a significant amount of scientific attention, to ensure their wide usage for enhancing groundwater recharge, there is a need for clear documentation outlining their benefits and barriers. In this study, a systematic literature review was carried out to evaluate the application of NbSs for managing groundwater recharge. First, NbS approaches were classified into two broad groups: managed aquifer recharge (MAR) and ancillary recharge methods (ARMs). MAR includes all activities that intentionally enhance the recharge of an aquifer for later recovery, while ARMs include all the remaining NbSs wherein recharge enhancement is a secondary goal. In 50 out of 61 reviewed studies, MAR was reported to be successful in increasing recharge. However, in the remaining studies, reductions in recharge rates were reported. Most of the NbSs that failed to improve groundwater recharge were from the ARMs group. This group had little consensus among studies regarding the effectiveness of NbSs on groundwater recharge. In this study, we also identified opportunities and challenges, such as gaps in our knowledge of NbSs’ effectiveness, their assessment in long-term, cost–benefit analysis and scalability. Addressing these challenges will further enhance the efficiency of NbSs, which indeed is a promising alternative for enhancing groundwater resources.