Guidance

Appendix 2: Nature-based solutions for water resources literature review

Published 17 August 2026

Applies to England

There are multiple parts of the water cycle, and locations within catchments, where nature-based solutions (NbS) for water resources could be implemented.

This is a literature review of the key NbS for water resources proposals, namely: 

• changes in water and land use 
• changes in infiltration and runoff splits   
• capturing runoff and changes in infiltration  
• changing floodplains and river restoration 

Methods and purpose

This systematic literature review was done by conducting journal searches using key search terms. This was supplemented with materials provided by the project steering group.

Evidence was taken first from UK-based research. In the absence of this, research from other countries was used. 

The focus of many sources used here is not on NbS for water resources. For example, work on infiltration basins has focused on their effectiveness in dealing with runoff rather than their effects on aquifer recharge.

This review aims to identify some key inferences for NbS for water resources, setting out key issues, common themes and general principles.

This literature review was completed on 1 July 2025.

This review is not comprehensive or exhaustive due to:

• the broad scope of NbS for water resources
• the limited literature which focuses on it

1. Water use and land use change 

Spatial variations in land use, or human interventions to land use, can significantly affect water uptake by altering:

• evaporation
• transpiration losses
• irrigation demands
• rainfall interception

These can lead to changes in groundwater resources and stream flow.

This section reviews literature on 3 land cover management practices:  

1. Changing to crops that have lower evaporation rates and water demand. 
2. Adding cover crops compared to bare soil. 
3. Changes in woodland management. 

Crop choice, evapotranspiration rates and water demand 

Key findings from this review are that:  

• there are large variations in evapotranspiration between crops and between crop management regimes
• converting to crops with lower water demands can increase infiltration rates, soil water storage and recharge – this can translate to increases in river flows, but the effect is less pronounced
• water demands change seasonally – crops with a critical growing season outside of drier seasons have reduced irrigation demands and could optimise water resources
• catchment characteristics (soil, geology, topography) can influence the impact of crop changes, emphasising the need for site-specific advice or catchment management approaches

Full literature review

Evapotranspiration comprises soil and plant surface evaporation and the water usage by a crop for growing and cooling, taken up by its rooting system from the soil root zone (Al-Kaisi, 2000). This will vary:

• from species to species
• with soil type
• throughout the year

Agriculture is responsible for around 70% of water abstractions globally (UN Environment-DHI, 2018). Therefore, changing to lower water demand crops can significantly benefit water resources. 

Change in crop type can significantly influence annual evapotranspiration rates. Studies have also evaluated how intentional crop changes affect evapotranspiration and stream flow. Holder and others (2019, USA) observed that converting grassland to Miscanthus species increased recharge by 40%, whereas short rotation coppice raised evapotranspiration rates and reduced water yield. However, these changes had a limited (but potentially significant) net effect on stream flow (plus or minus 5%). 

Zhang and Schilling (2005, USA) reported that between 1940 and 2003, stream flow in the Mississippi increased by 31 to 41%. This was due to more precipitation being routed as baseflow rather than storm flow, attributed to replacing perennial vegetation with seasonal row crops like soybeans. Reduced evapotranspiration and increased recharge may therefore have resulted in higher baseflow and stream flow (Zhang and Schilling, 2006). 

Qi and others (2011, USA) compared modified crop management with conventional crops. They found that modified crop management produced higher evapotranspiration rates but similar drainage, reducing surface runoff and likely decreasing stream flow. 

Groundwater modelling studies further support this. JBA Consulting (2023, UK) modelled the impact of converting arable land to horticultural crops in the River Wensum, River Cam and River Bure catchments. This was characterised by shallower root structures and intermittent growing seasons. The model results indicated that this would lead to:

• an 11% increase in interflow
• an 8% rise in baseflow
• a 5% increase in rapid runoff

It was found that river flows increased at all exceedance levels, with up to 14% increase in average flow (at Q35 to Q45). 

Specific crop transitions also influenced recharge:

• converting beet fields to potatoes increased recharge by 8 mm/year
• horticultural crops such as strawberries, apple trees and linseed added an additional 7 mm/year

Cropping regimes that promote earlier recharge initiation (for example, August) and lower water demand during winter provided optimal conditions for groundwater recharge. This suggests potential strategies for crop planning to maximise recharge. 

Seasonal changes in evaporation rates at different crop growth stages can also impact the seasonal pressures on water resources. Evaporation rates are highest after rainfall and irrigation, and will decrease as the crop canopy grows, covering wet soil and increasing the proportion of transpiration relative to evaporation (Al-Kaisi and Broner, 2014, USA).

The critical growth stages will also differ between crops, changing the timing of water usage. For example, the highest water use by corn occurs in July and August while water intense bloom occurs later for soybean crops, even though the total water usage is roughly the same (Al-Kaisi, 2000, USA). Therefore, one crop may have a greater pressure on water resources if it aligns with local periods of drought (Al-Kaisi, 2000, USA). Zhang and Schilling (2006) attributed the substantial impact of soybean production on Mississippi Valley stream flow over 40 years to the crop’s lower water demand and associated soil conservation practices. 

Reduction in water demand due to lower evapotranspiration or shorter crop seasons can reduce the irrigation requirements for abstraction of water. This can reduce the impact on low flows and increase resilience to drought (JBA Consulting, 2023, UK). This is a critical consideration in regions with limited water resources or during drought conditions.  

Catchment character can play a large role in determining the impact of these factors. A detailed study by Peskett and others (2023, UK) explored the interaction between land use changes and catchment characteristics in upland Scotland using isotopic and geochemical measurements. The study found that local soil, geology, and topography strongly influence catchment hydrology, often overshadowing the effects of land use changes. This highlights the importance of comprehensive hydrological assessments before proposing land use changes. Underlying physical characteristics may play a dominant role in shaping catchment water storage and flow dynamics. 

O’Connell and others (2004) also highlight the importance of local geology and temporal factors. Their study observed seasonal differences in soil moisture beneath arable crops during the growing season compared to grassland, but no significant differences in annual soil moisture in a chalk catchment. This highlights the need to consider site-specific conditions when evaluating the effects of crop selection on water resources. 

In summary, crop selection can influence water resources due to variations in:

• evapotranspiration
• total water demand
• seasonal water demand due to critical growing seasons of different crops

Catchment characteristics (soil, geology, topography) have also been found to have a greater effect on catchment hydrology than land use changes. 

Cover crops compared to bare soil 

Key findings from this review are that:  

• cover crops typically increase evapotranspiration, reducing soil moisture and groundwater recharge compared to bare soil
• in permeable catchments, the impact of cover crops on soil moisture relative to bare soils is less pronounced
• cover crops offer other recognised benefits relative to bare soils
• cover crops improve soil health, which can increase infiltration, and reduce damage during rainfall and runoff, improving water quality by reducing nitrate leaching and soil erosion

Full literature review

Cover crops are commonly recommended as part of land management practices to stabilise soils between seasonal or annual crop cycles. They are also proposed as NbS to reduce surface runoff and enhance soil health. 

Most studies (Meyer and others, 2019 - global; Qi and others, 2011 - USA; Blanchy and others, 2023 - Europe) found that cover crops:

• increase evapotranspiration
• reduce soil moisture
• decrease groundwater recharge compared to bare soils

The extent to which drainage and groundwater recharge are reduced varies, with no main determining factor found as to why based on meta-analyses of multiple studies (Meyer and others, 2019, global).  

The impact is less pronounced in permeable catchments. For example, Meyer and others (2019) and O’Connell and others (2004, UK) observed minimal differences in soil moisture under arable crops and grassland in a chalk catchment. O’Connell and others (2004) concluded that land use management had no measurable effect on aquifer decline in such contexts. 

While bare soils optimise recharge, they negatively affect water quality. Wang and others (2021, global), in a meta-analysis of 121 studies, found that cover crops could be integrated into NbS for water resource management. Though cover crops reduce soil water storage and rainfall retention compared to bare soils, they showed no adverse effects on crop yields. They also decreased evapotranspiration by 6.2% and improved crop water use efficiency (the ratio of plant biomass to water loss by transpiration) by 5%.  

Despite reduced groundwater recharge, cover crops offer recognised benefits relative to bare soils. Blanchy and others (2023, Europe), synthesising 161 studies, noted that ‘continuous living cover’:

• enhances carbon sequestration
• stimulates biological activity
• improves soil aggregation
• enhances bio-porosity

These improvements promote infiltration and reduce surface runoff. Cover crops also significantly improve water quality by limiting nitrate leaching into aquifers and reducing soil erosion. 

Changes in woodland management 

Key findings from this review are that:  

• woodland planting can increase water demand due to the high evapotranspiration rates of trees relative to other land use types – this can reduce groundwater recharge and stream flows  
• the impact varies with species, density, and woodland type – coniferous woodlands are generally more water intensive than deciduous woodlands 
• the impact of tree planting on water resources is scale dependent, with small-scale plantations having limited effects on streamflow 
• the complex impacts of woodlands demonstrate that not all NbS always benefit water resources
• woodland planting improves soil hydrological function over time (compared to pasture), storing water in soil and root systems, and slowly releasing it through seepage – expansion of woodlands may enhance water resources in the long term via soil recovery
• forests can intensify the hydrological cycle increasing the supply of precipitation leading to long-term increases in recharge
• understanding local geology, soil properties and topography is crucial for optimising woodland planting for water resource management 

Full literature review

Woodland planting is a widely adopted NbS due to its ecological and habitat creation benefits. It is also used to stabilise slopes and reduce flood risk as part of Natural Flood Management (NFM) strategies. Trees and forest soils can regulate water flows, storing water and slowly releasing it. The role of woodlands in attenuating surface runoff flows by slowing the velocity of runoff and reducing peak flows has been widely studied within a flood risk and ecological context (Pearson and others, 2025).  

However, the specific impact of afforestation for water resources appears to be highly variable and often negative, depending on cover, species, and density. This was reviewed as part of Multiple benefits of NbS: an evidence synthesis. This review considered the effect of woodland on water resources. Although in the short term it appears that woodlands, particularly conifers, have net negative impacts for water resources, broadleaf woodlands can have long-term benefits (Broadmeadow and Nisbet, 2024). 

Forest and forest crops generally have a high water demand. Fahey and Payne (2017, New Zealand) monitored forestry plantations over 22 years. They observed a 26% reduction in low flows and 33% reduction in annual water yields in forested plantations relative to unforested catchments. Similarly, a long-term monitoring study in Northern England found a 250 to 300mm annual streamflow reduction following the development of mature spruce forest over grassland (Birkinshaw and others, 2014, UK). Fennell and others (2023, UK) modelled changes in low flows in an upland Scottish catchment from tree planting and assessed the impact on local whisky production. The study found that tree planting led to reduced recharge which decreased low flows. This increased the number of days of low flows by 11 to 25 days relative to the baseline and impacting production at the distillery. 

Collins and others (2023, UK) highlights the negative impact that land-based NFM interventions may pose for water resources in groundwater-dominated catchments. Modelling the upper Thames catchment, they found that both tree planting and sustainable agriculture scenarios led to decreased low flows due to greater evapotranspiration. Broadscale planting of spruce had the greatest negative impact, due to high evaporative losses throughout the year. This led to lower recharge and a 39% reduction in Q95 flows, increasing drought intensity by 134% despite benefits for flood reduction. This highlights the importance of weighing up reduced low flows and recharge when considering flood risk management projects in permeable groundwater-dominated catchments. 

The type of woodland significantly influences water resources, with variations in:

• rooting depth
• density
• water uptake

Coniferous plantations generally have higher interception rates and greater water loss compared to deciduous woodlands (Robinson and others, 2003, Europe; O’Connell and others, 2004). Dunn and others (1995, UK) documented evapotranspiration rates for various land covers in the Tyne catchment (Table 1). Table 1 shows that evergreen woodland had over twice the annual evapotranspiration rate of felled woodland (bare soil), with an annual average rainfall of 752mm. Areas of dense evergreen woodland could therefore be expected to limit the contribution of rainfall to river flow. 

Table 1: Average evapotranspiration rate in the Tyne Basin (Dunn 1995) 

Land Cover	1985 to 1989 average evapotranspiration rates (mm)
Arable	554
Bracken	411
Conurbations	438
Deciduous	633
Evergreen	766
Felled	338
Grass	475
Heather	473

Calder and others (2002, UK) quantified recharge under different woodland types, estimating oak woodland recharge at 17% of rainfall, compared to 6 to 8% for Corsican pine. Archer and others (2012, UK) investigated the impact of different land use types in the Scottish borders and found that old mixed forests (500 to 180 years) had 5 to 6 times greater infiltration capacity than grassland. Although they found that soil moisture depletion rates may be greater and rewetting slower in the woodland (Green and others, 2006, UK). 

Despite these challenges, there is evidence that woodlands can have long-term water resource benefits. Woodland planting has been shown to enhance soil hydrology over time. Murphy and others (2020, UK) demonstrated that woodland planting improves soil structure which, over 15 years:

• reduced compaction
• increased macro-porosity
• doubled saturated hydraulic conductivity

These changes enhance infiltration and reduce surface runoff, aligning with Monger and others (2022) findings and suggesting that woodland planting could support long-term water resource improvement through soil recovery. Most reforestation studies consider short monitoring periods, which may not cover the timespan of soil restoration and detect any long-term water resources benefits. 

The effect of woodland planting on water resources and stream flow also varies depending on the scale of implementation. While large-scale planting, such as industrial forestry, is water-intensive, small-scale planting may have minimal impacts on water resources. Collins and others (2023) found negligible impacts on stream flow (0.2 to 2.6% peak flow reduction) from realistic small-scale tree planting scenarios in permeable catchments. However, broad-scale planting scenarios reduced Q95 flow by 39%, negatively impacting water resources. 

Local catchment properties can also have a more significant influence on water resource availability than land cover. In upland areas, soil hydraulic properties, geology and topography have been shown to exert a greater influence on catchment storage potential and spatial variability than woodland cover (Peskett and others, 2021, UK). 

Another complexity is added by the impact of forests on atmospheric moisture vapour availability due to evapotranspiration from forests. Ellison and others (2012, global) emphasise the importance of considering the impact of land use changes at multiple scales. They infer that while trees can decrease recharge at the scale of small catchments through high demand, they can increase the supply of precipitation and water availability. This can raise the recharge potential at a regional and global scale.  

Overall, careful targeting and a thorough understanding of catchment characteristics are essential before implementing or excluding woodland planting as a strategy for water resource management.  

 2. Runoff infiltration split  

Improvements in soil health and increased surface roughness at the point that rainfall lands can:

• reduce runoff generation
• improve infiltration into the saturated zone
• lead to increased aquifer recharge

This section reviews literature on 2 practices: 

1. Improving soil structure. 
2. Changes in surface roughness.

Soil structure 

Key findings from this review are that:  

• maintaining healthy soil structure can increase infiltration
• non-inversion tilling and the sustainable management of livestock can improve soil structure
• the impact will vary depending on underlying soils and geology – in saturated soils or those with low-permeability, soil improvements may not increase infiltration
• modelling can be a useful tool for assessing the potential effect of soil improvements for different geologies and catchments

Full literature review

Soil structure refers to the arrangement and organisation of particles in the soil which determines the pore spaces in the soil. Soil structure is influenced by factors such as (Hillel, 2003):

• climate
• biological activity
• soil management practices

There is a lot of literature indicating that soil structure is an important control on infiltration capacity, generally directed at flood benefits from sustainable farming practices. Palmer and others (2013, UK) note that nearly 40% of soils in south-west England are structurally degraded. This has led to increased runoff and decreased winter recharge to aquifers with the greatest degradation and runoff from maize, where soils are frequently bare and compacted. 

Soil structure can be improved by measures which reduce compaction or increase soil organic matter such as:

• low or zero till systems
• crop rotation
• cover crops
• mulching

Although cover crops have higher evaporation rates than bare soils, their role in improving soil structure provides infiltration benefits that may balance the increased evapotranspiration. 

Improving soil structure can increase runoff infiltration. The pore structure of healthy soil retains a greater volume of water in dry weather, and coarse-textured soils help rapid percolation to the aquifer. Wallace and Chappell (2019, UK) observed 7.5 times increase in permeability (Kfs) due to blade aeration. Basset and others (2023, global) review the range of experimental and theoretical literature on this topic. They emphasise that soil amendments, crop management and reductions in tillage significantly correlate with infiltration into soils. 

Griffiths and others (2018, UK and Netherlands) present evidence that infiltration rates are significantly higher in fields with non-inversion tilling than conventional tilling. However, the paper highlights the challenges of monitoring the impacts of soil improvements on infiltration in the field. They suggest that earthworm abundance can be used as a proxy for infiltration rates, as earthworm burrows can increase soil infiltration rates significantly in untilled soils.  

The outcomes of tillage method for infiltration rates are also not always the same and may vary with soil type and slopes. Lipiec and others (2006) observed 61% lower infiltration under no-till agriculture than conventional tilling.

Pastoral farming can also impact soil structure and infiltration rates. Dunne and others (2010, Kenya) observed the impact of different cattle stocking densities on infiltration on rangelands in Kenya and found reduced infiltration in some circumstances due to trampling. This is supported by a meta-analysis by Basche and DeLonge (2019) which found that livestock systems were more likely to contribute to an overall decline in infiltration rates through soils by approximately -21.3% (with a confidence interval of -50.3 to 7.9%, based on 24 English language studies). Conversely, hydraulic conductivity from deteriorated soils can naturally recover to 10 to 15cm from the exclusion of animals, through burrowing by macro-invertebrates (Drewry and others, 2006, NZ).  

Although sustainable farming and improved soils can be encouraged by increased standardisation of soil monitoring and clear communication to farmers, this may not be appropriate in all settings. Subsurface geology must be accounted for. Delin and others (2000, USA) investigated the impact of microtopography and soils on recharge in maize agricultural fields in Minnesota. They found that fine-grained laminations in the unsaturated zone retarded the flow path of surface water to groundwater, reducing recharge rates. Palmer and others (2013) observed limited soil degradation on chalk and limestone areas, and the impact of soil enhancements for recharge is likely to be negligible on low-permeability clay soils.  

Model studies indicate the relationship between improved soil infiltration and baseflow. Packman and others (2004, UK) discuss how the Flood Estimation Handbook (FEH) rainfall-runoff model and Flood and Agriculture Risk Matrix (FARM) can be used to evaluate the impact of land use and soil management on baseflow. These modelling techniques can be a useful guide to understand the potential effects of interventions.

Holman and others (2011, UK) modelled the effect of soil management on catchment baseflow. They undertook hydrological modelling using the WaSim model to predict changes to baseflow index in rivers due to enhancements to soil structure across the subject catchments across England and Wales. They ran a large array of models over 30 years for a range of variables reflecting catchments in England and Wales. WaSim is a water balance model which does not explicitly simulate groundwater processes, showing that it’s not always needed to effectively model water resources benefits. The study suggests that 10% increases in baseflow index can be achieved with reasonable improvements in soil structure in some catchments. Changes were more limited in southern and northern England highlighting the importance of soil type, superficial geology, land cover and climate on the potential to achieve baseflow increases.  

In summary, healthy soil with intact soil structure can increase infiltration. Basche and DeLonge (2019) state from their meta-analysis that practices promoting ground cover and continuous roots, both of which improve soil structure, were most effective at increasing infiltration rates. However, the impact can vary based on local soil and geology.  

Increasing surface roughness 

The key finding from this review is that increased surface roughness can attenuate runoff, increasing infiltration and recharge. However, the range of field studies available on this is limited. 

Full literature review

Increases in surface roughness through changes in soil roughness, buffer strips, and increased microtopography can slow and reduce runoff. However, there is limited literature addressing the connection between roughness and infiltration.

The study by Zhao and others (2013, China) found empirically that soils with rough surface textures had increased infiltration capacity compared to smooth soils on cultivated slopes in China. This suggests that improved microtopography and textured soils may have the potential for greater recharge. 

Cole and others (2020) reviewed the environmental impacts on buffer strips and noted increased infiltration and water retention where wooded buffer strips increased soils permeability due to their deep roots. This was also reflected in a report by Stutter and others (2020, UK) which showed that riparian buffers create a physical barrier which slows overland flow and increases infiltration to the soil. This has the overall effect of trapping and retaining pollutants before they reach watercourses. Both studies showed that buffer strips increase infiltration to the soils but there were no specific monitored changes to groundwater levels and water resources.  

Lu and others (2024, China) modelled the impact of microtopography in agricultural regions on surface-groundwater interactions. They found that microtopography has a significant effect on response times of groundwater. It slows the rate of infiltration but increases the range of response times and should not be neglected in models. 

Wetland and peatland restorations also lead to increased surface roughness.

This literature review found no reported studies in the following areas that would have been of particular interest: 

• comparisons between roughness and recharge on arable land versus grasslands 
• comparison within grassland types, especially between intensive and extensively grazing, and the impact of improvement measures 
• comparison of different methods of increasing roughness, such as leys, buffer strips, stubble and microtopography  

3. Runoff attenuation features (RAFs) and infiltration basins  

RAFs are landscape features designed to create a temporary flow storage, slowing the flow of surface runoff and facilitating infiltration. These include swales, ponds, bunds and barriers across flow paths designed to store water and slowly release over time.

Often the main aim of these features is for NFM. Infiltration basins direct surface water into groundwater.

This section reviews literature on 3 practises: 

1. RAFs.  
2. Purpose-built infiltration basins. 
3. Cross-slope interventions. 

Runoff attenuation features (RAFs)

Key findings from this review are that:  

• RAFs store water and slowly release it, allowing greater infiltration and recharge – they have been shown to increase baseflow and reduce the severity of low flows in downstream rivers and streams in small catchments 
• RAFs also have a range of other benefits, including reducing peak flows at lower cost and impact than traditional flood defences, improving water quality and supporting habitats
• modelling studies can be used to optimise the design and placement of RAFs at local and regional scales
• there is insufficient empirical evidence to fully understand the impact of RAFs on groundwater and residence times in larger catchments

Full literature review

The use of leaky dams to improve baseflows and recharge aquifers has historically been widely used in drier areas (Standen and others, 2020, global). Hut and others (2008, Kenya) highlight examples of structures used to maintain water supplies in a range of areas. They present a model to evaluate the impact of sand groundwater dams on groundwater levels in Kenya.  

RAFs that store water in times of high flows, such as offline ponds, can also be used to supply additional irrigation during periods of lower flows. They help to improve low flows in rivers and increase resilience to drought (Camnasio and Becciu, 2010, Italy). Although irrigation construction is beyond the scope of NbS, you should consider it in the holistic assessment of any project. 

Norbury and others (2021, UK) monitored changes in baseflow before and after RAF installation in the Pennine Uplands. These installations took the form of log jam leaky barriers, which are cost effective to install. The study observes both significant reductions in peak flows during flood events alongside 27% increased baseflow during dry periods. They summarised that ponding due to RAFs slows groundwater velocities, increasing storage during dry periods, while also allowing greater infiltration of surface water. This observation underlines the potential for RAFs to increase resilience of rivers to droughts, at least over short river stretches. 

The first large-scale monitored use of RAFs in the UK was undertaken at Belford, Northumberland, with a focus on flood prevention (Nicholson and others, 2020, UK). The project involved the construction of storage ponds and leaky barriers composed of bunds, vegetation and woody debris. Nicholson and others (2020, UK) measured 12% reductions in the size of peak flows during small flashy storm events. This evidenced that the RAFs provide a low impact and ecologically beneficial means of protection for properties at lower cost than traditional flood defences in this small catchment.  

Although the focus of study by Nicholson and others (2020, UK) at Belford was on flood prevention, it can be inferred that the drop in runoff may be linked to increased infiltration and groundwater recharge. However, the study highlights that infiltration at depths greater than 60cm is very limited, with the active hydrological zone in the Belford catchment restricted to the top metre of the subsurface. Wilkinson and others (2010, UK) present evidence that the RAFs applied in 2007 and 2008 reduced flooding during heavy rainfall in September 2008 by storing runoff for 8 hours. It may therefore be that RAFs are delaying the movement of runoff into the river over short periods, spreading out the volume of flow but not aiding infiltration. This could benefit from further field research in this catchment.   

A study by the James Hutton Institute at Elm Sike, Scotland, also highlights the importance of site parameters, design, and placement for the efficacy of RAFs (Addy and Wilkinson, 2017). They found minimal evidence of impacts on low flows from leaky barriers after 3 years of monitoring. This suggests that the potential for water resource benefits from RAFs is limited in steep confined streams. It can be inferred from the study that it is likely that sites with lower gradients and wider floodplains, with RAFs placed along flow pathways, have greater potential for off-channel water storage. 

Modelling can be used to understand the potential of RAFs in different catchments. Fennell and others (2022, UK) modelled the impact of nature-based RAFs on low flows in a Scottish catchment. They modelled the impact of the size and location of RAFs and found that the installed scheme led to a 0.1% increase in recharge and 4% increase in baseflow. They also found small increases in low flows and reduced high flows. Modelled RAFs were found to be most effective when distributed across a wide area, and on well-draining soils.  

In a 1km upland catchment feeding the Glenlivet distillery, Fennell and others (2023, UK) modelled a 3% increase in Q95 with RAFs positioned through the catchment. This was associated with a 25% decrease in the number of low-flow days and halving of the mean number of 7-day consecutive low-flow periods when the distillery may be unable to function. This makes the installation of RAFs cost effective, particularly when combined with modelling to optimise the location of RAFs.   

JBA and WSP have explored the impact of RAFs in regional models (WSP and JBA, 2023, UK). A range of RAFs were investigated:

• surface RAFs at field level
• catchment RAFs further down the drainage network
• RAFs at the boundary of Chalk and till deposits

Simple spreadsheet models, alongside 4R and MODFLOW modelling, have been applied to investigate the impact of size, type, density, and location of RAFs, particularly across catchments in East Anglia. In general, modelling indicated that increasing the number of RAFs led to increased baseflows and larger low flows. Superficial geology played a significant factor in the scale of response, with the greatest modelled impact at the boundary between till and Chalk deposits.

For guidance on simple spreadsheet models, including 4R and MODFLOW modelling, see guidance on NbS for water resources: plan your project.   

However, benefits for low flows may not last during extended periods of drought. Meinen (2022, Australia) investigated the impact of leaky weirs in Australia. The weirs increased recharge but also increased discharge. Although this led to increased stream flow at the start of the dry season, the rest of the dry season experienced lower discharge flows compared to pre-installation values. This suggests that RAFs may not deliver lasting benefits when drought is extended.   

Pilot studies and modelling appear to support the potential of RAFs. However, Quinn and others (2022, global) reviewed the evidence basis and emphasise the complex evidence required to prove the effectiveness of RAFs at lowering runoff rates and maintaining recharge. So far, there is a lack of large-scale projects that can empirically demonstrate:

• recharge changes downstream of RAFs in large catchments
• the connection between the residence time of water in RAFs and groundwater flows

Modelling can be helpful to understand the impact of RAFs on water resources, and underlies the current NFM evidence base, but is not yet supported by long-term observations. The Stroud Valley NFM Project should provide valuable insights to address these unknowns. Over 430 nature-based measures have been installed in the Stroud Valley. This includes a case study site at Cranham involving 10 leaky log dams at a cost of £280 per structure and 9 earth bunds costing £305 per bund. A long-term monitoring plan has been designed by Van Biervliet (2022, UK) to assess the impact on low flows, groundwater recharge and soil moisture content. This will help provide insights into RAFs and water resources in a larger catchment. 

Quinn and others (2022) argue that overall evidence seems to support the benefits of RAF interventions, particularly leaky barriers, for slowing flows, initiating recharge to depths over longer periods. These approaches seem applicable at both local scales and at large scales across a catchment, particularly alongside other NbS methods such as:

• peatland restoration
• managed floodplains
• afforestation

RAFs also provide a wider range of benefits including:

• flood attenuation
• ecological benefits
• reduced erosion
• limiting the movement of diffuse pollutants and sediments
• increasing overall landscape resilience

Infiltration basins 

Key findings from this review are that:  

• infiltration basins can significantly increase recharge of the saturated zone, with clustered arrangement more effective than distributed arrangements
• they can also improve surface water quality, reducing nutrient and sediment loading 
• infiltration basins require long term maintenance and may impact groundwater chemistry

Full literature review

There is a large range of monitoring studies on the impact of stormwater infiltration basins for mitigating flood risks in urban areas and recharging aquifers. Urban NbS measures are designed to remove water from the surface to ground and improve water quality. They include:

• permeable pavements
• retention ponds
• integrating reedbeds into storm and grey water drainage system
• green roof water harvesting

They can also improve community wellbeing, cool urban areas and help green the local environment (Jarvie and others, 2017, UK). 

Masetti and others (2015, Italy) discuss the recharge dynamics of a large (16ha) storm water infiltration basin in Northern Italy above sands and gravels. This was monitored with a 4-year field study and numerical model. They reported that groundwater levels can respond in minutes to recharge from the infiltration basins, with recharge up to 50 times natural levels. However, the study highlights that infiltration basins must be actively maintained to prevent clogging, otherwise recharge can become significantly reduced. Appleyard and others (1993, Australia) monitored urban and industrial stormwater infiltration basins in Perth, Australia. Groundwater recharged from the basins within minutes of rain events, with notable salinity reductions and increases in dissolved oxygen downgradient of the sustainable drainage systems (SuDS) basins. Although concentrations of toxic metals and nutrients were found to be low around the basin, they noted the accumulation of lead and plastics within basins. This highlighted the need for long-term maintenance. 

Lebon and others (2023, France) quantitatively studied the impact of stormwater infiltration basins on unsaturated zone chemistry and microbiology. The study found increased dissolved organic carbon leading to increased microbial biomass in regions of rapid transit from surface to groundwater. They found that nitrates decreased due to dilution where recharge was rapid. Regions with a thinner unsaturated zone also saw greater changes in phosphate. Understanding the influence of nature-based interventions on groundwater chemistry and biology is important.

Mathematical models can be used to understand potential groundwater recharge from infiltration trenches and basins, and the impact of spatial arrangement of basins. For example, Chahar and others (2012, France) present a numerical solution to quantify the movement of stormwater through SuDS infiltration trenches. Choat and others (2020) model flow dynamics in the unsaturated zone. They suggest that on short timescales the movement of water from infiltration to recharge and baseflow is controlled by partitioning between the unsaturated and saturated zone. They also suggest that a distributed arrangement of discrete infiltration basins results in greater storage in the unsaturated zone and reduced baseflow, compared to a clustered arrangement of basins. Therefore, clustering infiltration measures could lead to the greatest increase in groundwater recharge, although this is sensitive to the permeability and geology of the subsurface. 

Cross-slope interventions 

The key finding from this review is that hedgerows and woodland planted on catchment slopes can act to attenuate surface runoff but may reduce summer recharge due to evaporative demand. However, the range of field studies available on this is limited. 

Full literature review

The role of cross-slope woodlands at attenuating surface runoff, with mixed impacts for water resources, has been discussed in section 1.  

Hedgerows can also be used in a similar manner as RAFs, slowing surface flows by obstruction. However, there is limited quantitative field data on the impact of hedgerows for groundwater resources. Blanusa and Hadley (2019, UK) experimentally studied the influence of different hedgerow species on runoff. They demonstrate that common hedgerow species can significantly reduce the volume of rainfall runoff. Significantly higher permeability (𝐾fs) values have also been observed under hedgerows than arable land (Holden, 2019), which would facilitate recharge. Wallace and others (2021, UK) present empirical data on reduction in overland flow incidence in plots with hedge margins compared to pasture. This is due to higher infiltration and soil saturation levels but does not address the split between infiltration and evapotranspiration.  

Ghazavi and others (2008, France) highlight the significant impact of hedgerows on hydrological dynamics, reducing recharge seasonally. Hedgerows increased water uptake from the unsaturated zone in late summer, reducing summer recharge and drying soils. This can act as a barrier preventing lateral subsurface flow. The study observes that the influence of slope hedgerows on soil water extends across the area of rooting, up to 9m upslope. The loss of leaves in winter reduced rain interception and evaporative demand so recharge increased during winter due to reduced root water uptake. The influence of the hedgerows was insignificant, but it took longer to rewet soils where root density was highest.  

Vegetative barrier buffer zones also have been shown to increase vertical infiltration rates at a local scale with 10 times greater infiltration in fascines (brushwood stick bundles) with high biological activity than those without (Richet and others, 2017, France).  

4. Floodplain, wetland and peatland restoration 

Field and model scenarios highlight the range of benefits for water resources that can be achieved by peatland and floodplain wetland restoration and increased connectivity between rivers and floodplains.

This section reviews literature on 3 practises: 

1. River-floodplain connectivity 
2. Wetland restoration 
3. Lowland peat restoration 

Floodplain connectivity 

Key findings from this review are that:  

• increasing storage within floodplains can reduce high flows during flood events and maintain baseflow during periods of low flows 
• the creation of small ponds on the floodplain, particularly through beaver activity, has been shown to increase baseflow 
• the potential for improved groundwater resources depends on geology and the hydrological structure of the floodplain 
• modelling floodplain hydrology can guide restoration of floodplain-river connectivity under different meteorological and groundwater conditions, ensuring sufficient water resource benefits 
• floodplains also offer a range of co-benefits such as the settlement of nutrient rich sediments, improving water quality, and fertilising agricultural land – they have high biodiversity

Full literature review

Increased river to floodplain connectivity can:

• increase the frequency of floodplain inundation
• store water offline across a wider area
• slow the release of water from the floodplain during drought events

Through the expansion of canalisation, riverbank modifications and urban development, up to 90% of Europe and North America’s riparian floodplains are ‘functionally extinct’. This has severe consequences for freshwater biodiversity (Tockner and others, 2002). Channelisation also increases flow velocities, with negative downstream consequences for low flows and floods (Heritage and Entwistle, 2020, UK). 

Liu and others (2023, UK) highlight the trade-offs between water quality and water availability from different NFM measures, suggesting an optimisation framework to balance different management objectives. The study models urban and rural water management in Norfolk. They found that water availability is likely to be optimised by a combination of RAFs with floodplain restoration, with the benefits increasing with the scale of implementation.  

Clilverd and others (2016, UK) highlight that 40% of river length in England and Wales has been severely modified with a large loss in river-floodplain connectivity. They found that increasing river and floodplain connectivity by removing embankments increased subsurface groundwater storage and improved river drainage of the River Glaven, North Norfolk, alongside ecological benefits. Regular flooding along marginal areas increased groundwater recharge and water table levels, while aiding the development of riparian habitats through nutrient delivery. The removal of embankments also allowed the drainage of groundwater and surface water back into the river, increasing stream flow during periods of low flows. 

The interaction of rivers and floodplains is complex and these benefits will not be achieved in all locations or under all meteorological conditions. Clilverd and others (2016, UK) highlights the value of a modelling methodology to quantify the impact of restoration and plan suitable restoration sites. They explored the use of MIKE-SHE/MIKE 11 modelling to simulate the groundwater impacts of river restoration to more naturalised conditions. They conducted hydrological modelling coupled with hydraulic modelling to simulate conditions within a wet meadow grassland habitat in Norfolk following river restoration. They were able to demonstrate increased groundwater levels and aquifer storage. Peak flood flows were attenuated downstream with improved floodplain connectivity and enhanced biogeochemical cycling was inferred, offering potential water quality benefits.

Burt and others (2002, UK) conducted a numerical simulation of hillslope, floodplain and channel interactions for the River Severn, Shropshire. They look at the reversal of flows towards the hillslopes during flood events. This reduced the input of water from hillslopes to the riparian zones but depended on certain antecedent runoff and rainfall conditions. 

Beaver reintroductions can:

• increase the connectivity of floodplains and rivers
• increase surface water storage by forming ponds

The release of these surface pools can help maintain flows during dry periods, increasing drought resilience (Brazier and others, 2020, Europe). Raising water levels in the floodplain can therefore increase groundwater recharge. However, raised water tables also experience greater rates of water loss by evaporation. This balance appears to be in favour of improved groundwater resources. A study by Smith and others (2020, Germany) found a 5% increase in groundwater recharge in a river following riparian wetland restoration aided by beaver colonisation. 

While river-floodplain connectivity can reduce flood peak flows downstream and improve floodplain biodiversity, the storage of water on the floodplain does not necessarily lead to increased infiltration and stream flow. The underlying geology must also facilitate recharge. Acreman and others (2003, UK) model the impact of channel geometry on floods in the River Cherwell, Oxfordshire. They find that re-naturalising river channels within the floodplain can reduce flood peaks and increase floodplain inundation, storing water on the floodplains. However, as the catchment is underlain by very impermeable clays, the impact of this intervention would have a negligible effect on groundwater resources. 

Wetland restoration and creation 

Key findings from this review are that:  

• restoring wetlands on lowland floodplains can improve groundwater recharge and reduce surface runoff
• wetlands store water in the landscape and can reduce the intensity and length of the effects of drought events
• wetlands also provide multiple other benefits including biodiversity gains, as well as taking up excess nutrients, filtering out sediments and improving water quality  
• wetlands are also associated with increased evapotranspiration, and in some locations can reduce low flows, highlighting the variable impact on water resources dependent on location and geology 

Full literature review

Wetlands provide a range of ecological benefits and there has been growing interest in the use of floodplain wetlands for flood alleviation and improvement of water resources. Over the 20th century, wetland coverage declined by 64 to 71%, losing valuable ecosystem services (Davidson, 2014).  

In a literature review of non-floodplain wetlands, Lane and others (2018, US) considered the flow regulatory services of North American non-floodplain wetlands. They highlight that they can increase groundwater recharge and store water to maintain baseflows and reduce flood peaks.

Demissie and Khan (1993, USA) monitored stream flow in catchments with a variety of wetland coverage in Illinois. They observed that a 1% increase in wetland area led to an average 8% increase in low flows (Q95), while decreasing flood volumes. The wetlands increased the resistance to flow, slowing the input to streams, gradually releasing water from offline surface pools to groundwater and stream channels. However, the range of results spanned +22% to -5% changes in Q95, highlighting variation between wetland catchments due to differences in soils and geology.  

Wetlands have rough surfaces compared to other land uses, with hollows and hummocks forming microtopography. Frei and others (2010, Germany) modelled the impact of wetland microtopography on infiltration. They found that the surface depressions buffered the input of rainfall into recharge, leading to subsurface flow throughout the year except during severe storms. They found that a flat surface experienced flashy response to rainfall events, with greater runoff.  

Wu and others (2020, China) used a hydrological model to assess the impact of wetlands on watershed hydrology, within the Upper Nenjiang River Basin in Northeast China. The study highlights the significant role of wetlands in attenuating quickflow and supporting baseflow, by slowing surface runoff, with a stronger regulation impact during the summer season, mitigating droughts. The model also suggests that increasing wetland areas enhances their cumulative effect on flow regulation.  

Modelling the impact of wetlands for drought mitigation, Wu (2023, China and Canada) found that wetlands can:

• shorten and reduce the severity of droughts impacts
• accelerate recovery
• reduce the probability of hydrological droughts by approximately 18% in both a Chinese and a Canadian basin

However, wetlands can sometimes exacerbate droughts effects when water has been lost from wetlands by evapotranspiration. We can infer from this that the wetlands impact on hydrological droughts is altered by:

• the internal properties of soil vegetation and hydrology
• external controls by the basin hydrology, precipitation, anthropogenic influences and climate

Interventions to buffer wetlands against drought, such as changing the network of dykes to retain elevated water levels, can also be effective for improving their NbS function.  

Bullock and Acreman (2003, global), emphasise that the hydrological role of wetlands is not simple and varies with location and hydrological type. Floodplain wetlands often provide benefits for flood control, recharge and baseflow. However, in 47 out of 71 studies, wetlands have been shown to increase floods and reduce low flows, particularly headwater wetlands along river margins. Wetlands also have large evaporative losses compared to other land types, so can reduce downstream flows during dry periods. Many wetlands have insignificant interactions with groundwater as they lie on impermeable deposits, so have no recharge impact. 

In summary, wetlands do not offer universal benefits for water resources. You must consider the location of the wetland in the catchment and its geology. Protecting and restoring wetlands in the upper catchment can play a significant role in:

• attenuating surface runoff during heavy rainfall
• increasing baseflow
• mitigating droughts
• ecological benefits

Upland and lowland peatland restoration 

Key findings from this review are that:

• restoring upland peatlands can reduce surface runoff – this can maintain elevated water tables

• the impact on groundwater resources of lowland peatland is often complex and generally is not associated with increased baseflow in rivers – with impermeable soils it may be neutral, but increased evapotranspiration may reduce infiltration to groundwater 

• restoration of lowland peatland projects should consider the impact on groundwater resources from irrigation and utilise surface water retention over irrigation

Full literature review

Peatland refers to carbon-rich wetlands where waterlogged conditions prevent plant material from fully decomposing, forming soils of partially decayed material which builds up slowly over time. Peatlands are dominated by mosses, sedges and shrubs (UKCEH). In the UK, upland peatlands, often called blanket bogs, are large areas of peat found in the uplands fed primarily by rainfall. Lowland peat can be split into lowland peat and fens. Both are found in flat lowland areas, but lowland peat is primarily rainfall fed and fens are by groundwater, river water and rainfall.  

Restoration of upland peatlands by blocking drainage has been a major focus of NFM. Restoring lowland peatland on floodplains can raise water levels and increase surface roughness, reducing surface runoff (Goudarzi and others, 2024, UK). In the same study, sphagnum moss was identified as a key peatland species which can deliver significant flood reduction benefits due to its unique properties. It can hold 20 times its weight in water. 

Although increased surface roughness on the surface of peatlands can slow the movement of water towards rivers, this does not necessarily lead to increased recharge and baseflow. Holden (2002, UK) highlights that there is little vertical percolation through peat deeper than 10cm in upland blanket peat, so they generally do little to maintain baseflow. The roughness and retention of peatlands therefore makes them important sinks for water but limits the transmission of water into downstream rivers or aquifers.  

Gatis and others (2023, UK) also observed no increase in baseflow over 4 years of monitoring, following the blocking of peat gulleys and raising of water tables in peat in Dartmoor National Park. 

In lowland peatland, which is more directly connected to the water table, the water resource response to peatland restoration can be complex. Ahmad and others (2021) observed that rewetting and restoring fenland maintains high water tables, allowing peat accumulation and improving ecosystem functions. The study found increased evapotranspiration in restored fenland compared to the degraded fen, particularly in dry months, reducing groundwater levels due to elevated water levels and taller vegetation.  

JBA (North Somerset Council, 2024, UK) discussed the impact of methods of peatland restoration on water resources. Restoration requires the raising of water tables, often with a demand for irrigation at the start of projects. Although subsurface irrigation increases the stability of the water table across a wider area of peat, where peat is irrigated from the river the water demand can be as high as 3000m3/ha. This affects water resources across a wider area. Peatland restoration schemes that rely on storing excess winter rainfall through surface water retention can improve peat conditions and stabilise water levels without reducing water levels elsewhere. 

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