Basin scale effects on climate resilience

Reconnecting river channels with floodplains was found to be the most promising solution to promote a more resilient ecosystem and improve biodiversity. The following factors have led to disconnection of the floodplains: presence of embankments, reduced sediment supply, increased incision of the main riverbed, and hydropower dams reducing peak flows and increasing base flows. This mostly has resulted in disconnection of floodplains, wetlands and starvation of flooded forest, which forms an important natural ecosystem. Not only area is disappearing, also the quality of those areas is deteriorating, due to a lack of the right hydrological and biological conditions.  

Restoration efforts, such as adjusting levee positions, lowering floodplain elevations or installing culverts, are expected to activate floodplains and promote hydrologic connectivity. This activation is essential for improving spawning habitat availability for various fish species (Navodaru et al., 2005). Additionally, these measures bring extra benefits, including better retention of nutrients and suspended solids that is important to soil quality on floodplain (Schneider, 2002; Suciu et al., 2002).  

Flooded forests and wetlands facilitate the exchange of water, nutrients, and sediments between the river and the floodplain, supporting the overall health of the ecosystem. Moreover, they provide habitat for a diverse range of flora and fauna by harbouring a variety of plant species, including trees and aquatic vegetation, which, in turn, sustain diverse animal species, such as fish, birds, and mammals. Lastly, those flooded forests and wetlands provide an important nursery ground for commercially important fish species. The submerged vegetation and complex structure of the forest provide shelter and food resources for juvenile fish, contributing to the overall productivity of the aquatic ecosystem. 

At the basin level, recent estimates indicate that the economic value of Mekong fishery dropped by more than a third between 2015 and 2020. The estimated annual value of fish catch was estimated between USD 7.13 billion and USD 8.37 billion in 2019-2020. It is difficult to make an educated guess on the impact of NbS on fisheries income. But if implementing an NbS in 25% of the highly suitable areas would only result in a 1% increase in fisheries, the revenues would already generate about USD 77.5 million in annual benefits. 

Moreover, specifically for NbS 1, a reduction in chemical inputs reduces the cost of agricultural production. An intangible income-related benefit is that flood-based agriculture could provide a more stable household income as it is more climate and flood-resilient. Besides intangible social benefits from a more stable income, it could also support economic development. 

For NbS 2, studies have shown that products such as resin, bamboo, rattan, wild honey and fuelwood can be collected from forests in Cambodia. However, data on the amounts and values of these products is limited. A study by Sophanna et al. (2022) conducted a survey in 22 villages in the Tonle Sap Lake area that are located within 500 metres from a flooded forest to assess ecosystem services. They found the following annual economic benefits per person per year from flooded forests: fuelwood – USD 12; wild food – USD 8; traditional medicine – USD 1; honey – USD 1, hence in total USD 22 per person per year. However, while the amounts and value may be low, they can be very important for low-income households who can obtain these natural resources for free.  

For NbS 3, Invasive species can be used to make compost. Households can sell the compost or use it in their fields, which would reduce their production costs. As this would reduce the use of chemical fertilisers, it would also help to improve water quality.  

Last but not least, for all NbS, tourism can be a significant source of income, further supporting the diversity of income from food production. 

Restoring the natural function of floodplains will enable the system to facilitate the movement of aquatic organisms and transport of materials like sediment, minerals and nutrients that is important for soil on floodplains. The active floodplain plays a crucial role in maintaining water quality, mitigating flood stages, recharging ground water reserves, washing soil for excess salt, and it acts a natural pesticide. Re-establishing a connection with the floodplain is anticipated to improve livelihoods that depend on the quality of the floodplains. Additionally, active floodplains will also enhance resilience against droughts and extreme heat, because natural habitats and water mitigate heat. 

Wetlands and flooded forests support the water purification processes by allowing for natural filtering and nutrient cycling, helping to maintain water quality. 

Introduction of the NbS at the basin level would lead to flood risk reduction downstream, as the storage capacity for flood waters increases. In addition, increased inundation would result in deposition of sediments, partly mitigating the effects of land subsidence and aiding in maintaining the elevation of the delta (mostly relevant for Vietnam).  

Almost every year floods cause damages to agriculture, infrastructure and buildings and lead to loss of lives, which might be reduced with greater water storage upstream to reduce peak flows. Moreover, reduced flood levels could result in lower required protection levels, and hence lower costs for flood protection infrastructure. 

Assessing the quantitative impact of implementing for example flood-based agriculture at the basin level on flood damage and flood protection is complex, hence only a very rough indication of the benefits can be given. The average annual costs of floods in the Lower Mekong Basin ranges between USD 60 to 70 million. Assuming the project would contribute to a 1% reduction in damages, this would be USD 600,000 to 700,000. 

Closely related to reduction of flooding, the NbS would also lead to increased resilience to drought through groundwater replenishment. Groundwater replenishment is very important to mitigate further subsidence, and thereby also indirectly affects flood risk in a positive way. 

Moreover, the quality of the ground and surface water is likely to increase, because less chemicals and fertilisers are used. Cleaner water would lead to health benefits. Theoretically, health benefits could be quantified and monetised as reduced medical expenditures, avoided loss of working days due to illness, or increase in expected healthy living years. 

Reduction in climate change and associated effects can be valued through carbon sequestered by vegetation and a social price for carbon. Measuring this would require an estimate of carbon sequestration in the project area without and with the project. For this, newly established vegetation, restored degraded forest and avoided deforestation could be considered. Using a ballpark calculation, an indication of the benefit could be obtained. Assuming carbon sequestration is 5.5 tCO2 ha⁻¹yr⁻¹ and the value of a tonne of CO2 is USD 5, benefits would be USD 27.50 ha⁻¹yr⁻¹, or USD 6,875 per year for the 250 ha planted area. At the basin level, a planted area of 12,325 ha would generate USD 338,938 as a ballpark figure. 

In the study, costs and benefits of the selected NbS have been quantified where possible. Hence, a caveat in comparing the different NbS is that not all costs and benefits have been quantified.  

From cost perspective, restoring flooded forests is the most expensive measure, which is mainly caused by the costs for acquisition of land. Reconnecting riverine wetland ecosystems also requires the purchase of land, though this is a relatively smaller area needed to create connections. Construction of culverts is the main costs for restoring floodplain dynamics through flood-based agriculture.  

The benefits are largely determined by the impact on basin-level fisheries of the interventions. Restoring the flooded forest ecosystem (NbS 2) and riverine wetland connections (NbS 3) will have a larger impact on basin-level fisheries than flood-based agriculture (NbS 1), and hence generate more benefits.  

Comparing the economic metrics (NPV, BCR, IRR) at basin level, investing in restoring riverine wetland connections would be the most effective NbS, followed by restoring the flooded forest ecosystem. However, at the local level these measures do not generate enough benefits to make them viable projects (which is also a main cause why these projects are challenging to implement). For flood-based agriculture the reverse is true: assuming the profit from flood-based agriculture for farmers can match triple rice cropping over time, the local benefits exceed the basin benefits (making them easier to implement).