This project aims to generate measurable, additional, and verifiable volumetric water benefits in a priority watershed in southern Africa through hydrological restoration based on the removal of invasive exotic plant species. These species, such as pines (Pinus spp.) and acacias (Acacia mearnsii), have spread extensively in mountainous and headwater ecosystems, exerting extreme pressure on water availability due to evapotranspiration rates significantly higher than those of native plant species.
The intervention proposes a systematic ecohydrological restoration of the invaded areas, directly improving soil infiltration capacity and the volume of water recharging shallow and deep aquifers.
Through a rigorous technical approach, this intervention enables the recovery of key ecosystem services, reduces water loss due to evaporation, and contributes significantly to closing water availability gaps for environmental, community, and industrial uses.
The project replicates a practice validated and quantified in Cape Town by The Nature Conservancy and local partners, where over 55 million cubic meters were recovered over ten years, and proposes its adaptation to a new critical zone, integrating continuous monitoring, ecological restoration, and alignment with international frameworks such as VWBA 2.0, Act4Water CAPs, and verification under the AWS standard. This solution is considered cost-effective, with long-term permanence, and suitable for integration into corporate water replenishment strategies as part of Water Positive commitments.
The watershed faces a state of severe water stress, the result of a multifactorial imbalance in the regional hydrological cycle. Among the main contributing factors are the intensive overexploitation of aquifers for agricultural, industrial, and urban use; the sustained decline in precipitation and the extreme climatic variability attributable to climate change; as well as the alteration of natural vegetation cover. This last factor has been especially exacerbated by the proliferation of invasive exotic species such as Acacia mearnsii and Pinus radiata, which exhibit evapotranspiration coefficients more than 30% higher than native species, according to studies by the Council for Scientific and Industrial Research (CSIR).
These species act as biological pumps that extract water, decreasing the effective recharge of aquifers, reducing base flows in perennial streams, and degrading habitats of endemic species. The replacement of native vegetation with invasive monocultures alters soil structure, reduces its infiltration capacity, and increases surface runoff, which further diminishes the availability of accessible freshwater in the watershed and accelerates the degradation of critical ecosystem services.
The proposed intervention consists of a two-phase technical-ecological strategy. First, the selective removal of invasive exotic plant species using manual and mechanical techniques adapted to the type of vegetation and terrain slope (e.g., deep uprooting on gentle slopes and basal girdling in inaccessible areas), prioritizing catchment and recharge zones identified through spatial hydrological analysis. This action enables a significant reduction in the evapotranspiration coefficient (ETc), by eliminating species with annual water consumption exceeding 1,000 mm/year, compared to native species which range between 350–600 mm/year.
The second phase consists of assisted revegetation using a set of native species selected for their soil water retention capacity, low water consumption, ecological compatibility, and contribution to local biodiversity (such as Protea repens, Restio spp., Erica cerinthoides). This vegetation restoration aims to recover soil structure and functionality, increasing infiltration capacity and reducing surface runoff. The deep root systems of these species facilitate percolation to groundwater layers, resulting in a measurable improvement in base flows of rivers and springs.
The water benefit is estimated through a simplified water balance model that considers the difference in ET before and after the intervention, multiplied by the intervened area and adjusted according to the effective soil recharge coefficient. Based on previous studies by the Western Cape Invasive Species Programme (WC ISP) and hydrological validations by The Nature Conservancy, a net gain of over 500,000 m³/year is projected, with the benefits expected to last over 20 years, subject to a biannual maintenance plan and adaptive control of regrowth.
The execution of the project is organized into four sequential and complementary stages, each with specific objectives, technologies, measurement methodologies, and differentiated monitoring schemes, all aligned with the VWBA 2.0 framework and the Act4Water verification standard.
Stage 1 : Hydrological diagnosis and planning (duration: 6 months): During this phase, a detailed mapping of areas with the highest density of biological invasion is conducted using high-resolution satellite imagery (Sentinel-2, PlanetScope) and geographic information systems (GIS). A multitemporal NDVI analysis is applied to detect changes in vegetation cover, and these data are cross-referenced with digital elevation models to identify priority catchment areas. The evapotranspiration (ET) baseline is established using local automatic weather stations that record parameters such as solar radiation, relative humidity, temperature, and wind speed, along with satellite estimates (MODIS, FAO ET-Look). In parallel, a spatialized water balance model (using tools such as SWAT or WEAP) is developed to simulate scenarios with and without intervention, thereby defining zones of maximum hydrological effectiveness. This diagnosis is complemented by field validation (ground-truthing) through linear transects and control points with differential GPS. Planning incorporates risk analysis, accessibility, slope, and ecological sensitivity to prioritize intervention areas.
Stage 2 : Removal of invasive exotic species (duration: 18 months): This stage implements direct intervention on invasive plant cover using methods adapted to the species type and terrain morphology. In accessible areas with gentle slopes (<20%), mechanical uprooting techniques are applied using manual tools (shovels, axes, saws) and light machinery such as mini-excavators with hydraulic arms. In difficult-to-access areas, basal girdling and subsequent drying are used. Each treated plot is documented through georeferencing and standardized intervention forms. Effectiveness control is carried out with drone flyovers every six months, equipped with multispectral cameras, to monitor ET reduction through spectral indices (NDVI, SAVI, EVI). Additionally, regrowth is monitored through field inspections and time-lapse photo sensors installed at fixed points. The remaining invasive cover density is controlled using systematic sampling plots (1 ha) and circular quadrants. The traceability of actions is managed through a digital platform based on environmental blockchain (e.g., OpenForest Protocol or a local equivalent).
Stage 3 : Ecological restoration with native species (duration: 12 months): Once removal is completed, assisted revegetation begins through the planting of native species selected for their ecohydrological compatibility, low water requirements, and capacity to enhance infiltration (e.g., Protea repens, Restio spp., Erica spp.). Planting is done in staggered strip designs to maximize diffuse runoff capture, and includes bioengineering measures such as wattling and retention barriers to stabilize slopes. Seedling survival rates (establishment percentage) are measured through counts in permanent monitoring plots, and the annual increase in vegetation cover is compared using drone-processed aerial imagery. Improvement in infiltration is indirectly measured with soil moisture sensors (TDR or FDR type) installed at depths of 20 and 50 cm. Pests, competition with secondary invasive species, and ecological succession are controlled with adaptive management according to monitoring results. Each restored site is integrated into a geospatial database containing the intervention history, growth curves, and local climate records.
Stage 4 : Hydrological monitoring and benefit validation (duration: 10 years): This stage ensures the traceability and permanence of water benefits. A monitoring network is installed, consisting of: (1) soil moisture sensors, (2) automatic weather stations that report variables for calculating actual ET, and (3) digital piezometers to record water table levels in recharge zones. Monthly ET monitoring is conducted using energy balance and calibrated satellite models, integrating data from MODIS, Sentinel-2, and Landsat 8. Vegetation cover is monitored every six months using NDVI series validated through drone flyovers. Additionally, external audits are conducted every two years to verify benefit permanence, identify secondary effects (such as opportunistic species or changes in water quality), and validate benefit accounting using water balance models. Monitoring is integrated into platforms such as Aqua Positive and enables the generation of standardized reports in accordance with ESRS E3 and CDP Water Disclosure requirements, also facilitating Water+ certification under Act4Water.
This project aims to generate measurable, additional, and verifiable volumetric water benefits in a priority watershed in southern Africa through hydrological restoration based on the removal of invasive exotic plant species. These species, such as pines (Pinus spp.) and acacias (Acacia mearnsii), have expanded widely in mountainous and headwater ecosystems, exerting extreme pressure on water availability due to evapotranspiration rates that are considerably higher than those of native plant species. In many South African mountain watersheds, the invasion of these species has radically altered the hydrological regime.
These plants, introduced for timber or stabilization purposes, have displaced native vegetation and disrupted evapotranspiration and runoff patterns. As a result, aquifer recharge and base flows in rivers have been drastically reduced, affecting both ecosystems and human water supply. The project proposes a systematic ecohydrological restoration of the invaded areas, directly improving the soil’s infiltration capacity and the volume of water recharging shallow and deep aquifers. It is based on the adaptation and scaling of a practice already validated by The Nature Conservancy in Cape Town, where over 55 million cubic meters of water were recovered in 10 years, and incorporates continuous monitoring, adaptive ecological restoration, regrowth control, and verification under the Act4Water standard and VWBA 2.0 methodologies.
The priority watershed identified belongs to the Western Cape Water Supply System (WCWSS), one of the 100 most water-stressed basins in the world according to the CEO Water Mandate. Sources of stress include overexploitation of aquifers, loss of native cover, and sustained decline in rainfall in the context of climate change. The initial diagnosis combined satellite imagery from Sentinel-2 and PlanetScope with field validation to map invasive species cover.
Hydrological modeling using SWAT was used to simulate the water contribution of different vegetation cover types and to calculate the baseline evapotranspiration (ET) and effective recharge. This analysis allowed the identification of areas with the highest invasion density and the greatest potential for water benefit if intervened. It was estimated that in some sectors, the replacement of Acacia mearnsii with native species could generate savings of up to 700 mm of water per hectare per year.
The intervention was designed to maximize hydrological benefits without compromising soil stability or creating unintended negative impacts. First, spatial prioritization criteria were established, considering invasion density, slopes under 30%, logistical accessibility, and proximity to recharge zones. Selected areas were grouped into restoration blocks ranging from 50 to 150 hectares. For each block, an ET and soil moisture baseline was established. The intervention was structured into four sequential phases: hydrological diagnosis and planning, removal of invasive species, revegetation with native species, and long-term monitoring.
The methodology used to quantify the benefits follows the VWBA 2.0 framework, specifically method A-2 for ET reduction, multiplied by the restored area and adjusted by effective recharge coefficients. Additionality is ensured because the intervention areas were not included in any prior government restoration plan and would not have been restored without this project.
In the first stage, invasion density was measured using NDVI and SAVI analysis from satellite imagery, calibrated with field observations. Reference (ETo) and actual (ETc) evapotranspiration were estimated using automatic weather stations and validated with MODIS data. The accuracy of the modeling was checked through historical climate series back-analysis. The monitoring plan included soil moisture sensors at 30 and 60 cm depth, portable meteorological stations, and drone observation systems with multispectral sensors.
In the second stage, mechanical techniques such as uprooting and basal girdling were applied, with adaptive control depending on species type and slope. Progress in removal was tracked using drones and differential GPS, and regrowth was monitored monthly. Technologies used included electric chainsaws, manual tools, georeferenced treatment records, and mobile reporting stations.
The monitoring plan included visual inspections every 3 months and drone flyovers every 6 months. In the third stage, species such as Protea repens and Restio spp. were selected for their adaptability, low ET, and ecological compatibility. Establishment was measured through counts in permanent plots, and competition with invasive species was monitored using NDVI-based vegetation cover tracking. Drones were used to verify planting uniformity, moisture sensors guided supplemental irrigation, and hemispherical photographs were used to estimate canopy cover. Monitoring was conducted quarterly.
Finally, in the fourth stage, annual water savings are measured through water balance comparing pre- and post-intervention actual ET. On-site sensors, MODIS/Sentinel satellite data, automated piezometers, and weather stations are used. Benefit permanence is ensured through external audits every 2 years. The monitoring system includes dashboards connected to IoT sensors, satellite imagery comparisons, and traceability via the Aqua Positive platform.
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