Isometric’s new Rice Methane Reduction protocol
This is a new issue of newsletter from Deloitte Tohmatsu Sustainacraft.
Methodology Updates is a series covering carbon and biodiversity credit methodologies. This article introduces Isometric’s latest Rice Methane Reduction protocol that is currently in public consultation period.
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Author: Nick Lau (Applied Scientist)
Summary
Isometric has released a new protocol draft for reducing methane emissions from rice cultivation through alternate wetting and drying (AWD), a water management practice that has been studied extensively and is already reflected in several existing carbon credit methodologies. AWD reduces methane emissions by periodically interrupting flooded conditions in rice fields, limiting the anaerobic processes responsible for methane production.
Methodologies for AWD are already established under Verra, Gold Standard, and Japan’s Joint Crediting Mechanism (JCM), and the overall accounting approach is largely consistent across these frameworks. Each defines a flooded baseline, introduces drainage-based interventions, and estimates emission reductions based on changes in water regime. These similarities reflect a shared scientific foundation and understanding of methane dynamics in rice systems.
Isometric’s protocol builds on this foundation while introducing a set of design choices that affect how projects are implemented and how emission reductions are translated into credits. These include the use of field-level evidence for baseline definition, explicit conditions on water levels during AWD implementation, and a structured approach to quantification and uncertainty. Measurement, monitoring, and verification requirements are also defined in a way that links observed field conditions directly to crediting outcomes.
This article first reviews the scientific basis of AWD and how it affects greenhouse gas emissions in rice systems. It then outlines the structure of Isometric’s protocol, including eligibility, baseline definition, quantification, and monitoring requirements. Finally, it compares the protocol with existing methodologies to highlight where they align and where they diverge in implementation.
Stacy Kauk, P.Eng.Chief Science Officer
1. Scientific basis of Alternate Wetting and Drying (AWD)
Rice is typically grown under flooded conditions. Standing water limits oxygen in the soil, creating an environment where microorganisms break down organic matter anaerobically and produce methane.
AWD changes how water is managed during the season. Instead of keeping rice fields continuously flooded, irrigation is periodically stopped so that water levels decline before being restored. A typical AWD cycle involves:
- Flooding during early crop development
- A drainage period where water levels gradually fall
- Re-flooding once the water table reaches 5-15 cm below the soil surface
- Repeating this cycle several times over the season
This alternation between wet and less-wet conditions changes the soil environment. When water recedes, oxygen enters the upper soil layers. This suppresses methane-producing microorganisms and allows methane in the soil to be broken down. The result is a reduction in net methane emissions.
The magnitude of this reduction depends on local conditions, particularly drainage frequency, soil type, organic matter inputs, and climate. Residue incorporation tends to increase methane production, while more frequent drainage generally increases reductions. Across different studies, methane emissions are commonly reduced by 30-70%.
2. Isometric AWD protocol
2.1 Eligibility and applicability
The protocol defines the conditions under which AWD can be applied and credited. These requirements focus on ensuring that water management changes are measurable, attributable, and consistent with existing farming systems.
Projects must take place in irrigated lowland rice systems where water levels can be actively controlled. Fields that depend entirely on rainfall or uncontrolled flooding are not eligible. Eligible interventions include:
- Introducing multiple drainage events during the growing season.
- Reducing the overall duration of flooding.
- Transitioning from transplanted rice to direct-seeded systems, where early flooding is
These interventions must represent a change from existing practices. Each field must demonstrate that it follows an eligible baseline water regime (typically continuous flooding) within the five years prior to project start. Evidence can include remote sensing data, field records, or regional statistics.
The protocol also constrains how fields can be managed under the project:
- The number of cropping seasons must remain unchanged.
- Yields must not decline due to the intervention.
- Any new cultivars must not require additional changes in management practices.
These conditions ensure that the project isolates the effect of water management rather than introducing broader changes to the production system. Projects must also demonstrate that the practice is not already widely adopted in the region and that it is not required by regulation.
2.2 Baseline and stratification
The baseline represents the conditions against which project emissions are compared. Under the protocol, this corresponds to irrigated systems characterized by prolonged flooding, including continuous flooding or, where applicable, a single drainage event. Baseline conditions are defined at the field level and include:
- In-season water regime.
- Pre-season flooding.
- Organic amendment practices.
- Typical number of growing seasons.
- Rice variety selection
To account for variability across the project area, fields are stratified and grouped into strata with similar characteristics. Stratification typically considers soil type, water regime, and management practices. Emission estimates are then applied within each stratum, improving consistency and reducing measurement variance.