What is Interannual Variability?

Water is a critical resource in the mining process, influencing operational performance, regulatory compliance, environmental management, and closure outcomes. However, the challenge is not just how much water is available, but also how much that availability fluctuates from year to year.

The International Council on Mining and Metals (ICMM) recently highlighted this challenge in its Global Mining and Metals Water Dataset. It’s reported that 16% of mining and metals facilities globally are exposed to high interannual variability, with 74% of facilities in Oceania exposed to high or extremely high interannual variability (ICMM, 2026).

In regions such as New South Wales (NSW), Australia, extreme seasonal events, including storms, can also significantly impact mining operations, requiring mine sites to conduct seasonal risk assessments (NSW Resources, 2024).

This raises an important question for mine operators across Australia and New Zealand: how can you plan and effectively manage water when rainfall, storm events, evaporation, and runoff look different each year?

In this month’s Conversation on Closure, we explore why interannual variability matters when planning for climate resilience throughout operations and into closure, and discuss practical strategies for managing water quality and quantity uncertainties and designing resilient mine water management systems.

What is Interannual Variability?

Interannual variability is defined as the “year-to-year fluctuation in available water supply, capturing how unpredictable water availability is from one year to the next” (ICMM, 2026). Unlike baseline water stress and depletion, which look at water pressure under average conditions, interannual variability reflects the reliability of water supply over time (ICMM, 2026).

The higher the interannual variability, the greater the fluctuations between the years (ICMM, 2026). This makes interannual variability a critical consideration because mine site landforms (including mine rock stockpiles, tailings storage facilities, and water infrastructure) and cover systems must perform reliably during both extremely wet and dry years, not just under average conditions.

What are the Risks and Uncertainties with Interannual Variability?

When the average annual precipitation decreases, it can lead to even larger reductions in runoff, especially in arid and semi-arid regions (Zheng et al., 2026). In other words, wet years produce higher streamflow and dry years lead to lower streamflow.

Water availability then becomes less predictable and reduces the reliability of water supply required for mine operations and closure. Not considering climate variability can result in landform and cover designs that are not fully optimized or adaptable (INAP, 2017).

For example, during a drier cycle, tailings can generate dust emissions that may affect air quality and human health, while tailings storage facilities (TSFs) can potentially crack due to desiccation in the upper portion of the impoundment beach, increasing the risk of erosion (GISTM, 2025). Under one drier year with intermittent wet months, a TSF may avoid contributing to poor air quality and maintain erosion processes under control. However, the system can be stressed beyond its capacity if multiple drier cycles occur successively.

On the opposite side of the spectrum, if major rainfall or storm events aren’t considered when designing tailings storage facilities, there’s a potential risk of increased metal leaching from tailings that can impact nearby soil and water quality, as well as increased risk of flood overtopping and tailings liquefaction (Enze et al., 2025; Okane, 2024).

For soil cover systems, if a 1-in-10-year average precipitation storm event is calculated using a drier cycle dataset and is used to inform the optimal cover system thickness for supporting vegetation, the cover may not be able to sustain vegetation throughout the entire life of the cover system (INAP, 2017). Alternatively, during drier years, a diminishing water cover for TSFs can reduce tailings saturation and increase oxygen ingress, which can promote sulfide oxidation and increase the risk of metal leaching. Figure 1 shows the volumetric water content within a soil profile during storm and dry events.

Likewise, when designing wetlands as part of the post-mining land use, it is important to understand whether the watershed has enough water supply during drier months to support wetland functions (INAP, 2017). If precipitation variability isn’t considered in the design, the wetland system is less likely to be resilient to changes in water supply (INAP, 2017).

How to Plan for Interannual Variability: Okane’s Approach

Many processes that influence operational and closure performance, including infiltration, seepage generation, solute transport, oxygen ingress, and contaminant loading, respond to climate at daily or sub-daily timescales. When planning for interannual variability, it’s important to identify periods when multiple climate oscillations amplify dry periods or wet periods to better understand how current water management approaches fit into the overall context of the closure timeframe and inform cover system and landform designs (INAP, 2017).

The key questions we often ask ourselves and our clients are:

• What is the probability of poor performance due to climate oscillation?
• How much risk can be tolerated as a result of the poor performance of each facility?

At Okane, we first use the Köppen-Geiger Climate Classification to account for the seasonality of climate conditions — including extreme events during wet periods of the year — so that we can consider the dynamic environmental factors that affect closure landform and cover system performance (Okane, 2024).

This Köppen-Geiger Climate Classification is then integrated into our modelling practice, where we develop physically-based simulations driven by time-series climate data rather than reliance on long-term average annual conditions. We routinely develop site-specific historical and climate change databases at daily resolution to support numerical modelling of closure landforms, cover systems, water balances, and contaminant transport processes. Using current climate science and statistical downscaling methodologies, we develop databases that enable us to evaluate the effects of seasonal fluctuations, multi-year wet and dry periods, and projected climate change on long-term performance.

Our team of experts then embed our climate database into our soil-plant-atmosphere (SPA) modelling framework, where climatic forcing data are coupled with hydraulic, thermal, airflow, and vegetation processes to simulate water and energy balances. Using tools such as GeoStudio, HYDRUS, DRAINS, and GoldSim, we evaluate dynamic processes including snow accumulation and melt, evapotranspiration, runoff generation, infiltration, net percolation, interflow, and flow velocity, as well as thermal and oxygen transport, through cover systems and mine waste landforms. These analyses provide us a quantitative basis for assessing closure performance under variable climatic conditions rather than a single deterministic scenario.

Several projects illustrate our approach, including:

• Tasmania, Australia

At a mine site in Tasmania, Okane was engaged to reimagine closure at the site by developing stable landforms that reduce the volume of contact water requiring treatment to meet regulatory requirements. To inform the closure strategy, we used event-based modelling to design the infrastructure to withstand extreme weather events (i.e., a 1-in-200-year Annual Exceedance Probability rainfall event). We also assessed infrastructure performance under long-term, synthetically generated rainfall that reflect climate variabilities between seasons.

While the event-based simulation determines the appropriate sizing and geometry of the infrastructure, the long-term climate-based model informs the probability of exceeding the capacity of the infrastructure over the life of mine and beyond. From the long-term model, we learned that the interannual variability from the climate input has a significant impact on the water table fluctuations, which further impact water quality from the backfilled open pit. The event-based model is unable to produce a fluctuating water table due to short duration and lagged response from the backfill. On the other hand, the long-term model provides insights on when and where monitoring and maintenance effort should be focused during the cover system construction period, filling in a gap due to the limitation of event-based models.

Our assessments and models helped inform a mine closure and water management strategy that prioritized a source control approach to mitigate long-term acid and metalliferous drainage (AMD) impacts on the surrounding environment. We improved upon the previous closure and water management strategy, where the cover system construction would potentially result in an increase of AMD following continuous wetting up and drain down processes from high rainfall events.

• South Australia, Australia

At a polymetallic mine in South Australia, Okane applied SEEP/W and CTRAN/W modelling to evaluate salinity transport through closure cover systems. Our numerical models coupled transient water movement with advective and diffusive solute transport processes, providing insight into long-term salinity migration under climate-driven seepage conditions.

• New South Wales, Australia

At a gold and copper mine in New South Wales, our team combined GoldSim-based field response (inverse) modelling, GeoStudio SPA modelling, and climate change assessment to calibrate hydraulic properties against observed field performance and evaluate future closure performance under changing climatic conditions. By integrating monitoring data with climate-driven simulations, our work reduced uncertainty in long-term predictions and improved confidence in closure design decisions, as well as water management decisions.

Across these projects, Okane’s objective is not to simply model average future conditions, but to understand the sensitivity of water management and closure systems to climate variability and uncertainty. The systems can behave differently if the site experiences a succession of wet years versus alternating wet and dry years.  Through the integration of daily climate datasets, calibrated numerical models, probabilistic methods, and climate change scenarios, we provide defensible predictions of long-term performance and support risk-informed closure decisions that remain robust under a broad range of future climatic conditions.

If your site is exposed to interannual climate variability and you’re looking for a risk-informed approach to designing resilient water management and closure systems, connect with our team at info@okaneconsultants.com.

References

International Network for Acid Prevention. (2017). Global cover system design: Technical guidance document. https://www.inap.com.au/wp-content/uploads/global-cover-system-design.pdf

International Council on Mining and Metals (ICMM). (2025). Tailings management: Good practice guide (2nd ed.). https://www.icmm.com/website/publications/pdfs/innovation/2025/tailings-management-good-practice-guide-2nd.pdf

ICMM. (2026). Global mining and metals water dataset. International Council on Mining and Metals. https://www.icmm.com/website/data/2026/research_water-dataset.pdf 

New South Wales (NSW) Resources. (2024, November 13). Managing storm season risks in NSW mines. https://www.resources.nsw.gov.au/news-articles/managing-storm-season-risks-nsw-mines

Okane Consultants. (2024, March 21). Water stewardship in mining through innovative store-and-release cover systems. https://okaneconsultants.com/ideas/water-stewardship-through-store-and-release-covers/

Zheng, H., Chiew, F., Post, D., Robertson, D., & Charles, S. (2026). How will higher interannual precipitation variability intensify water stress under a drying climate? Water Resources Research, 62, e2025WR042767. https://doi.org/10.1029/2025WR042767