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Incorporating Climate Variability into Water Balance Modelling to Help Inform Water Management Design: The Pebble Mine Project

Author: Alana Shewan, Jaime CathCart
Conference: Mine Water Solutions 2018
Date: June 12-15, 2018

ABSTRACT
Water plays a key role in the operation of mining projects, and it is essential that water management facilities be designed to manage the inflows and outflows required for proper operations, as well as accommodate any unpredictable runoff resulting from storms events. The amount of water available varies by year, season, and day, so how do you design for this? Establishing a predictive water balance model during the mine design stage is one of the most important considerations for minimizing water management problems during the operation and closure stages for a project. The water balance model needs to simulate mine operations under normal climate conditions and also under unusually wet and dry periods of varying durations. Key water management planning may be driven by the spring melt of a winter snowpack in cold regions, the variation in snowpack accumulation, and the timing of the freshet season.

The current industry standard for introducing climate variability into water balance modelling is to model monthly precipitation values as distributions, which are typically based on the mean and standard deviation values of historical monthly precipitation. The water balance model is then run for thousands of iterations using Monte Carlo simulation techniques to produce a large range of potential precipitation conditions and corresponding results. This procedure is effective, but it has a major shortcoming in that it simulates precipitation in every month as being completely independent of precipitation in any proceeding or following month. Areas that experience strong climate cycles resulting in extended dry or wet periods may not be correctly represented by this type of modelling.

This paper presents a case study for the proposed Pebble Mine project (Pebble) in Alaska and highlights the importance of a climate variable water balance model for informing the water management plan and design. The Pebble water balance model utilizes a 68-year monthly time-series of temperature and precipitation that was developed using a long-term regional dataset in combination with the extensive hydrometeorological dataset collected for Pebble. The time-series data were stepped through the model incrementally by year for the planned life of the project, thereby preserving the inherently cyclical nature of the climate record including wet and dry cycles, while creating 68 unique sets of water balance results for each month of each year of the project. The results were used to develop a robust water management strategy that supplies sufficient water to maintain full mine operations, even during prolonged dry periods, and maintains downstream flow requirements for aquatic habitat and resources.

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