**1. Introduction**

A variety of management tools are required to effectively understand, predict, and optimize surface water quality. This includes traditional information such as data from welldesigned monitoring programs, and increasingly relies on the use of water quality models. Water quality models are critical for assessing situations that cannot be directly measured, to better understand system constrains, and for examining scenarios, including operational changes. Reservoir management has benefitted especially from the growing application of water quality models to the problems of regulated flowthrough and anthropogenic water demands. These models have been applied globally to issues such as selective withdrawal strategies for optimizing water temperatures and quality e.g., [1–4], and water quality simulations e.g., [5–7]. Other uses include modelling nutrient loading from the watershed [8,9], impacts of pisciculture [10], reservoir contaminant scenarios [11], and impacts of changing climate and streamflow on reservoir water quality [12].

Canada has one of the highest rates of water availability per capita per year [13]. Canada also has one of the world's highest per capita water use [14]. The Prairie region contains 80% of Canada's agriculture [15], with agriculture accounting for an 86.5% share of surface water use in the South Saskatchewan River Basin [16]. The climate of the Prairies is highly variable with an extreme temperature range and periodic floods and droughts [15]. The landscape is semi-arid, and many users compete for limited water resources [17]. Population growth and irrigation expansion will place further pressure on Prairie reservoirs showing signs of water stress [18].

**Citation:** Terry, J.; Davies, J.-M.; Lindenschmidt, K.-E. Buffalo Pound Lake—Modelling Water Resource Management Scenarios of a Large Multi-Purpose Prairie Reservoir. *Water* **2022**, *14*, 584. https:// doi.org/10.3390/w14040584

Academic Editor: Fernando António Leal Pacheco

Received: 18 January 2022 Accepted: 10 February 2022 Published: 15 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

#### *Buffalo Pound Lake*

Buffalo Pound Lake (BPL) is the water source for approximately 25% of the provincial population of Saskatchewan, Canada. Water demands from BPL include municipal, industrial, and agricultural abstractions, and the lake is an important recreational waterbody. The importance of BPL has resulted in numerous scientific studies over the years e.g., [19–24], and organizational reports from environmental consultancy firms and government agencies. The water quality of BPL is a continuing concern. The lake is situated in an area that experiences high runoff variability associated with its variable climate and is therefore subject to periods of both high and low runoff. BPL is located within a former glacial-melt channel in the Upper Qu'Appelle River Basin. Glacial till on the prairies has abundant nutrients and salts and most prairie lakes are therefore naturally eutrophic [25]. In contrast, BPL's supply reservoir Lake Diefenbaker (LDief) is located in the South Saskatchewan River Basin. LDief receives much of its water from the Rocky Mountains and acts as a nutrient and sediment trap [26,27]. Water sourced from LDief is therefore generally lower in nutrients, salts, organic carbon, and sediment relative to runoff received from BPL's local watershed.

Several recent studies have evaluated options for augmenting the water supply from LDief to BPL to meet increasing water demand from proposed irrigation expansion projects e.g., [28–31]. However, there are physical constraints on how much water can be diverted to BPL. Water levels in the Qu'Appelle River watershed are managed as a whole system that includes multiple downstream lakes—one of which is a lake tributary to the Qu'Appelle that can act as a large surge tank during periods of high water. The diversion of flow into BPL from LDief is also dependent on channel capacity and the amount of upstream runoff from BPL's local watershed in any given year.

Given the complexity of managing the water system, there is a need to better understand how BPL water quality is affected under different inflow scenarios—notably in years with high runoff from its local watershed versus drier years when the majority of water is transferred from LDief. This in turn can form the basis for developing tools and information to help inform decisions on flow management.

Water quality models are an emerging tool for managing water resources on the Prairies. Within the last few years, the first complex water quality models have been applied to BPL as part of a targeted collaborative research strategy. The most recent study, that tested the sensitivity of a BPL CE-QUAL-W2 (W2) model to its boundary data, concluded that flow management strategy may be the most important aspect of water quality management in BPL [32]. The W2 model was calibrated to a historical period due to the empirical data available at the time the model was developed. Regardless, the results clearly implied that any proposed change in flow regime was anticipated to impact BPL's water quality. This model offers a highly relevant platform for assessing whether inter-basin water diversion from LDief, during periods of high local watershed inflows, makes a difference to water quality.

In this paper we use the W2 model to evaluate the effect of different water diversion volumes on the water quality of BPL. Analysis of diversion scenarios provides insight into how flow management in a variable climate environment impacts water quality. This research updates the pre-existing calibrated W2 model, extending the calibration period by including an additional 6.5 years of recent data. The lake now benefits from an expanded monitoring program including water quality profile data, and the reinstatement of an upstream flow gauge. This new data covers a natural period of several wet years, with large watershed runoff events, followed by several relatively dry years where source water shifted to a higher percentage from LDief. The goal of this research was to test whether the updated model provided suitable outputs to assess different water management scenarios. A secondary goal was to undertake scenario assessments using W2. These would determine how different transfer rates would have affected water quality in BPL after the lake received a substantial amount of poor water from local runoff. We anticipated that increasing flows from LDief would result in higher water quality, as defined by lower levels of nutrients in

BPL. The type of modelling approach used in this study can provide an informative tool for investigating this important management question in other aquatic systems.

Section 2 describes the base model set-up and calibration, and details of how the water diversion scenarios were developed. Section 3 presents the results of the base model calibration and results of the scenarios.
