Lake Diefenbaker is a fresh water reservoir in the Prairies of southern Saskatchewan that formed following the establishment of the Gardiner and Qu’Appelle dams over 50-years ago. The reservoir, a primary storage facility in the South Saskatchewan River Watershed, both manages water flow (flood-drought control) and ensures a reliable water supply for municipal, agricultural, industrial, and recreational users. The provincial government recently announced a decision to move forward with a $4-billion project to irrigate water from the reservoir , and committed $22.5 million this year towards preliminary engineering and initial construction. The multi-phase plan proposes to construct two new canal systems that will annually extract more than 850 million cubic metres of water to irrigate nearly 500,000 acres of specialty crops between Moose Jaw and Biggar over the next 50 years. With such a large-scale project over a long time period, there is a pressing need to understand its environmental impacts, particularly on the availability and quality of surface water and stability and health of agricultural soils.
The region is characterized by a semi-arid climate that carries abundant sunshine and warm weather throughout the summer and early fall, and a landscape that is largely composed of flat areas with low permeability soils. While these features together result in an area that has exceptional farming conditions, they also cause excessive evaporation. The risk of evaporation has always been prevalent in the Prairies with several significant droughts over the past century and their frequency and severity is increasing more recently. Current estimates indicate that evaporation from the surface of Lake Diefenbaker accounts for more water loss than all other withdrawal sources combined, such that a single millimetre of vaporized water is equivalent to as much as 430,000 cubic metres.
The amount and rate of evaporation are physically governed by: (1) the surface area of an exposed water body or a wetted soil, and (2) atmospheric variables (wind speed, temperature, humidity, and sunshine). A bigger surface area means a larger amount of water is available to evaporate; while warm, sunny, windy, and dry conditions lead to a faster rate of evaporation. Estimating evaporative losses from exposed surfaces is complex, especially in large-scale settings. This has prompted the authors at the University of Regina to develop specialized equipment for simulating atmospheric conditions.
The plan to move substantial water flow via open canals and to irrigate large areas of land will increase the exposed surface area and, as such, increase the amount of water available to evaporate. Similarly, the predicted rise in air temperature and extreme weather events (drought severity and intensity) reported by the Intergovernmental Panel on Climate Change are expected to continue, thereby increasing the rate of evaporative loses. Likewise, the planned change in irrigation practices can lead to soil salinization because the water is known to contain dissolved salts. Salinization occurs as dissolved salts precipitate in the soil when irrigated water leaves the surface by evaporation instead of infiltration. When soil salinity reaches a certain point, the salts become toxic to plants and hinder nutrient interactions and water uptake by roots, all of which results in poor plant health and a loss of yield. Such situations have occurred in the vicinity of large reservoirs and similar climates elsewhere around the globe.
Given the likelihood of possible environmental changes, we need to further investigate the associated short-, medium- and long-term impacts of this project. For example, what effects can open-water canals have on the quality and amount of water lost to the atmosphere? Will faster evaporation rates lead to more rapid salt accumulation in arable lands? Can increased irrigation alter microclimates in the region enough to either hinder or improve farming? How often will farmers need to rely on groundwater for irrigation in lieu of reservoir water during low-flow periods caused by drought? These and similar issues need to be identified and addressed at various stages in the lifespan of the project. By engaging Saskatchewan communities of researchers, stakeholders, and the public, we can develop the right information to guide us as we embark on this expansion project and ensure that it serves us well for the next 50 years.