Photo by @Camrin Dengel courtesy of Teton Watershed Aquifer Recharge Project, Friends of the Teton River
WATER AND IRRIGATED AGRICULTURE
and the timing of groundwater availability throughout the year. In addition to the maintenance of existing aquifers, irrigation has also produced new shallow aquifers, all of which soak up irrigation water and release it at a later time, potentially influencing streamflow. Thus, in areas with extensive flood irrigation, a novel pattern of human-influenced streamflow and groundwater hydrology has developed, and this pattern still exists today. Downstream water users, including irrigators, municipalities, fish and wildlife, recreationists, and individuals on wells, heavily depend on this legacy of agricultural activity and the many aquifers that are maintained or enhanced by irrigation.
Over the past 50 years, there has been a trend toward new and more efficient irrigation methodsᵇ that result in a lower volume of water diverted for irrigation, a larger proportion of diverted water consumed by crops, and a smaller proportion of that diverted water seeping into the ground to recharge aquifers. Commonly referred to as increasing irrigation efficiency, (Box 1) these changes often lead to on-farm gains in profitability and can yield specific hydrologic and ecological benefits (Table 2). However, increased irrigation efficiency can also result in unintended consequences at the watershed level, including increased water consumption, reduced groundwater levels, and undesirable changes to surface water availability.ᶜ To prepare for changing water regimes and to develop informed, adaptive, and place-based management strategies, it is critical that we consider the complex, watershed-level implications of changes in irrigation efficiency.
Water is our most valuable natural resource, and is used to support the demands of industry, agriculture, hydroelectric power generation, and municipalities. Water also sustains Montana’s booming recreation and tourism economy and maintains the diverse freshwater ecosystems that provide natural goods and services and promote human well-being. As our population continues to grow, and the collective demand for water increases, it is imperative that we carefully assess how our water is used, as well as how changes in water distribution, management, and governance are likely to influence its availability in the future. This is especially important in the context of a changing climate.
Here, we focus on irrigated agriculture – specifically, the topic of ‘irrigation efficiency’ – because irrigation represents the largest consumptive use of water in the state (~67%¹). As a consequence, irrigation systems, and any changes therein, will have significant and lasting effects on how water moves across the landscape and through our river valleys, with important implications for agriculture, wildlife, policy, and society.
More than 150 years of irrigated agriculture has altered the natural water balance in many of Montana’s river valleys. Prior to agricultural development, intact floodplains and large beaver populations maintained regular exchange between surface water and groundwater, and alluvial aquifers were widely distributed in river valleys. Over the past century and a half, thousands of miles of seeping irrigation canals, ditches, and saturated farm fields have created a new hydrologic system, which maintains alluvial ᵃ aquifers but alters their geographic distribution
Figure 1 | The multidisciplinary nature of irrigated agriculture
Building a Collective Understanding
Irrigation is a vital tool for water management, and the topic of irrigation efficiency is multifaceted, requiring careful consideration of social, political, ecological, and hydrologic perspectives (Figure 1). In 2019-2020, the Montana Water Center convened a technical working group to better understand how changes in irrigation methods affect groundwater recharge, streamflow, and local and regional water supplies. The group comprised a diverse set of stakeholders including irrigators and irrigation managers, university scientists, agency scientists and managers, legislators, non-governmental organizations and tribal representatives (see List of Contributors, p. 37).
A specific effort was made to represent different perspectives and areas of expertise. Through a series of webinars, workshops, meetings, and other outreach efforts, this group worked to (1) distill the core concepts and knowledge gaps associated with changes in irrigation efficiency, and (2) explore ways to mitigate any unintended consequences associated with such changes. In this document, we summarize the current state of knowledge and attempt to broaden understanding of this complex topic to support decision making by legislators, natural resource agencies, funding entities, and irrigators. We begin with a brief overview of irrigated agriculture in Montana, including a short description of changes in irrigation method that have taken place over the past 50 years. Next, we explore the hydrology of irrigated agriculture in alluvial river valleys and what we know about the impacts of changes in irrigation method. We focus on irrigation from surface water sources, as surface water provides 99% of irrigation water withdrawals in Montana.d,2 Next, we provide an overview of Montana water policy as it relates to changes in irrigation method, focusing on key aspects of policy that merit consideration in light of climate change and increasing water demand. Finally, we present creative ways to support the resilience of irrigated agriculture in Montana while addressing current and future changes in our water supply.
BOX 1 - DEFINING IRRIGATION EFFICIENCY
In the standard interpretation of efficiency, resources that are used but do not contribute to production are considered lost or wasted, and reducing this waste or loss is the goal of efficiency improvements. Herein lies the challenge of using this concept to assess irrigation water use (or water conservation) beyond the field scale. Water that is not delivered to the field or consumed in production is often not lost or wasted; it simply moves to another part of the watershed where it may serve another purpose, such as recharging aquifers or providing downstream water for agriculture or streamflow. In these cases, maximizing irrigation efficiency may have unintended consequences elsewhere in the system. In some situations, water not consumed by crops evaporates from soils, is consumed by non-crop plants, or flows to saline sinks, the ocean, or aquifers so deep they are considered inaccessible; in these cases, maximizing efficiency may lead to water conservation.
Efficiency is often described as the capability of a specific effort to produce a specific outcome with a minimum amount of waste, expense, or unnecessary effort. Not surprisingly, increased efficiency is pursued as a goal in many arenas (e.g. economic markets, machinery, work flow, industrial processes). Traditional agricultural engineering has embraced the concept of irrigation efficiency when designing and evaluating irrigation projects. In this context, irrigation efficiency (IE) is defined as the ratio of water consumed by crops to diverted water: IE = Consumed water ÷ Diverted water.
If 50% of diverted water is consumed by the crop, then the field, farm or irrigation project is considered 50% efficient.
If 100% of diverted water is consumed, then the project is 100% efficient.
Total irrigation efficiency is determined by the combination of conveyance efficiency (delivered water ÷ diverted water)and application efficiency (consumed water ÷ applied water).
Table of Contents | Key Messages | Water and Irrigated Agriculture | Irrigated Agriculture in Montana | The Paradox of Irrigation Efficiency | Hydrology of Irrigated Agriculture | Assessing Consequences of Changing Irrigation Methods | Water Policy and Irrigated Agriculture | Adapting to Change | Conclusion | List of Contributors | Glossary | Footnotes | References