Photo by @Camrin Dengel courtesy of Teton Watershed Aquifer Recharge Project, Friends of the Teton River
ASSESSING CONSEQUENCES OF CHANGING IRRIGATION METHODS
downstream irrigation or streamflow that enhances recreation or fisheries. The same is true for water applied to a field in excess of what is used for crop transpiration: the fraction of applied water that remains as liquid water and seeps into the underlying aquifer is not lost, it has also simply moved within the system. Thus, if a watershed is the system in question, water is only truly removed when it evaporates, is transpired during plant growth, flows out of the watershed, or is impaired to the extent that it is not considered viable for reuse.
Changes in irrigation and conveyance technology are often proposed with the goal of reducing water losses and thereby saving water for other purposes. However, if field or canal seepage was already serving the purpose of aquifer recharge, and that water was being reused downstream (as is the case in many watersheds in Montana), then the water was not actually lost in the first place. Thus, much of the time, shifts in irrigation technology are not truly saving water, they are simply changing the timing and location of its availability in the
Moving from hydrologic concepts to the implications of changing irrigation methods requires consideration of spatial and temporal scale and a wide variety of site-specific factors.
The importance of spatial scale
Spatial scale refers to the geographic extent being considered, whether it is a field, farm, or a watershed. Clearly delineating the scale of the system in question influences the way hydrologic processes or pathways are defined or described. For example, the traditional term conveyance loss refers to water diverted from a river into a ditch that does not make it to a field. To clearly understand and quantify how much water is actually ‘lost’ (i.e., unavailable for further reuse), it is essential to define the scale of the system in question. If the scale of the system is defined as a short stretch of canal, seepage from this canal can accurately be considered a loss from that system. However, if we define the scale as a larger irrigation district of a watershed, most of the water that seeps into the aquifer is not lost, it has simply moved to a different part of the hydrologic system and may be available to support other uses such as
FIGURE 6 | Spatial scale influences calculations of irrigation efficiency. Defining the spatial scale or boundary of a system affects the calculation of irrigation efficiency, i.e., the proportion of diverted water that is eventually consumed. If the defined system is a single farm or small group of farms (grey dashed boundary), a substantial proportion of the diverted water - especially under flood irrigation – may not be consumed by crops within that system. Water not consumed may seep into groundwater and flow downgradient, becoming available to water users at lower positions in the watershed. Due to the potential for water reuse, when the larger watershed is considered (orange dashed boundary), a much greater proportion of the total diverted water may be consumed.
translated or equated to a different scale. This perspective is critical for understanding that changes to irrigation at small spatial scales may influence water users at larger scales. In addition, decisions made at a farm or canal scale to achieve a certain objective may have different, and unintended, consequences at the watershed scale.
Even when spatial scale is taken into account, describing irrigation water use in terms of high or low irrigation efficiency can lead to confusion for two reasons: (1) the term irrigation efficiency does not describe the fate of diverted water that is not consumed by crops, and (2) as the term efficiency implies savings of some kind, water conservation and irrigation efficiency are often conflated. This misconception has led to the proposal of new terminology based on fractions instead of efficiencies (Table 3). The use of fraction-based terminology can lead to greater clarity and more accurate understanding of water use.²¹
larger watershed,k which may also be an important goal (Box 3).
How we calculate and understand irrigation efficiency also depends on the spatial scale being considered.19-20 For example, if water is diverted to a farm from a stream or river and only half of that volume is consumed by crop transpiration, this would traditionally be considered a 50% irrigation efficiency (Figure 6). However, water not consumed on one farm may pass through surface water or the shallow aquifer to be reused and consumed by crops on a downstream farm. In this way, multiple farms within a watershed that are each ~50% efficient can together lead to an irrigation system in which a much larger percentage of the water diverted is consumed. This increase in efficiency is generally proportional to the volume of water reuse within the system.
It is important to emphasize that the relevant spatial scale to consider in calculations of irrigation efficiency is dependent upon the question being considered, and that efficiency at one scale cannot be automatically
TERMINOLOGY
| TRADITIONAL TERMINOLOGY |
SUGGESTED TERMINOLOGY |
DESCRIPTION |
|---|---|---|
| Beneficial consumption* (Crop irrigation consumption) |
Intended consumption |
Water evaporated or transpired for the intended purpose, such as crop growth. |
| Non-beneficial consumption | Unintended consumption |
Water evaporated or transpired for unintended purposes – for example evaporation from fields or water surfaces, weeds, ditch vegetation. |
| Field (farm) loss (i.e., application in excess of plant needs leading to deep percolation and surface runoff) Conveyance loss (i.e., evaporation, seepage, plant transpiration from canals, faulty headgates, unneeded diversion |
Groundwater recharge Irrigation runoff (tailwater) Unintended consumption Non-recoverable seepage |
Water delivered to a field that does not contribute to intended consumption and conveyance water that does not make it to the field are often considered losses. This water actually has four potential fates and, from a watershed-scale perspective, only the latter two result in water ‘lost’ or removed from the system: Groundwater recharge – seeps into the underlying aquifer and becomes available for reuse downstream Irrigation runoff (tailwater) – flows over fields to rejoin a surface water body, or flows in canals to ponds, wetlands or back to the source Unintended consumption – is evaporated or transpired for purposes other than the intended use Non-recoverable seepage – seepage that does not make it to the aquifer or back to surface water, or becomes too contaminated to be considered viable for reuse downstream. |
| Irrigation efficiency (Total) |
Consumed Fraction (CF) Reusable Fraction (RF) Non-Reusable Fraction (NR) |
Using terminology that refers directly to the fractions defining irrigation efficiency and the fate of diverted irrigation water can provide an accurate and value-free means to convey these concepts. Consumed Fraction (CF) is synonymous with Total Irrigation Efficiency. CF = Evapotranspiration (ET) ÷ Diversions Reusable Fraction (RF) is the remainder. RF = 1-Consumed Fraction OR (Diversions - ET) ÷ Diversions For more accuracy, the Non-Reusable Fraction should also be figured into the equation: CF = ET + Non-Reusable Fraction (NR) ÷ Diversions RF = 1 - CF OR (Diversions - ET - NR) ÷Diversions Conveyance Efficiency and Application Efficiency can also be calculated and defined by Consumed and Reusable Fractions. |
*This term implies consumption that benefits the user but is not defined by law, as is the term “beneficial use”.
Table 3 | Terminology. The use of more precise terminology to describe irrigated agriculture systems could help promote a broader and more accurate understanding of how water moves through these systems. Some of the frequently used terms are also value-laden, which, positive or negative, can affect how accurately a term is understood. Alternative terminology that is precise and value-free is suggested above. This list represents a selection of important terminology but is not exhaustitive.
Photo by Whitney Lonsdale
influence streamflow during the same irrigation season and in the same watershed. In contrast, return flow from another farm may not affect streamflow for many months or longer, or may appear far downstream in a larger watershed. Therefore, understanding the specific temporal and spatial consequences of changes in irrigation is critical, and these consequences should be considered carefully in the context of identified watershed goals.
Site specificity
The important site-specific variation and unique characteristics of individual farms and geographic regions have a large influence on the spatial and temporal implications of changes in irrigation. For instance, hydrogeologic and geographic factors such as soil characteristics, underlying geologic material, depth to the aquifer, field slope and shape, distance of a field or canal from a stream or river, and elevation/position in a watershed, all influence how water moves through the system and how groundwater and surface water interact. In addition, cropping systems, water rights’ status and administration, cooperative agreements among water users, and individual decision-making influence water use and how changes in methodology will impact water availability. Therefore, while a conceptual discussion of actions and impacts is important and informative, it is difficult to generalize how changes in irrigation or conveyance method might influence water availability at different sites or watersheds. Clearly, site-specific understanding of potential impacts and outcomes, considered in relation to watershed goals and priorities, will be crucial for effective decision-making for individual water users and communities.
Irrigation influences the spatial and temporal availability of water
Understanding when and where water is available and needed is a critical element in water planning and balancing water supply and demand. As discussed above, changes to irrigation method or technology can alter the timing and location of water availability within a watershed.22 For instance, conversion from flood to sprinkler or lining of canals may reduce irrigation diversions, leaving more water instream at the time and place of diversion. In the spring, this may help to maintain high streamflow that is important for aquatic habitat and species that depend on flushing flows and gravel transport (Box 4). During later summer, this could help restore connectivity along a stream or between headwaters streams and mainstem rivers, which is essential to spawning and other lifecycle events of aquatic species. However, conversion to sprinkler or lining canals may also increase consumption and reduce aquifer recharge and irrigation return flows, which in turn could cause downstream reductions in late summer and early fall streamflow and reduce the stream cooling provided by groundwater return flow. Conversion may also affect the existence and distribution of wetlands,23 which provide critical habitat for migratory waterfowl,24 support high diversity and abundance of other plants and animals, and also provide important services to human communities such as erosion control, enhanced water quality, and flood control.
Individual farms, fields, and sections of canal contribute varying amounts of aquifer recharge; return flow from each will affect streamflow at different locations and times of the year. For example, return flow from one farm may
box 4 - STREAMFLOW AND AQUATIC ECOSYSTEMS
flows can influence the ecology of streams because these conditions tend to elevate both stream temperature and nutrient concentrations, both of which control species physiology and growth. Extremely low flows during drought years can lead to temperatures that exceed thermal tolerance limits for many fishes, causing recreational fishing closures (i.e., ‘hoot-owl days’) and mortality events. Warm temperatures and high concentrations of nutrients can also stimulate large blooms of algae and plants, and because these organisms consume oxygen at night, these blooms can produce critically low nighttime oxygen levels that can jeopardize fish and lead to permanent changes to river communities.
Flow also plays a critical role in moving and redistributing river sediments, providing the ‘fuel’ that shapes and reinvigorates riverbeds and floodplains over time. These processes not only influence the formation and maintenance of riparian vegetation, floodplain wetlands, and fish and bird habitat, but also sustain critical hydrologic connections between rivers and their floodplains, and between surface water and groundwater.
Irrigation practices have a clear influence on the flow regime of local rivers and streams (Figure 8), but the specific effects - and the timing of these effects - vary depending on the scale and type of irrigation and the environmental and spatial context. As we work to sustain and balance both healthy freshwater ecosystems and the livelihoods and well-being that depend on them, it is important that we explore irrigation water management options that can benefit both producers and aquatic ecosystems
Rivers and streams exhibit natural flow regimes that are characterized by the amount, temporal pattern, and predictability of streamflow throughout the year.25 In a large portion of western Montana, the flow regime is strongly influenced by snowmelt, with peak runoff during the spring, and low and relatively steady flows in the summer and early fall (Figure 7).26 Further east, other patterns of flow emerge, including early runoff of low-elevation snow in the plains or more erratic summer flows in response to localized rain events. Organisms that inhabit rivers, including microbes, plants, invertebrates, amphibians, and fishes have adapted to these local patterns of flow over long periods of time, and have thus evolved various behaviors, life-cycle events, or physiological processes that are directly tied to different components of the flow regime.27
For some species such as rainbow and cutthroat trout, rapidly rising spring flows and high peak flows are important because they trigger movement to spawning sites and successful spawning events. Other fish species such as bull and brown trout spawn during fall months and thus require adequate and relatively stable minimum flows during this time for movement, spawning, and the successful development of eggs. Aquatic insects, an important component of river food webs, are also influenced by patterns of flow and associated changes in river temperature and light penetration. Many of these species require specific environmental cues, such as changes in day length or rapid shifts in temperature, to complete larval development and emerge from the water as adults.
During the summer and fall, particularly low
Figure 7 | Importance of flow regime to aquatic ecology. Generic flow regime of a snowmelt-dominated river in Montana. The inset boxes and photos highlight portions of the flow regime that are particularly influential for aquatic ecology. (Fish illustrations by Joseph Tomelleri.)
Figure 8 | Conceptual differences in flow regime as a result of irrigation management practices. Flood irrigation may reduce streamflow during the spring and summer due to large diversions. Irrigation return flow can bolster baseflow in late summer, fall, and winter, with timing varying considerably. The solid and dotted red lines represent hypothetical variation of the influence of flood irrigation return flows on seasonal streamflow. Sprinkler irrigation may have a smaller effect on maximum and minimum flow but results in less return flow contribution to baseflow. Note that this figure is generalized and does not represent the effects of irrigation at all locations.
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