Photo by @Camrin Dengel courtesy of Teton Watershed Aquifer Recharge Project, Friends of the Teton River
HYDROLOGY OF IRRIGATED AGRICULTURE
About half of the total annual flow in typical Montana streams is supported by groundwater,5 and some portion of this is influenced by irrigation. Irrigation ditches and canals interact with the aquifer in similar ways to a stream or river: water can seep from a ditch or river to the aquifer, contributing to aquifer (groundwater) recharge; water can also flow from the aquifer to the ditch or river, bolstering flow through aquifer discharge.
Irrigation ditches, if unlined, generally increase groundwater recharge in a watershed. Diverted water flowing through the vast network of irrigation ditches in a given watershed spreads surface water over a large geographic area and results in greater recharge than would take place if water flowed only in current riverbeds or stream channels.g Water applied to a farm field in excess of what is consumed by plants, and which does not evaporate, also seeps into the ground and contributes to aquifer recharge. Much of this groundwater recharge from fields and ditches in turn supports surface water streamflow when aquifers discharge into streams and rivers. Because there is a lag time between aquifer recharge by irrigation and aquifer discharge to surface water, this can be especially important for bolstering streamflow in the low-water months of the year.
With 10.5 million acre-feet of water per year diverted for irrigation, and 2.6 million acre-feet of that water consumed by crops, it is important to understand what is happening with the nearly 8 million acre-feet of water that is diverted but not consumed. Although this water has historically been considered a loss or a product of inefficiency, it has nonetheless come to play a critical role in the hydrologic regimes of our present-day irrigated valleys (Figure 5 and Box 3).
In irrigated valleys in central and western Montana, individual parcels of land such as irrigated fields, subdivisions, and municipalities are hydrologically connected by surface water systems (streams, rivers, irrigation ditches and canals) and by the aquifer that underlies the watershed. Thus, the spatial and temporal variation in surface water supply is inextricably linked to groundwater. Recognition of this connection has grown in Montana over the past 50 years and in 200715 the state reinforced the ‘conjunctive’ management of groundwater and surface water as a single resource and began requiring acknowledgement of this status in any water resource decision-making. Groundwater and surface water naturally interact in riparian systems as water moves between shallow aquifers and rivers and streams, and groundwater plays a crucial role in sustaining streamflow throughout the year.
Figure 5 | Pathways of water flow in irrigated agriculture
box 3 - Irrigation HYRDOLOGY
proportion of diverted water that makes it to the farm. It is important to note that the amount of water delivered to the farm may change over the growing season depending on water supply conditions, priority of a water right, and the frequency and extent of crop harvest on an individual farm.
Once water is delivered to the farm, it either (a) is consumed by evapotranspiration of crops (i.e., intended consumption) or evapotranspiration from soils, sprinklers, and non-crop plants (i.e., unintended consumption), (b) flows overland and rejoins a surface water body, or (c) seeps into the ground. Much of the water that seeps into the ground makes its way into the aquifer, recharges groundwater, and can contribute to irrigation return flow ⁱ and subsequent downstream use (recoverable seepage). A small percentage of seepage may percolate to such deep aquifers it is not reasonably recoverable,18 or may become too saline or contaminated by agricultural chemicals or fertilizers to be considered reusable (non-recoverable seepage).ʲ The amount of water consumed by crops is influenced by various environmental factors (e.g., weather, soil type) and farm practices (e.g., crop or seed type, method of application). Consumption by non-crop plants or evaporation from fields or sprinklers is influenced by crop phenology/timing, crop density, weed management, and the environmental factors listed above. The intended and unintended consumption both contribute to total consumptive use, which, combined with nonrecoverable seepage, represents the total quantity of water that is unavailable for reuse (colored red in Figure 5)
Figure 5 portrays pathways of water flow in an irrigated agriculture system, including a surface water source (stream or river), an irrigation canal, an irrigated field, domestic buildings/homes on a well, and the alluvial aquifer that underlies the entire system. We use contemporary agricultural engineering terminology16-17 that focuses on the fate of water to describe pathways of water in an irrigated basin; this framework avoids value implications (e.g. farm loss, beneficial consumption, efficiency) and forms the basis for hydrologic tools, such as a water budget, that can be used to understand effects of conversion on local and downstream hydrology.
Water that is diverted from the surface water source has one of two fates. It either makes it to the place of use (i.e., application on the farm or field) or it doesn’t. If water does not make it to the place of use, it either (a) remains in the ditch and ends up in a pond, wetland, or back in the original source, (b) is unintentionally consumed via evapotranspiration in ditches (i.e., unintended consumption), or (c) seeps into the ground as canal recharge. Much of the water that seeps into the ground enters the aquifer and may become available (recoverable seepage) for subsequent downstream use.ʰ The specific fate of water that does not make it to the place of use is strongly influenced by how water is delivered to the farm, including whether it is by pipe or ditch, and if and to what extent the ditches are lined. Additional characteristics of the conveyance system, such as length, adjacent vegetation, maintenance history, underlying geology, and elevation relative to the water table, all play a role in determining the
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