Gold Creek Return Flow Pilot Study: Evaluating Potential Benefits of Early Season Center Pivot Irrigation on Local Hydrology and Production

The project site is Thomas Herefords Ranch in Gold Creek, Montana. The ranch is located near the mouth of Gold Creek, and the project will focus on three pivot-sprinkler-irrigated fields. These three pivots irrigate roughly 260 acres, two on the east side of the creek and one on the west side (see attached project map). Grass is grown on one of these fields while the other two fields are split between grass and alfalfa/orchard grass mix.

The project will occur over three years. In year 1, we will install the monitoring equipment and collect baseline data. In years 2 and 3, we will modify the pivots on the east side of the creek (modified irrigation technique) to apply as much water as possible early in the season by increasing the overall amount of time the pivot is run, not increasing the flow rate of the system. The west side pivot will operate as normal (control). Comparison data will include previous years’ hay production, baseline data from year 1, and the west pivot, which will control for inter-annual variability. To ensure that the timing and volume of water diverted will not negatively impact fisheries, Bruce will consult with staff from Montana Department of Fish, Wildlife & Parks (1 pre-irrigation meeting per year; follow up communications as needed).

We have already secured financial, equipment, and/or staff time support from several state entities (Natural Resource Damage Program, Department of Natural Resources and Conservation, DNRC, and Department of Environmental Quality, DEQ), which will partially cover the costs of project monitoring. The focus of this grant request is to cover the remaining costs for equipment, supplies, and electricity costs.

Although Objectives 1 and 2 overlap, they measure different aspects of the modified irrigation technique (feasibility vs. effects) so can be assessed independently. Objective 3 relies on the results of 1 and 2, but can be completed regardless of outcomes; i.e., if electricity is expensive or return flows are undetectable, results still provide valuable knowledge.

Objective 1. Demonstrate the feasibility of modified irrigation technique.

Producers may anticipate challenges or barriers to using the modified irrigation technique, so it is important to demonstrate feasibility and address potential questions. Specifically, we will assess:

• Electricity costs. Higher costs incurred from increased pumping will be tracked and compared with normal operations.
• Mud on wheel tracks. Muddy conditions may bog down the pivot wheels; this will be mitigated by installing directional nozzles.
• Shifts in species composition. Producers may worry about inadvertently creating wetlands on their fields. Species composition will be monitored using vegetation plots (1x1 meter, 3 times per season) or comparable approach. Water application rate and volume will be adapted if compositional shifts occur.
• Changes to production. Increasing the water volume may retard plant growth, and production may be lower next to the wheel tracks depending on the functioning of the directional nozzles. We will visually monitor plant growth and adapt water application as necessary; changes to production will be monitored as part of Objective 2.

All photos (before, after, during), lessons learned, costs, and adaptations from this experiment will be recorded and compiled as part of the demonstration of feasibility.

Objective 2. Quantify effects of modified irrigation technique on the hydrology, soil, and agricultural production and develop ranch-scale water balance.

We will use a robust monitoring network to quantify the effects of the modified irrigation technique on hydrology, soils, and agricultural production. Specifically, we will measure:

• Volume of water pumped from creek and applied using the pivots.
• Streamflow using three stream gages. One gage is already installed near the center of the project area (incidentally, this gage is located at the site of a retired stream gage that operated during the 1960s when the entire basin was flood irrigated, so it will provide interesting comparison data). Two others will be added on Bruce’s property: one where the creek enters and one where it leaves.
• Soil moisture using six multi-depth probes
• Surface water quality in the creek at two locations (above and below) monthly throughout the season. Runoff (erosion and/or fertilizer) will be evaluated by analyzing samples for forms of nitrogen, phosphorus, and suspended solids, as recommended by DEQ.
• Evapotranspiration and precipitation using a Mesonet station (collaboration with the Montana Climate Office).
• Soil infiltration using a soil infiltration ring. Tests will be performed monthly; before, during and after sections of each field are irrigated.
• Shallow groundwater near the creek using hand-dug wells with twelve piezometers (groundwater level monitoring wells).
• Deep groundwater using five, 4” monitoring wells to be drilled. We were unable to procure a drilling contractor, so, instead, we built and installed five lysimeters to measure vadose zone water flux, and we hand-augered 18 additional piezometers across the site to monitor groundwater levels and fluxes.

Streamflow, groundwater, and other hydrologic monitoring data will allow John to use a MODFLOW-based numeric groundwater model to develop a ranch-scale water balance. The water balance will quantify the amount of water being added to the local aquifer and contributing to return flows, allowing us to directly quantify how the modified irrigation system impacts local hydrology.

Effects on crop production and soils will also be assessed. Quantity of hay produced will be compared with previous years, and regrowth will be estimated using established DNRC methods for consumptive water use. Root depth (as a proxy for microbial activity and soil health) will be estimated using soil cores (6 per pivot; sampled consistently in late season) or other comparable method.

Collectively, documenting changes in crop production, soil moisture, and rooting depth will quantify effects of the modified irrigation technique on agricultural production and soil health, while hydrologic monitoring will quantify changes to groundwater and return flows. Estimating all these aspects and any impacts to surface water quality is critical to providing other producers enough information to feel comfortable adopting this approach in their operation.

Objective 3. Synthesize results for dissemination.

We will synthesize the data and observations from Objectives 1 and 2 to develop salient and understandable summaries. Results will be integrated into compelling visuals (e.g., figures, diagrams, and/or infographics) that are accessible to producers and demonstrate the costs and benefits of the modified irrigation technique.