Tailwater Recovery and Reservoir Storage Benefits for Farm Profits and Aquifer Sustainability Emerge When Considering Field-Aquifer Interaction in Long Planning Horizons

Investigators: Nicolas Quintana Ashwell, Drew Gholson

Date: 2021

Project Summary

Benefits of Capturing, Storing and Using Pluvial and Irrigation Runoff

Tailwater recovery and storage reservoir systems are capable of capturing runoff from the fields. The system includes a large tailwater ditch which captures all runoff and stores significant amounts of water themselves depending on their design—see Figure 1. When the ditch fills to a prescribed level, the excess water can be pumped to a storage reservoir, back on the fields or be allowed to overflow into drainage canals. The first and more evident benefit is the capture of water that would otherwise leave the area (a type of consumptive use); which can be re-used for irrigation and reduce the amount that would otherwise be pumped from the alluvial aquifer. Because this can be a closed-loop, on-farm structure, the grower has total control, which reduces the growers risk of facing adverse consequences of groundwater use limitations. Another benefit that is less visible and more difficult to quantify and valuate is the retention of sediments and nutrients that are not discharged into receiving streams--and ultimately affect the hypoxic zone in the Gulf of Mexico. This feature benefits the farmer (internal benefit) in terms nutrient retention and reapplication on their fields but it also benefits society including other farmers (external benefit) in terms of the environmental benefits and the preservation of alluvial aquifer. Table 1 summarizes the main non-exploitative benefits of tailwater recovery and storage structures. The benefits in Table 1 do not include potential yield gains or fertilizer cost reductions derived from re-application of nutrient. Researchers from NCAAR, MSU and USDA ARS are collecting data to quantify this potential effect.

Costs of Capturing, Storing and Using Pluvial and Irrigation Runoff

This significant capital investment involves the leveling of the affected fields as a catchment area from which to collect the water to be stored and reused in irrigation. Table 2 summarizes the investment and opportunity (land) costs associated with the baseline design used by NRCS of 160 acres of cropland/catchment and10 to 12 acres of ditch and reservoir capable of supplying all irrigation needs for 80 acres. Not included are the costs of land-leveling and tributary ditches. The largest upfront cost is earth-moving: the establishment of the levees for storage and excavation. However, these works retain 90% or more of their value at the end of the useful life, resulting in relatively low annuity cost-equivalent. The pumping plant is the second most expensive component but due to the low recovery value at the end of its useful life, it is the largest annuity cost-equivalent item. The opportunity cost of the land occupied by the ditch network and reservoir is valued at $1,308 per acre— equivalent to profits from highly productive land. However, the design can take advantage of the farm topography and occupy marginal or low-productivity tracts. The cost of relifting water from the tailwater ditch and applying it to the fields is lower than the cost of pumping groundwater. Estimates from NCAAR put the cost of lifting one acre-ft. of groundwater at approximately $0.54 per foot of lift. In contrast the cost for relift and application of tailwater is estimated at approximately $3.76 per acre feet. This implies that pumping from wells more than 7 ft-deep would be more expensive than pumping water stored in the system.

Need to Think on an “Aquifer Life” Planning Horizon Rather Than on a “Farm Operations” Planning Horizon to Reveal the Economic Merits

Our research reveals that the true benefits of tailwater recovery, storage and reuse occur over an extremely long planning horizon partly because of the large recovery value of the structures. Furthermore, when the practice is evaluated in terms of optimal aquifer management rather than seasonal profits over a given horizon; the benefits in terms of aquifer conservation become more evident. Relatively short planning horizons emphasize the upfront costs. Employing a simplified hydro-economic model to Sunflower county, MS, we show that aquifer-related benefits are virtually invisible when the time horizon is less than 30 years and the extraction behavior follows periodic individual farm profit maximization (green curve in Figure 2). The model anticipates that farmers would progressively adopt on-farm water storage (OFWS) as the alluvial aquifer depletes (without added incentives) but not at the optimal levels (blue curve in Figure 2). NRCS does offer incentive programs to develop tailwater recovery and storage facilities including incentive payments and technical assistance. Our results show that up to $520 million of additional farm-level profits across Sunflower County may be achieved over the 150-year planning horizon when the optimal level of OFWS is employed while saving more than 4 million acre-ft. of groundwater over the same period—not including the retention benefits listed in Table 1.

Conclusion

Expansion of OFWS can result in large gains derived from taking advantage of off-season precipitation and keeping pumping lift distances low (i.e., high water table in the aquifer). Additional benefits of the practice that affect the quality of receiving streams and the hypoxic zone justify aggressive incentives to encourage growers to develop these structures which provide them with complete control of an important source of water for irrigation.

References

This report is based on ongoing research, including the following publication: Quintana-Ashwell, N., & Gholson, D. (2021). Optimal Groundwater Management with Pluvial and Irrigation Runoff Recycling. Poster presented at the Agricultural and Applied Economics Association annual meeting, Austin, TX.00

Cited work

Falconer, L., Tewari, R., & Krutz, J. (2017). Cost analysis of water management scenarios for the Mississippi Delta. In Delta Sustainable Water Resources: Monitoring and Modeling (pp. 25-37). Mississippi Water Resources Research Institute.

Kovacs, K. F., & Durand‐Morat, A. (2020). The influence of lateral flows in an aquifer on the agricultural value of groundwater. Natural Resource Modeling, 33(2), e12266. Omer, A. R., Henderson, J. E., Falconer, L., Krӧger, R., & Allen, P.J. (2019). Economic costs of using tailwater recovery systems for maintaining water quality and irrigation. Journal of Environmental Management, 235, 186-193.

Project Photos

• Topic:
• Irrigation Scheduling
• Irrigation
• Technologies