Recirculating Irrigation Water to Manage Pollution in a Rice-Soybean Rotation

Chang Jeong  |  1/29/2018 3:40:48 PM

Changyoon Jeong and Ernest Girouard

Recirculating tailwater, or irrigation water that runs off fields, is recommended as a best management practice (BMP) to improve irrigation efficiency and to reduce excess nutrient loading in watersheds. A tailwater recovery system, also referred to as circular irrigation, collects and recirculates tailwater. It consists of a recovery reservoir to store water, ditches to capture and deliver water to the reservoir, and a pumping system to distribute tailwater to irrigate crop fields.

The tailwater recovery reservoir at the lowest point of crop fields collects surface runoff from rainfall and irrigation events for future agricultural irrigation. The reservoir is an engineered system built using quantitative approaches based on ecological principles, which makes the systems akin to natural wetlands. The recovery reservoir acts as a bio-filter through a combination of various physical, chemical and biological functions capable of removing a wide variety of contaminants — including sediment, nutrients, pesticides and bacterial pollutants — from field runoff and nearby waterways.

The water from the reservoirs can be reused for irrigating crop fields, which improves water use efficiency while recycling the nutrients to the fields for beneficial agronomic use. Additionally, the system can reduce the amount of sediment and nutrients that flow into water bodies such as lakes, rivers and coastal marine environments, mitigating eutrophication and algal blooms.

This project demonstrated the potential of a recirculating irrigation water management system as a BMP to reduce phosphorus and nitrogen losses, sediment accumulation and underground irrigation water consumption. The project was conducted in 523 acres of rice and soybean fields northwest of Kaplan in Vermilion Parish, on the farm of Christian Richard. Read more about this project in the article titled Farmer Designs Unique Conservation System to Grow Rice.

Surface runoff from these fields drained to a tailwater recovery reservoir via drainage ditches and was then recycled to rice and soybean fields through underground pipe for irrigation.

Water quality was monitored at nine spots in the crop fields and their drainages by automated water samplers with sensors. Additional samples were collected by a grab method: four at the drainage from crop fields, two from rice, two from soybean fields and one from the tailwater recovery reservoir. Using standard methods, water samples were analyzed for pH, electrical conductivity, total suspended solids, phosphate, total phosphorus, nitrate as nitrogen (NO3-N) and total nitrogen.

The results showed the nitrate concentration from samples collected in two rice fields were 24 percent and 17 percent higher, respectively, than concentrations measured from the drainage ditches. This is because the rice fields were closed systems where fertilizer was applied (Figure 1, top). Irrigation water, which contains nutrients that can be easily utilized for crop growth, stays in rice fields for most of the growing season. The rice fields functioned as a filtering system, allowing the rice crop to utilize applied nutrients to produce grain, thus mitigating excess nutrient in effluent water.

At the tailwater recovery reservoir, the soluble reactive phosphorus (PO4-P) concentration was 66 percent lower than the concentration from the ditch where all drainage merged (Site 1, Figure 1, bottom). However, the soluble reactive phosphorus concentration in runoff water from a soybean field was 75 percent higher than the level measured from the drainage ditch connected to the soybean field and was captured to be recirculated to the reservoir, then re-used in rice fields. The analysis of drainage water in this study indicates there is an excellent opportunity to recycle nutrients, which can reduce the recommended fertilizer application for crop production.

The benefits of circular irrigation systems are not limited to conserving underground water resources. They also can reduce pollutants leaving agricultural watersheds. Rice fields in this project demonstrated a supplementary filtering system to improve the quality of irrigation water, which typically is not often drained to watersheds. This irrigation system is highly recommended as an additional, novel BMP to control nonpoint-source pollution and to serve as a source of irrigation water instead of pumping groundwater.

Ernest Girouard, now retired, was the coordinator for the Louisiana Master Farmer Program. Changyoon Jeong is an assistant professor at the Red River Research Station in Bossier City.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

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Figure 1. The top chart shows the average nitrate as nitrogen (NO3-N) concentration in recirculating irrigation system. The bottom chart shows the average soluble reactive phosphorus (PO4-P) concentration in recirculating irrigation system. Site 1: Drainage A site + all other drainage; Site 2: Tailwater recovery reservoir; Site 3: Soybean field west; Site 4: Rice field west; Site 5: Soybean field east; Site 6: Drainage B off-site 80%; Site 7: Rice field east; Site 8: Drainage C 80% off-site: Site 9: Drainage D 100% off-site.

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Christian Richard, who farms rice, crawfish and soybeans in Vermilion Parish in southwest Louisiana, uses a unique system to manage water and nutrients in his rice-farming operation. It's called a tailwater recovery system because he captures the water from the fields and reuses it so none of the fertilizer is lost in runoff. Photo by Bruce Schultz

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This is another view of the tailwater recovery canal before being filled. Photo by Bruce Schultz

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