By Vinayak Shedekar and Elizabeth Schwab, The Ohio State University
The February issue of Ohio’s Country Journal featured an article by Greg LaBarge on “What is drainage water recycling?” (https://ocj.com/2023/02/what-is-drainage-water-recycling/). I decided to continue this conversation, especially for those who may have started wondering what it would take to implement drainage water recycling (DWR) and irrigation for field crops. Let’s try and address some additional questions on DWR.
What sites are suitable for drainage water recycling? The short answer is: a site that can benefit from both improved drainage and supplemental irrigation. If you know how to access the USDA-NRCS’s Web Soil Survey, you can look up the suitability of the field’s soils for drainage as well as irrigation. The surface topography and the layout of the field’s subsurface drainage system will play an important role in deciding the location of the storage pond, as well as the appropriate irrigation method (subirrigation versus overhead or drip).
For example, a relatively flat field with somewhat narrow drain spacing would be an ideal candidate for subirrigation, while a sloping field or a field with wide drain spacing may be better served by overhead or drip irrigation systems. If you recently split the spacing in a field with an existing drainage system, you just improved its suitability for subirrigation in the near future. Note that subirrigation is usually effective in soils that have low lateral seepage and have a soil layer of low permeability located below the subsurface drains. The permeability of the restrictive layer should be less than one-tenth that of the soil in the crop root zone. Also, a soil with a shallow restrictive layer (severe compaction or fragipan) may not be suitable for subirrigation. A site with a pre-existing pond or reservoir and adequate space for retrofitting or expansion would be an ideal candidate. Wherever possible, routing the drainage water through a preliminary treatment system such as a wetland may provide additional ecological and water quality benefits. And note that a drainage water recycling system can easily be designed to receive surface as well as subsurface drainage water.
How large does the pond need to be? The size of the storage pond depends on two things: how the size of the area draining to it, and the area of land you want to irrigate. So, how much area can you irrigate with, let’s say, an acre of pond? That would be the wrong question to ask. You should start by asking how much water would be needed to irrigate an acre of cropland. This question was answered in detail in a previous OCJ article (https://ocj.com/2022/12/can-drain-tiles-do-more-than-drainage/).
To recap, net irrigation demand for corn in Ohio is 3 to 6 inches in a growing season. So, let’s assume we need to store 6 inches (0.5 ft) of water to irrigate a 20-acre field. That amounts to a storage volume of 10 acre-feet (20 acres x 0.5 feet). For storing 10 acre-feet of water in an acre, the pond would need to be about 10 feet deep. A 2-acre pond with the same storage capacity would need to be only 5 feet deep. Thus, the ratio of storage area to irrigated area can range from 1:10 (10%) to 1:20 (5%) depending upon the feasible depth of the pond. Scaling this calculation up on a larger irrigated and drained acreage will further help minimize the area required for storage. Another way to shrink the size of the storage pond would be to supplement the pond water with another source, such as a low-yield groundwater well or another surface water source.
What are the costs associated with DWR? The costs for DWR can vary widely for each of the system components. Typically, the most expensive part of a DWR project would be the storage pond construction. A rough estimate for pond construction cost is $1,000 to $3,000 per acre-foot of storage. Expenses for additional components would include those associated with conveyance, pumping from a drainage outlet, design, control structures at the inlet and outlet, and technical services such as design and permitting. For the irrigation component, depending upon the method, you may have relatively lower costs if only simple retrofits are needed to facilitate a subirrigation system, or costs may be as high as $1,000 per acre for an overhead irrigation system. The value of land taken out of production should also be factored into the calculations for return on investment.
Will such a capital-intensive practice pay off? I will not give you the standard “it depends” answer; rather, it will pay off…especially if you master the art of irrigation scheduling and drainage water management.
There are many more questions about DWR that need to be answered, especially for Ohio’s soils under future crop and climate scenarios. Meanwhile, a factsheet on the Transforming Drainage Project website is a good resource to consult (https://transformingdrainage.org/practices/drainage-water-recycling/).
ANNOUNCEMENT: The 2023 Overholt Drainage School (March 13-17, 2023 in Wooster): If you are interested in learning about planning, design, and installation of drainage, conservation drainage practices, and advanced technology applications, you should consider registering for the upcoming Overholt Drainage School. Topics will include drainage law, economics, planning, topographic mapping, GPS surveying, drainage design and installation, drainage design software, machine control, and conservation practices to manage water quality. More information can be found at https://go.osu.edu/drainageschool23.
This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, International Program for Water Management in Agriculture, Ohio Agricultural Research and Development Center, and the College of Food, Agricultural, and Environmental Sciences. Technical information and data from Frankenberger et al. ABE-156-W (Questions and answers about drainage water recycling for the Midwest).
Dr. Vinayak Shedekar is the Assistant Professor of Agricultural Water Management and director of the Overholt Drainage Research and Education Program in the OSU Department of Food, Agricultural and Biological Engineering and can be reached at email@example.comElizabeth Schwab is a Research Associate and coordinator of the Overholt Drainage Research and Education Program in the OSU Department of Food, Agricultural and Biological Engineering and can be reached at firstname.lastname@example.org.