Irrigation research looks at drip system With drought becoming more the rule than the exception in the lower Southeast, research in north Alabama's Tennessee Valley is focusing on new irrigation options and management strategies.
A non-traditional off-stream storage reservoir was first constructed at the Tennessee Valley Research and Extension Center in Belle Mina to harvest abundant water available in the winter and spring for irrigation during the summer. Using this water source, several cotton irrigation research projects have been under way since 1996.
One such project is evaluating the potential of subsurface drip irrigation on Alabama cotton.
"This is our fourth year of data from drip irrigation in the Tennessee Valley and our second year in the Wiregrass region of south Alabama," says Larry Curtis, Auburn University Extension engineer. "We've been pleased with the results in terms of yield response, but we're not to the point of recommending it as a general practice. We just don't have enough experience with it. But the concept of drip irrigation is appealing."
In one experiment, irrigation drip tubing with emitters located every two feet along the tubing was buried permanently at a depth of 15 inches between every other row (controlled traffic). It also was buried perpendicular to rows (random traffic) with the same number of outlets and the same amount of tubing on a per-acre basis.
Irrigation was applied daily based on calculated pan evaporation provided by the Alabama Weather Information Service for that location. Irrigation amounts equaled 30, 60 and 90 percent of pan evaporation after full canopy with corresponding percentage adjustments prior to full canopy.
Pan evaporation is a measurement of daily evaporation from a standardized U.S. Weather Service device and relates closely to daily water use by growing cotton plants.
These rates result in amounts of approximately 0.1 inch per day, 0.2 inch and 0.3 inch per day, respectively, during periods of peak water use. Significant yield differences occurred in 1998 and 1999 with the most dramatic differences occurring in 1999.
A second subsurface drip irrigation study initiated in 1998 is comparing five drip irrigation tape products with a fertigation component included. This study was installed in an area where continuous crops have been produced for many years.
Each of the five tapes in the study have emitters spaced every two feet along the tape and the tape is buried 15 inches between every other row. Rows 340 feet in length were used to better simulate field conditions.
Each tape product is being evaluated with conventional fertilizer and fertigated treatments. A tape product also is used on the surface with a conventional fertilizer treatment.
Little difference was observed in fertility treatments during 1998. Sufficient rainfall occurred late in the 1998 growing season so that fertilizer in the upper layers of the soil was more readily available.
In 1999, significant yield differences were observed for the fertigated versus conventional plots. Extremely dry conditions in the upper layers of the soil profile made conventionally applied fertilizer less available, resulting in significant yield reduction compared to fertilizer applied through the irrigation system.
Significant yield differences were observed each year between non-irrigated plots and tape plots with fertility treatments. To date, only minimal differences have been observed between the different drip irrigation tape products.
Results of these two studies indicate that, while yield results for subsurface drip irrigation are attractive, questions remain related to the cost, longevity and suitability of products currently available for the rolling farmland found in the Tennessee Valley, says Curtis.
"We haven't seen much in the way of water savings from drip irrigation. It takes about the same amount of water as a center pivot system. It does, however, have some energy-saving features because drip irrigation operates at low pressures," says the engineer.
The real concern, notes Curtis, is having a tape product that will perform satisfactorily over a long period of time in the rolling terrain of Alabama.
"On rolling ground, we run into problems such as back siphoning and reverse flow that increase the probability of the system becoming clogged. Design considerations will be required to overcome that in rolling terrain, including extensive surveys and plans based on topography. We need a tape product that won't require all of this."
Subsurface drip irrigation wouldn't be suitable, he says, in fields where heavy equipment tends to bog down during periods of wet weather. A combine could become mired in the field and damage the tape product in the ground. It would be more suitable, he adds, on upland soils.
Subsurface drip irrigation is not a cheaper method of irrigating, cautions Curtis. "The cost of subsurface drip irrigation could run anywhere from $750 to $1,000 per acre, and that's just for the drip tape, filters and other components."