Sub-surface drip irrigation has feasibility on Delmarva (Choice Voice)
(Editor’s note: James Adkins is an irrigation engineer with the University of Delaware, Carvel REC.)
Sub-surface drip irrigation is a type of micro-irrigation system using drip tape or tube typically installed 10-16 inches below the soil surface.
Unlike the shallow or surface drip systems typically used for vegetable production, SDI tape is thicker allowing for a semi-permanent installation intended to last 20-plus years with proper maintenance.
There are documented installations and research plots in the Great Plains still functional after 30 years of use.
The concept of SDI is not new, despite being only adopted in the Mid-Atlantic over the last 15 years.
SDI provides some opportunities to farmers in the Mid-Atlantic to irrigate fields that were previously impractical to water with overhead sprinkler irrigation due to field shape. In larger fields, SDI can pose an advantage over pivots as there are no dryland corners. The greatest concentration of SDI installations has taken place in the Great Plains region from Texas up through Nebraska with federal programs encouraging farmers to replace pivots with drip systems.
The primary driver for the adoption of SDI in the plains has been water conservation.
The high winds, low humidity, high temperatures and very limited rainfall promotes a situation where the soil evaporation in the growing season may be two to three times what we typically see on our humid coastal plain.
The semi-arid Great Plains climate combined with the ever-strained water availability in the Ogallala aquifer create a major challenge for surface and sprinkler irrigation to provide adequate water for crop production.
Their deep, high moisture holding capacity soils are ideal for the slow, low volume application of irrigation directly to the root zone provided by SDI while the high evaporative losses from the soil are mostly avoided as the soil surface remains dry.
Some of the water conservation advantage seen elsewhere is lost in the Mid-Atlantic as the soil evaporation rate is much lower due to the high humidity and our very thin, sandy topsoil greatly limits the soil’s water-holding capacity.
Furthermore, in a dry May-June, a significant volume of water can be lost to deep percolation while attempting to use SDI to push water vertically to shallow rooted crops.
As a result, there are seasons where the cumulative irrigation requirements on SDI fields can and have exceeded that of neighboring center pivots.
In 2012, with financial and in-kind support from the Natural Resource Conservation Service and Vincent Farms, the University of Delaware installed a 42-zone SDI research station with the primary objective of refining SDI management recommendations for Delmarva.
The research field at the UD Warrington Farm has a range of soil types from a very coarse deep sand with zero hardpan to a loamy sand with heavy clay base enabling us to evaluate practices in a replicated fashion using the multiple zones across soil types.
Our installation used 16-mil tape with low flow emitters spaced at 12 inches down the tape.
Real Time Kinematic autosteer was used to install three rows at a time on 60-inch centers at a depth of 16 inches. In hindsight, this system should have been installed shallower (12-14 inches) based on the soil type and typical rooting depth, but at the time we were concerned about retaining the ability to perform tillage if necessary.
We quickly learned that the coarse sands and heavy clay required pulsed irrigations not exceeding three hours in runtime. Pulsing promotes soils capillary action to wick the moisture away from the tape in the sand reducing deep infiltration while in the clay preventing the water from being “perched” and puddling on the surface.
The soybean and wheat yields using SDI have been nearly identical to the yields obtained with the pivot system on the adjacent field.
We had hopes that SDI could improve wheat yields by permitting irrigation during flowering (not recommended via overhead for head scab prevention) but no significant yield increase was measured.
So far, we have been unable to achieve as high of a corn yield with the SDI as with the pivot (typically 82-87 percent of pivot yield though the first two years were closer to 95 percent).
We also found a significant yield advantage to sidedressing nitrogen fertilizer over fertigating or a 50-50 sidedress/fertigation split.
The disadvantage of fertigation is likely caused by the drip tape being located at the lower boundary of the crop root zone; thus any large rainfall event or over irrigation can result in leaching beyond the range of the crop roots.
The design of an SDI system is rather flexible and can be customized to suit the soil, crop, and water availability of each specific field.
Drip lines can be installed at depths of 10-18 inches (shallower is possible but not typical) with typical line spacing of 30, 45 or 60 inches (even wider in heavy and deep soil types). The drip emitter spacing and flow rate within the line can range from six to 30-plus inches to match the available pump flow rate to the installation.
Zone sizes can also be adjusted to fit the field shape and pump flow.
While an SDI system can be designed to run with very low pump flows, the fundamental water demand of the crop does not change and the recommended minimum of 6.6 gallons per minute of pump flow per acre irrigated is still required to fully irrigate the crop during extended dry spells.
This requirement is contrary to what many of the western states install. The difference is they can begin irrigating weeks before a crop is even planted to “bank” or build up the soil-water profile.
Unfortunately, the sandy, shallow soils on Delmarva and frequent profile-filling rain events does not make “banking” possible, necessitating the SDI system to be capable of matching the daily crop water use demands.
The foremost challenge to a SDI system is water quality. While most water issues can be corrected with intense filtration and chemical injection, some issues like high elemental iron can reduce the longevity of the drip emitters to the point that the system is not practical.
Another challenge is the limited ability to rotate SDI fields into processing vegetables.
Unlike agronomic crops that can wait until the field conditions are suitable for harvest, processing vegetable need to be harvested based on maturity and rutted fields are very common. The rut created by a sweet corn harvester or pea viner can permanently destroy the SDI system below.
Finally, SDI requires a fundamental change in irrigation scheduling methods from traditional sprinkler irrigation.
When it comes to scheduling irrigation, most pivot operators allow the soil moisture to drop to a designated limit then refill the soil profile with the sprinkler system.
SDI does an excellent job of maintaining soil moisture at 75 percent of available water and above but once the soil profile drops to the levels typical for a pivot it becomes impossible for an SDI system to catch back up without rain.
This inherent inability to replace depleted soil moisture requires the SDI system to maintain high moisture levels all season reducing the ability to capture and store rainfall. While this typically isn’t a problem in the plains where average rainfall is 12 inches a year, it does reduce the water use of efficiencies of SDI in the humid east.
Overall, SDI provides an opportunity to irrigate fields previously considered too expensive to install sprinkler systems. Improved and simplified controllers enable farmers enumerable possibilities to schedule irrigation, fertigations and chemical injections, reducing in-season workloads and stress.
Lower system pressures will require smaller pumps and reduce energy consumption. However, the limitations on tillage practices, crop rotations and difficulties in recognizing in-season leaks and plugs are recognized challenges.
At this point, I see SDI as an option for grain farmers to irrigate farms with small and odd-shaped fields that typically would require a three-span or smaller center pivot.
I do not foresee Mid-Atlantic farmers replacing four span and larger pivots with SDI on a widespread basis any time in the immediate future.