J. Enciso-Medina, Irrigation Specialist, Fort Stockton, TX

Warren L. Multer, EA-IPM, Glasscock, Reagan, Upton, Garden City, Texas

Robert Scott, CEA-AG, Reagan County, Big Lake, Texas A&M University

Raymond Quigg, CEA-AG, Upton County, Rankin, Texas A&M University

Steve Sturtz, CEA-AG, Glasscock County, Texas, Garden City, Texas

A study was planned to study the effect of irrigation frequency and drip-line spacing in cotton irrigated with subsurface drip irrigation systems. The experiment consisted of sixteen treatments and three replications. The treatments were: to space the drip-line every 30 and 40 inches, two row spacing (ultra-narrow row spacing, every 15 in, and 30 and 40 solid cotton rows), two irrigation frequencies (1 and 3 days), and 2 irrigation amounts (15 and 19 inches). The water use efficiencies for the 30 in drip spacing and ultra-narrow cotton were significantly greater (P<0.05) than the 40 in drip spacing for the solid and ultra-narrow cotton and for the 30 in drip spacing and solid rows. The water use efficiencies ranged between 86 and 106 lbs./ac-in for the 30 in drip-line spacing and ultra-narrow rows, and 50 lbs./ac-in for the rest of the treatments. The conclusions of the study were that 1. There was no statistical difference between 1 and 3 days irrigation interval and that 2) three ultra-narrow rows with 30 in drip spacing resulted in the highest water use efficiency.

When well capacities are low and water is limited, subsurface drip irrigation (SDI) systems are preferred over other irrigation methods because they can apply small amounts of water over the entire field with very high uniformity. Even after SDI systems have been installed, some producers continue to manage irrigation, scheduling with SDI systems on the cotton crop as they did with surface irrigation systems, when water is limited. Often they are irrigating with long irrigation intervals, between 12 and 15 days. Some farmers justify long intervals by explaining that they have deep soils with high water retention characteristics (medium to fine textured soils) and when water is very limited, the water that is lost to deep percolation and runoff is negligible. Other farmers feel that they need to irrigate more often to increase their yields and profits even under deficit irrigation, and they can do that with electric valves and an electronic controller. By installing automatic valves, irrigation times can be controlled more precisely, all the irrigation sections of the system can be irrigated with the same time, and irrigation intervals could be shortened. There has been an ample discussion about the benefits of more frequent irrigation, some of the benefits are the reduction of water stress with its consequent yield increase, and the increase of water use efficiency. The method generally used to determine irrigation frequency is to use a soil water balance model in which a pre-established balance is maintained in the root zone. The soil water should not fall below a certain deficit level to avoid stress and yield reduction. By using a water balance model with a fully irrigated crop, water lost to percolation and runoff are minimized, and water use efficiency is increased, because the soil profile is never overfilled. In contrast, when deficit irrigation is practiced losses are minimal and the crop is deliberately stressed to a certain profitable value.

Several irrigation strategies have been used to increase the water use efficiency with subsurface drip irrigation methods in the arid and semi-arid areas of the world when water is limited. Most of the irrigation strategies that have been used have been directed toward reducing evaporation either by reducing the row spacing and pattern (Enciso et al., 1999; Unruh et al,. 2000) or by using different tillage practices (Bordovsky et al., 1994). High frequency irrigation has been used to reduce percolation losses. The effect of higher water use efficiencies due to higher frequencies could be due to less percolation losses or could be due to physiological reasons. Radin et al. (1991) explained that drip irrigation and mid-cycle supplements increased midday leaf water potential and apparent hydraulic conductance of the plants for an extensive period during fruiting, enhancing the water uptake and transport capacity of the cotton plant. They concluded that increasing the number of irrigations for a short period during peak fruiting increased yield when the same amount of water was applied. It is uncertain if high frequency would help in very water limited conditions because there is little water lost to water percolation. Bordovsky et al. (1992) compared the effects of high frequency irrigation on the Southern High Plain where the average annual rainfall ranged between 450 and 500 mm. They found that frequencies shorter than 3 days with a LEPA system increased the water use efficiency. Later Bordovsky et al. (1998) studied the effect of high frequency with a SDI system, using maximum delivery rates of 2.5, 5.1, and 7.6 mm per day. They found that daily irrigation substantially increased yield and water use efficiency for both delivery rates. Several agricultural users in water limited areas of South West Texas have delivery rates of 1.5 mm/day, and the precipitation rates vary from 350 to 400 mm annually, (Henggeler J.C. 1998). According to Martin (1990) one of the benefits of high frequency irrigation with deficit irrigation is that a high soil water content is maintained in the upper root zone where the plant nutrients are generally in greater supply. He also mentioned that the systems more suitable for high frequency irrigation are Center Pivots and Subsurface Drip Irrigation Systems, because they can spread the water more quickly and uniform over the entire field. In a fully irrigated condition, enough moisture needs to be provided to the crop to attain its maximum evapotranspiration (Etm). The ET replacement fraction depends on the soil and may vary between growth periods, (Doorenbos and Kasam, 1986). In a deficit irrigation condition with inadequate water capacities, the ET replacement fraction is determined by the design capacity of the system (the area that can be irrigated with the well flow rate). The optimum design capacity is determined by maximizing the net return per unit of water applied, and this decision is generally made before the season starts. When irrigating with limited water supplies, the soil is generally depleted and the actual evapotranspiration (Eta) always falls below Etm. More information is needed to support the idea that more frequent irrigation under water limiting conditions can improve cotton lint yield and quality.

1. Identify if irrigation frequency affects cotton lint yield when grown at deficit irrigation levels using SDI.
2. Determine if variable dripline spacing of SDI, relative to the planted row, affects cotton lint yield.

The experiment was carried on a previously installed Subsurface Drip Irrigation System in the St. Lawrence Texas region of the southern high plains. The farm chosen to carry the experiment was the one of Kenneth Braden. All the cotton was planted on May 18th, 2001 with Delta Pine 458 Bollgard/Roundup Ready. The experiment consisted of sixteen treatments and three replications. The treatments were:

1. 30 in drip line spacing - amount 1 – frequency 1 – ultra-narrow cotton

2. 30 in drip line spacing - amount 1 – frequency 2 – ultra-narrow cotton
3. 30 in drip line spacing - amount 2 – frequency 1 – ultra-narrow cotton
4. 30 in drip line spacing - amount 2 – frequency 2 – ultra-narrow cotton
5. 30 in drip line spacing - amount 1 – frequency 1 – 30 in cotton
6. 30 in drip line spacing - amount 1 – frequency 2 – 30 in cotton
7. 30 in drip line spacing - amount 1 – frequency 1 – 30 in cotton
8. 30 in drip line spacing - amount 1 – frequency 2 – 30 in cotton
9. 40 in drip line spacing - amount 1 – frequency 1 – ultra-narrow cotton
10. 40 in drip line spacing - amount 1 – frequency 2 – ultra-narrow cotton
11. 40 in drip line spacing - amount 2 – frequency 1 – ultra-narrow cotton
12. 40 in drip line spacing - amount 2 – frequency 2 – ultra-narrow cotton
13. 40 in drip line spacing - amount 1 – frequency 1 – 40 in cotton
14. 40 in drip line spacing - amount 1 – frequency 2 – 40 in cotton
15. 40 in drip line spacing - amount 2 – frequency 1 – 40 in cotton
16. 40 in drip line spacing - amount 2 – frequency 2 – 40 in cotton

The amounts of water applied were 12 and 19 in. A pre-irrigation amount of 8 inches was applied for both treatment amounts. The amount 2-treatment received 4 inches of in-season water more than the amount 1. The frequencies studied were 1 and 3 days. The ultra-narrow cotton was planted spaced every 15 in. The data was analyzed with a general linear model (GLM) with mean separation by both Duncan’s multiple range test and least square difference (SAS Institute, 1991).

The effect of drip and row spacing on water use efficiency are presented in Fig. 1. From this figure it can be observed that the 30 in drip spacing and ultra-narrow cotton resulted in the highest water use efficiency. The water use efficiencies ranged between 86 and 106 lbs./ac-in for the 30 in drip spacing and ultra-narrow cotton. It was also significantly greater (P<0.05) than the 40 in drip spacing for the solid and ultra-narrow cotton and for the 30 in drip spacing and solid rows. There was no statistical difference for the 30 in drip spacing-solid row and the 40 in drip spacing-ultranarrow and solid rows, which resulted in water use efficiencies of about 50 lbs./ac-in. The effect of irrigation frequency on water use efficiency can be observed from Fig. 2. It can be observed that the difference between 1 and 3 days irrigation frequency was negligible. The greatest difference was observed for the ultra-narrow row treatments. For the narrower rows, the 1 day irrigation interval produced slightly higher water use efficiencies. The same trend was observed with higher water amounts. However, there was no statistical difference between 1 and 3 days interval and 15 and 19 in water amounts.

Fig. 1. Effect of drip spacing (30 and 40 in) in row spacing (40 in and 15 in). Saint Lawrence, TX. 2001.

Fig. 2. Effect of drip spacing (30 and 40 in) in row spacing (40 in and 15 in). Saint Lawrence, TX. 2001.

1. There was no statistical difference between 1 and 3 days irrigation interval.
2. The ultra-narrow rows with 30 in drip spacing resulted in the highest water use efficiency.

Bordovsky, J.P., W.M. Lyle, R.J. Lascano, and D.R. Upchurch. 1992.Cotton irrigation management with LEPA systems. Transactions of the ASAE 35(3):879-884

Bordovsky, J.P. and W.M. Lyle. 1998. Cotton irrigation with lepa and subsurface drip irrigation systems on the Southern High Plains. Beltwide Cotton Conference Proceedings. San Diego California. Jan 5-9, 1998

Bordovsky, J.P. W.M. Lyle and J.W. Keeling. 1994.Crop rotation and tillage effects on soil water and cotton yield. Agronomy Journal 86(1):1-6

Doorenbos, J and A.H. Kassam. 1986.Yield response to water. FAO Irrigation and Drainage Paper 33. FAO, Rome

Enciso J, D. Martin, and Bryan Unruh. 1999. Effect of row pattern and spacing on water efficiency for drip irrigated cotton. Presented at the 1999 ASAE annual international meeting. Toronto, Canada. ASAE Paper 99-250

Henggeler J.C. 1998. Managing cotton when water is limited. Proceedings of the Beltwide Cotton Conference. Volume 1:641-645

Martin D.L., E.C. Stegman, and E. Fereres. 1990. In Management of farm irrigation systems. ASAE Monograph published by the American Society of Agricultural Engineers (1^st edition), ed. G.J. Hoffman, T.A. Howell, K.H. Solomon, 155-207. St. Joseph, MI.

Radin, J.W., L.L. Reaves, J.R. Mauney, and O. F. French. 1991.Yield enhancement in cotton by frequent irrigations during Fruiting. Agronomy Journal. 84: 551-557

Unruh, B.L., W. Multer, and J. Enciso. 2000. Narrow And Conventional Row Pattern Yield Response To Limited Subsurface Drip Irrigation. 2000 Beltwide Cotton Physiology Conference, San Antonio, Texas.