Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2004
Publication Date: 2/23/2005
Citation: Enciso, J.M., Colaizzi, P.D., Multer, W.L. 2005. Economic analysis of subsurface drip irrigation lateral spacing and installation depth for cotton. Transactions of the ASAE. 48(1):197-204. Interpretive Summary: West Texas has a favorable climate for cotton production and high yields are possible with irrigation; however, the region has very limited water resources that come almost exclusively from pumping underground aquifers. In order to stretch irrigation water supplies, a few cotton producers began using subsurface drip irrigation (SDI) beginning in the early 1980s. SDI is a highly efficient, but expensive, irrigation technology. In SDI, water is delivered to the plant root zone through a flexible plastic line called a drip lateral. The drip lateral is about the size of a garden hose and is embedded with small plastic drip emitters. The drip emitters are spaced anywhere from a few inches to several feet along the drip lateral, and the drip lateral is buried a few inches to a few feet below the soil surface. SDI is advantageous over other irrigation technologies, such as flood or sprinklers, because evaporation of water from bare soil (a non-beneficial use of irrigation water) is greatly reduced or eliminated. The main disadvantages of SDI, besides the high initial investment, are steep maintenance and operation learning curves, and high maintenance and management requirements. One of the main costs of SDI systems is the drip laterals (also called drip tape). In the first SDI installations, laterals were buried under every plant row, where plant rows were spaced 30-40 inches apart. Later, the practice of burying the laterals in every alternating furrow became most common. This reduced SDI installation costs 30-40% because it required only half the laterals compared to placing them under every plant row. However, in dry years when there is little rainfall during the winter and spring months before planting, the alternate furrow lateral spacing requires large amounts of water to push the wetting front from the lateral to the seed bed to ensure germination. Depending on the soil, sometimes more water moves below the lateral (by gravity) than horizontally toward the seed bed (by capillary action), and seeds fail to germinate. Hence there is some risk associated with the less expensive alternate furrow lateral spacing. The objective of this research was to compare the economics of two drip lateral spacings (every row and alternate furrow) and two lateral installation depths (8 and 12 inches). An experiment was conducted on a commercial farm in St. Lawrence, Texas during the 2001, 2002, and 2003 growing seasons. In the first two seasons (2001 and 2002), net returns were highest for the every row lateral spacing, whereas in the third season (2003), net returns were highest for the alternate furrow lateral spacing. In all three years, the 12 inch lateral depth resulted in greater net returns than the 8 inch lateral depth. In the 2002 season, half of the rows in the alternate furrow lateral spacing did not germinate, and we think this was related to the tractor wheels compacting the soil around the drip laterals as the wheels passed through the furrows. The compacted soil may have impeded water movement from the drip laterals to the seed bed. Otherwise, net returns probably would have been the same for every row and alternate furrow spacing. In 2003, we did not allow wheel traffic in these furrows, and this appeared to solve the germination problem. These results suggested the optimal drip lateral spacing in this environment were inconclusive. A newer plant row configuration is beginning to be adopted in West Texas, where twin plant rows are closely spaced on either side of the drip lateral in a wide bed. The so-called wide bed design has been used successfully in other regions, and we are presently testing this for several crops.
Technical Abstract: Cotton lint yield, seed mass, fiber quality parameters, gross return, and net return were compared for subsurface drip irrigation (SDI) lateral spacing and installation depth in a clay loam soil in western Texas for three seasons. Drip laterals were spaced either in alternate furrows (2 m) or beneath every planted bed (1 m), and installation depths were either 0.2 m or 0.3 m beneath the soil surface. Net return was gross return minus fixed and variable costs. Fixed costs included financing (5.00% interest over 10 years) the initial investment of SDI materials and installation, which were $210 and $337 ha-1 for the 2 m and 1 m lateral spacing, respectively. Variable costs were those associated with cotton production and were similar for the two drip lateral spacings. Lint yield, seed mass, and gross and net returns were significantly greater for the 1 m lateral spacing in the first two seasons, but these parameters were significantly greater for the 2 m lateral spacing in the third season. These parameters were consistently greater (either numerically or significantly) for the 0.3 m lateral depth in all seasons. Most fiber quality parameters were not significantly different, and no consistent trends were observed. Lint yields ranged from 640 to 1,635 kg ha-1, and net returns ranged from -$395 ha-1 to $1,005 ha-1. The low lint yield and resulting net loss were due to a germination failure in the second season for the alternate furrow spaced laterals.