G. A. Clark

AN ECONOMIC COMPARISON OF SUBSURFACE DRIP AND CENTER PIVOT SPRINKLER IRRIGATION SYSTEMS

In the U.S. Great Plains region many irrigation systems have been converted from traditional furrow to more efficient center pivot sprinkler irrigation. Irrigators are also expressing interest in use of subsurface drip irrigation (SDI) but are concerned about the economics of its use on major field crops, such as corn. A study was conducted to analyze SDI profitability relative to center pivot sprinkler cropping systems, focusing on continuous irrigated corn production in western Kansas. Results indicated that for 65 ha fields, SDI had a distinct disadvantage in net returns of

SUBSURFACE DRIP IRRIGATION USING LIVESTOCK WASTEWATER: DRIPLINE FLOW RATES

54/ha. As field size declined, per ha investment costs for center pivots increased markedly, whereas SDI system costs adjusted proportionally. As a result SDI net returns were approximately equal to center pivot sprinkler systems for 25.9 ha fields, and greater for 13 ha fields (a

Water Requirements and Crop Coefficients of Drip-irrigated Strawberry Plants

28/ha SDI advantage). These results are very sensitive to SDI life. SDI was unprofitable relative to center pivot sprinklers for SDI life of less than 10 years. Changes in corn yield and price, and dripline costs also affected the relative profitability of SDI.

DEVELOPMENT OF A BEST MANAGEMENT PRACTICE FOR NITROGEN FERTIGATION OF CORN USING SDI

Using subsurface drip irrigation (SDI) with lagoon wastewater has many potential advantages. The challenge is to design and manage the SDI system to prevent emitter clogging. The objective of this study was to measure the flow rates of five types of driplines (with emitter flow rates of 0.57, 0.91, 1.5, 2.3, and 3.5 L/h/emitter) when used with lagoon wastewater. A disk filter with openings of 55 µm (200 mesh) was used and shock treatments of chlorine and acid were injected periodically. During the 1998 growing season, 530 mm of wastewater were applied through the SDI system and 390 mm were applied in 1999. During the growing seasons, the two lowest flow rate emitter designs decreased in flow rate, indicating that some emitter clogging had occurred. The magnitudes of the decreases were 15% and 11% of the original flow rates in 1998 and 22% and 14% in 1999 for the 0.57 L/h/emitter and 0.91 L/h/emitter driplines, respectively. After the winter idle period, the flow rates of both driplines returned to the initial flow rates. The three emitter designs with higher flow rates showed little sign of clogging; their flow rates decreased by 4% or less through both growing seasons. Observations showed that the disk filter and automatic backflush controller performed adequately in 1998 and 1999. Based on these preliminary results, the use of SDI with lagoon wastewater shows promise. However, the smaller emitter sizes (0.91 L/h/emitter or less) may be risky for use with wastewater and the long-term (greater than two growing seasons) effects are untested.

SENSITIVITY OF THIN-WALLED DRIP TAPE EMITTER DISCHARGE TO WATER TEMPERATURE

Studies were conducted for three production seasons between 1988 and 1991 to measure water use of drip- irrigated strawberry plants and to evaluate the effects of soil tension management level on fruit yield and drip irrigation water requirement. Water balance lysimeters were designed and installed to measure monthly water requirements of drip- irrigated strawberry plants. Drip-irrigated strawberry plots that were established next to the lysimeter facility used tensiometers to schedule irrigations with treatment management levels of 5, 10, and 15 kPa of soil tension.

Effect of Dripline Flushing on Subsurface Drip Irrigation Systems

A four-year study was conducted in western Kansas on a deep, well-drained, loessial Keith silt loam (Aridic Argiustoll; fine silty, mixed, mesic) to develop a Best Management Practice (BMP) for nitrogen (N) fertigation for corn using subsurface drip irrigation (SDI). Residual ammonium- and nitrate-N levels in the soil profile, corn yields, apparent nitrogen uptake (ANU), and water use efficiency (WUE) were utilized as criteria for evaluating six different N fertigation rates, 0, 90, 135, 180, 225, and 275 kg/ha. A BMP was developed indicating an N fertigation level of 180 kg/ha with the total applied N including other N-sources of approximately 215 kg/ha. The BMP also states that irrigation is to be scheduled and limited to replace approximately 75% of crop evapotranspiration (ETc). Corn yield, ANU, and WUE all plateaued at the same level of total applied N that corresponded to the 180-kg/ha N fertigation rate. Average yield for the 180-kg/ha N fertigation rate was 13.4 Mg/ha. Corn yield to ANU ratio for the 180-kg/ha N fertigation rate was high (53:1). Results emphasize that high-yielding corn production also can be efficient in nutrient and water use. The BMP can be used for managing SDI fertigation of corn on the deep silt loam soils of western Kansas.

VEGETABLE PRODUCTION ON VARIOUS BED WIDTHS USING DRIP IRRIGATION

One of the primary goals in the design of microirrigation systems is to have a hydraulic balance to ensure uniform emitter discharge. However, while most design processes focus on pressure distributions and changes associated with friction and elevation differences, elevated water temperatures will change the physical properties of the water and may change the physical properties of some emitters. Laboratory studies were conducted to measure the effects of water operating temperature on the sensitivity and discharge rate of emitters from thin-walled drip tape (collapsible emitting hose) products. Two different product types (Robert’s Ro-Drip, RD; and T-Tape, TT,) each with two wall thicknesses, were evaluated. The RD product included wall thicknesses of 0.20 mm (8 mil, RD-08) and 0.38 mm (15 mil, RD-15), whereas the TT product included wall thicknesses of 0.25 mm (10 mil, TT-10) and 0.38 mm (15 mil, TT-15). Additional characterization tests included a standard operating pressure/emitter discharge rate test and a tubing tensile stress (elongation) test. All tests were conducted in accordance with ASAE Standard S553, “Collapsible Emitting Hose (Drip Tape) — Specifications and Performance Testing.” Increases in water operating temperature from 21.C to 50.C resulted in an 18%, 44%, and 97% increase in emitter discharge from the RD-08 product at operating pressures of 55, 69, and 83 kPa, respectively. Emitter discharge rate changes in the RD-15 product were not as great (10 to 12% increase) for similar water temperature changes. Effects of water temperature on the discharge rate from the TT products were quite different from the RD products. Emitter discharge rate increased slightly(<5%) with water temperature at 55 kPa, but decreased by up to 7% at 83 kPa. TDR values (also referred to as a “temperature flow rate index”) relate the emitter discharge at each measured temperature value (qt .) to the emitter discharge at the initial base temperature (q20 .) [TDR = (qt .)/(q20 .)]. In this work, the RD-08 product had a quadratic relationship between temperature discharge ratio and water temperature, while the RD-15 product and both TT-10 and TT-15 products had more linear relationships. Designers of microirrigation systems need the hydraulic performance characteristics of the products that they are considering in a system design. Such information should come from the manufacturers of the various collapsible emitting hose (drip tape) products. Product information should clearly provide physical characteristic data such as the emitter exponent “x,” constant of proportionality “k,” temperature discharge ratio values, and maximum recommended operating temperature.

MUNICIPAL SOLID WASTE COMPOST (MSWC) AS A SOIL AMENDMENT IN IRRIGATED VEGETABLE PRODUCTION

The velocity of dripline flushing in subsurface drip irrigation (SDI) systems affects system design, cost, management, performance, and longevity. A 30-day field study was conducted at Kansas State University to analyze the effect of four targeted flushing velocities (0.23, 0.30, 0.46, and 0.61 m/s) for a fixed 15 min duration of flushing and three flushing frequencies (no flushing or flushing every 15 or 30 days) on SDI emitter discharge and sediments within the dripline and removed in the flushing water. At the end of the field experiment (371 h), the amount of solids carried away by the flushing water and retained in every lateral were determined as well as laboratory determination of emitter discharge for every single emitter within each dripline. Greater dripline flushing velocities, which also resulted in greater flushing volumes, tended to result in greater amounts of solids in the flushing water, but the differences were not always statistically significant. Neither the frequency of flushing nor the interaction of flushing frequency and velocity significantly affected the amount of solids in the flushing water. There was a greater concentration of solids in the beginning one-third of the 90 m laterals, particularly for treatments with no flushing or with slower dripline flushing velocities. As flushing velocity and concurrently flushing volume increased, there was a tendency for greater solids removal and/or more equal distribution within the dripline. At the end of the field study, the average emitter discharge as measured in the laboratory for a total of 3970 emitters was 0.64 L/h. which was significantly less (approximately 2.5%) than the discharge for new and unused emitters. Only six emitters were nearly or fully clogged, with discharges between 0% and 5% of new and unused emitters. Flushing velocity and flushing frequency did not have consistent significant effects on emitter discharge, and those numerical differences that did exist were small (<3%). Emitter discharge was approximately 3% less for the distal ends of the driplines (last 20% of the dripline). Although not a specific factor in the study, the results of solids removals during flushing and solids retention within the different dripline sections suggest that duration of flushing may be a more cost-effective management option than increasing the dripline flushing velocity through SDI system design. Finally, although microirrigation system components have been improved over the years, the need for flushing to remove solids and reduce clogging potential has not been eliminated.

Drip-irrigation Management for Watermelon in a Humid Region

Field studies were conducted over two seasons to evaluate the effect of production bed width on yield and quality of nine vegetable cultivars used with drip irrigation and fertigation. Bed widths of 41, 61, and 81 cm (16, 24, and 32 in.) were arranged in a 12 plot randomized block design experiment on an EauGallie fine sand at the Gulf Coast Research and Education Center, Bradenton, Florida. Cucumber (Cucumus sativus, L.), eggplant (Solanum melongena, L.), muskmelon (Cucumus melo, L.), pepper (Capsicum annuum, L.), summer squash (Cucurbita pepo, L.), tomato (Lycopersicon esculentum, Mill.), cherry tomato [Lycopersicon esculentum var. cerasiforme (Dunal) Alef.], watermelon and icebox watermelon [Citrullus lanatus (Thunb.) Matsum, and Nakai] were grown on black polyethylene mulched beds during the 1989 and 1990 spring seasons. Only summer squash yields from 1989 increased linearly with bed width. No differences were detected in yield or average fruit weight of all other vegetables from both seasons nor in soluble solids content of melon fruit. These studies indicated that yield and quality of drip-irrigated vegetables would not be compromised by reduced bed widths on sandy soil production systems.

Dripline Flushing Velocities for SDI

A field study was conducted to evaluate the water conservation aspects for vegetable production associated with field incorporations of municipal solid waste compost (MSWC). In June 1992, MSWC was incorporated into a sandy soil as a soil amendment at the University of Florida Gulf Coast Research and Education Center, Bradenton, Florida. Drip-irrigated and subirrigated vegetable production studies were conducted during autumn 1992, spring 1993, autumn 1993, and spring 1994 seasons. Green peppers were grown during the autumn seasons and fresh market tomatoes were produced during the spring seasons. MSWC was applied at various rates in drip-irrigated and subirrigated plots. Drip irrigation was applied based on crop water use estimated from reference evapotranspiration data and crop coefficients. A fully enclosed subirrigation system was used in the subirrigated plots with the water table controlled at an average depth of 0.60 m below the soil surface. Nitrogen fertilizer was applied to each of the MSWC, drip-irrigated, and, subirrigated plots. Incorporated MSWC was still immature (based on measured C/N ratios > 30) and reduced autumn 1992 drip-irrigated pepper yields and reduced plant growth. By spring 1993 the incorporated MSWC had field matured and resulted in significantly increased tomato plant size and fruit yields in the spring of 1993 and all other subsequent drip-irrigated trials. The 134 t ha –1 MSWC rate increased spring 1993 and 1994 tomato yields by 27% and 18%, respectively, and autumn 1993 peppers by 17% over the non-amended plot yields. Drip-irrigated pepper yields were not affected by irrigation rate or applied nitrogen level. While drip irrigation rate did not affect total tomato fruit yield in any of the seasons, yield of extra large tomato fruit was greater with higher levels of applied water in the spring of 1994, and the higher applied nitrogen levels increased marketable fruit yields by 13 to 14%. In general, amending a sandy soil with MSWC significantly improved plant growth and yield in drip-irrigated vegetable production. However, applied nitrogen rate in the subirrigated fields did not affect fruit yield in any of the seasons. Autumn 1992 pepper yields and spring 1994 tomato yields in the subirrigated plots were reduced by the addition of MSWC and autumn 1993 pepper yields were greater with the addition of MSWC. MSWC increased yield of extra large tomato fruit in the spring 1993 season and reduced fruit size in the autumn 1993 pepper trial, but had no statistical effect on fruit size in any of the other seasons. Therefore, the subirrigated system results were not as conclusive as the drip-irrigated results. Furthermore, immature MSWC products should be incorporated into fields with sufficient maturation time prior to planting.