Although the cost for water is one of the largest expenditures in a grower's budget in Arizona, many growers in the state still over- irrigate their fields to assure that there will be no yield losses. These excess irrigations usually do not cause any negative effects to the crop, they can cause the loss of available nitrogen to the plant and the potential for nitrate contamination of groundwater resources. To assess the impact that excess irrigation may have on cotton yields and the potential for groundwater contamination, a drainage lysimeter study was initiated at the Maricopa Agricultural Center, Maricopa, Arizona. Drainage lysimeters are large, open- topped steel boxes filled with soil and placed underground in the experimental field. Crops are grown directly above the lysimeters and the water that moves through the soil profile is collected at the bottom of the lysimeter and analyzed. In this study, three lysimeters were installed. The lysimeters were 80" wide (two row widths), five feet long, and six feet deep. They were placed 18 inches below the soil surface and filled with soil as to best represent the soil in its natural condition. The data presented in this paper are from two years of an ongoing experiment. Throughout the growing season, water samples were taken from the lysimeters in the field. Nitrogen applications were made according to field conditions and weekly petiole sampling. Irrigations were made according to field conditions and using the AZSCHED irrigation scheduling program. Treatment one was irrigated according to the schedule recommended by AZSCHED. The amount applied was equal to the total crop water use since the last irrigation. In treatment two, the timing was the same as treatment one, but the amount of irrigation water applied was 1.25 times more. Treatment three was also irrigated at the same time but with 1.5 times more water. Yield samples were taken at the end of each season and showed no significant differences between treatments, with yields averaging about 1100 lb./acre of lint in 1995 and 940 lb./acre of lint in 1996. The drainage amounts ranged from 9.5" in treatment three to 2.5 inches in treatment one. The corresponding nitrate-N losses were 56.9 lb. N/acre for treatment three and 33.4 lb. N/acre for treatment one. Monitoring continued during the winter to assess the impact of winter rainfall. In the last two years, there has been no significant winter rainfall.

A single field study was conducted in 1996 at the Maricopa Agricultural Center (1,175ft. elevation) to evaluate the effects of two dates of irrigation termination on the yield of a common Upland cotton variety (DPL 5415). Planting date was 11 April (667 HU/Jan 1 86/55° F thresholds. Two dates of irrigation termination (IT1 - IT2) were imposed based upon crop development into cut-out, with IT1 (14 August) being provided such that bolls set at the end of the first fruiting cycle would not be water stressed and could be fully matured The second termination (IT2) date was 10 September, which was staged so that soil moisture would be sufficient for development of bolls set up through the first week of September. Lint yield results revealed no differences between IT1 and IT2.

A newly installed subsurface drip system was compared to a conventional furrow-irrigated cotton production system in the Marana Valley in 2000. Regular measurements included soil moisture, flower tagging, general plant growth and development measurements, and lint yield. Results indicate that an increase in lint yield of approximately 250 lbs. lint/acre was obtained under the drip irrigation system. Approximately 1/3 less irrigation water was used under the drip irrigation system. Pounds of lint produced per acre-inch of water applied provide the most dramatic results. In the furrow-irrigated system approximately 25 lbs. of lint was produced per inch of water applied while the drip system ranged from 70-80.

Data was collected for a third season to determine the effects of high frequency irrigation on irrigation uniformity in cotton production. The past two seasons indicated that high frequency irrigation worked better on sandier soils than soils containing high clay contents. Although no significant differences were found, higher yields were obtained on a site with a relatively high sand content. A field located at the Maricopa Agricultural Center was split into two treatments. Treatment 1 was irrigated at approximately 35% depletion of available water in the plant rootzone. Treatment 2 was irrigated at approximately 50% depletion in the crop rootzone. Although the yield data from Treatment 1 was higher on the average, statistically, there was no difference between the two treatments.

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