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. Although these over 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 what impact over 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 steel boxes with the top open 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 35.7 lb/acre for treatment three and 21.3 lb/acre for treatment. Monitoring continued during the winter to assess the impact of winter rainfall. In the last two years, there has been no significant winter rainfall.