An irrigation scheduling trial was implemented on both long and short staple cotton on the Safford Agricultural Center in 1993. It is a continuation of studies initiated in 1991, where plots were irrigated when they reached 40 %, 50% and 60% soil water depletion level as predicted by the AZSCHED software. Results for this study are given as well as a summary of the three year study.

Three different irrigation scheduling techniques were compared in this study; aerial infrared, hand -held infrared, and neutron moisture gage measurements. There were four treatments with three replications of each. Treatment one was scheduled using aerial infrared imaging and analyzes performed by Agrometrics, Inc. Treatment two was scheduled using a hand -held infrared gun. Irrigations for this treatment were initiated at a crop water stress index value of 0.3. Treatments three and four were scheduled using neutron probe measurements. Treatment three was irrigated at 45% depletion of the available soil water. Treatment four was irrigated at 45% depletion of the available soil water until mid-bloom, when the strategy was changed to irrigate at 35% depletion. Yield results showed no significant difference between the treatments.

A two year study was conducted to determine the optimum timing of the initial post plant irrigation using leaf water potential (LWP) measurements. A short - season Upland cotton (Gossvpium hirsutum L.), variety DPL 20, was planted on 19 April 1993 and 15 April 1994 at the Marana Agricultural Center on a Pima clay loam (Typic Torrifluvent) soil. Treatments, designated Tl , 72, and T3, were such that the initial post plant irrigation would be applied when the midday LWP of the uppermost, fully- developed leaf exposed to full sunlight measured -15, -19, and -23 bars, respectively. All treatments received the same irrigation regime following the initial post plant irrigation. Basic plant measurements, including plant height, mainstem node number, fruit retention, number of nodes above the uppermost white bloom, fresh bloom count within a 166 -ft1 area, and percent canopy cover, were taken weekly from each plot. Soil -water data was collected at 10 inch depth increments, to a depth of 60 in. , from access tubes located in each experimental unit. Yields were 1112, 1095, and 977 lbs lint/acre in 1993 and 1082, 1035, and 964 lbs lint /acre in 1994 for T1, 72, and T3, respectively. Yields were reduced when the midday LWP was allowed to fall below -19 bars, however, reduction was significant (P 5 0.05) only in 1993. At the time of the initial post plant irrigation for each treatment, approximately 83, 62, and 32 % of the total plant available water was present in the upper 60 in. of the soil profile for Ti, 72, and T3, respectively.

One key component in good irrigation management is the measurement of soil moisture to help determine when to irrigate. In this study, resistance blocks and tensiometers were compared to neutron probe readings to assess how well these devices followed soil moisture and whether the resistance blocks and /or tensiometers could be used to schedule irrigation in cotton production. The resistance blocks were placed at 6, 18, and 30 inches. Tensiometers were placed at 18 and 30 inches. The readings from the resistance blocks and tensiometers were compared to neutron probe readings taken at 6, 18, and 30 inches. The resistance blocks compared well with the neutron probe readings at the 6 inch and 30 inch depth. At the 18 inch depth, there was much scatter in the data. The tensiometers also showed good comparisons at 30 inches and poor comparisons at 18 inches.

Irrigation scheduling, by keeping track of irrigation applications, soil storage and crop water use, has been computerized by a number of different individuals. A key component of the computerized methods is the estimation of a reference crop evapotranspiration rate. Complaints about one such method, AZSCHED, led the authors to compare the reference crop evapotranspiration values calculated by AZSCHED with those calculated by a second procedure available used by AZMET. Results of the comparison indicated that no significant difference existed between methods, for either a traditionally "long season", or a contemporary "short season" growing period. AZSCHED did estimate crop water use to be about 5% - 8% more than AZMET, an amount that is not of importance considering the irrigation inefficiencies created by field non-uniformities. Experience by the authors indicates that inappropriate selection of irrigation efficiencies and/or soil water holding capacity may be the main cause of user complaints.

Lint yield response to differing irrigation treatments based on maximum allowable soil moisture depletions was tested in an indeterminate (D +PL 5816) and a determinate variety (D+PL 5415) selection. The Arizona Meteorological Weather Network (AzMet) was used to summate evapotranspiration demands with irrigations triggered at 35 (wet), 50 (med), and 65 % (dry) maximum allowable soil moisture depletion levels. Soil water holding capacity was gravimetrically measured to a depth of four feet on one foot increments. The study consisted of three treatments replicated four times utilizing a complete block split plot design. When the allowable depletion level was attained, the water volume necessary to refill the effective root zone was delivered. This was accomplished by manipulating irrigation set times and flow rates. Irrigation volumes were 66.7, 57.2, and 46.9 acre - inches for the wet, medium, and dry treatments respectively. Lint yields were significantly reduced when the maximum allowable soil moisture depletion exceeded 50% in the determinate variety selection while there were no significant lint yield differences in any of the irrigation treatments with the more indeterminate variety. Water stress sensitivity is increased with the determinate variety while the indeterminate variety is more forgiving. With variety selection shifts in the Central Arizona desert towards a reduced season approach and utilization of more determinate varieties, water management strategy should be modified to minimize or eliminate any water stress during the flowering period. The AzMet weather network information offers cost effective (free) and reliable water use information. The system can be used to assist with irrigation scheduling if a producer is willing to attempt to characterize differing soil water holding capacities on the farm and manage accordingly.

A field trial was conducted at the Maricopa Agricultural Center to observe the effects of four irrigation efficiencies (65 %, 75 %, 85 %, and 95 %) on the lint yield produced from two upland cotton varieties (DP 5415 and DP 5816). Nitrogen requirements for the crop were determined using pre -season soil samples and in-season petiole samples with data collected from crop monitoring at weekly intervals. AZSCHED was used as a guide to the irrigation timing and amount of water applied during the season. The irrigation efficiency did not have an effect on the lint yield of the cotton crop regardless of variety, but there was a significant difference in yield between the varieties. Lint yields ranged from 1165 #/acre to 1299 #/acre for DP 5415 and 869 #/acre to 986 #/acre for DP 5816.

We tested the hypothesis that small frequent irrigations during the July cotton peak fruiting stage would result in better fruiting and higher cotton yields than the same amount of water applied less frequently. Over three years under a short - season production system, irrigation intervals of every 5-d with 42 mm of water applied at each irrigation increased cotton lint yield by 5-11 % compared to irrigation intervals of 10- and 15-d with 80 and 130 mm of water applied at each irrigation, respectively. The results show that small, frequent furrow irrigations during cotton fruiting are highly effective in reducing water deficit during critical growth stages and improved lint production in a short - season cultural system. Soil salt content in the top 15 cm of soil was not increased after three years of study.

Upland cotton lint yield response to several soil moisture depletion levels was measured in 1996 and 1997 for four Upland cotton varieties including DP 5415, DP 33B, DP 5816, and STV 474. In 1996, depletion of plant available soil water (PAW) irrigation treatments consisted of 35%, 50%, 75%, and 90%. In 1997, treatments were 35%, 50 %, 65 %, and 80% depletion of PAW. In 1996, the 35% and 50% treatments were significantly different than the 75% and 90% treatments (P < 0.05) and resulted in the highest lint yields of 1374 and 1438 lbs. lint/acre respectively. A lint yield reduction was measured with the 75% and 90% PAW treatments of 713 and 329 lbs. lint/acre, respectively. The 75% and 90% treatments were significantly different than the 35% and 50% treatments and were significantly different from each other. In 1997, all PAW depletion treatments were significantly different with the 35% PAW treatment resulting in the highest average lint yield of 1880 lbs. lint/acre. The 50%, 65%, and 80% PAW treatments resulted in 1410, 1123, and 248 lbs. lint/acre respectively. There was no significant (P < 0.05) difference between varieties within all PAW treatments in 1996 or1997.

A field trial was conducted at the Maricopa Agricultural Center to observe the effects of four irrigation efficiencies (65%, 75%, 85%, and 95%) on the lint yield produced from two upland cotton varieties (DP 5409 and SG 125). Nitrogen requirements for the crop were determined using pre- season soil samples and in season petiole samples in conjunction with crop monitoring data collected at weekly intervals. AZSCHED was used as a guide to the irrigation timing and amount of water applied during the season. This year there was a lint yield response to the different irrigation efficiencies, and a slight difference in yield between the two varieties. Lint yields were significantly lower in the 95% irrigation efficiency plots. Lint Yields ranged from 1448 # lint /acre (SG125 at 75%) to 1220 # lint/acre ( DP5409 at the 65% irrigation efficiency).