Cotton (Gossypium hirsutum L.) grown under drip irrigation was evaluated over a two year period for physiological and morphological responses. Three water levels representing, 103, 93 and 87% of estimated consumptive use (63.6 ha-cm) were used in Marana, AZ. in 1983. In 1984, cotton was grown under eight drip irrigation treatments corresponding to 120, 100, 80 and 60% of the estimated consumptive use (79.5 ha-cm) in Stanfield, AZ. These volumes of water were applied as small daily amounts and larger weekly amounts for a total of eight irrigation treatments. The experimental design was a randomized complete block with four replications. Mean leaf area ratio (LAR), leaf area index (LAI), mean net assimilation rate (NAR), mean relative growth rate (RGR), mean crop growth rate (CGR), plant height and the number of mainstem nodes were determined using the growth analysis method. Transpiration, diffusive resistance, leaf and ambient temperatures were measured with a steady state porometer. Apparent photosynthesis (APS) was determined in 1983 with an infrared gas analyzer which measured CO(,2) concentrations. In 1983, the cotton plants from the 103% irrigation treatment had greater transpiration, lower diffusive resistance and lower APS than the 93% treatment plants. In 1984, no significant differences were observed between the seasonal transpiration rates from the eight irrigation treatments. Cotton plants grown under the 120% treatment showed superior diffusive resistance responses than those from the 60% treatment. Temperature differentials were higher in the 120% treatments than in the 60% treatments. No significant differences were found between LAR, NAR, RGR and CGR during 1983 and 1984. Even though there were no differences between the total number of flowers produced in the three treatments in 1983, the 93 and 87% treatment plants produced more seed cotton than the 103% treatment plants. In 1984, the seed cotton yield from the 60% daily treatment was significantly the lowest. Due to the problems related to the late initiation of treatments and excessive rainfall, the physiological and morphological responses of cotton were inconsistent across the various water levels in 1983. Regression analysis confirmed the erratic responses of cotton plants from the weekly treatments across the wide range of environmental conditions in 1984.

Studies were conducted on commercial farms in 1982 and 1983 to study the effects of above surface drip irrigation, below surface drip irrigation, and furrow irrigation on flowering, boll set, yield, and growth of cotton (Gossypium hirsutum L.). Two varieties were used in 1982, 'Deltapine 62' and 'Deltapine 90', using both above and below ground drip irrigation and Deltapine 90 was grown under furrow irrigation. In 1983 the study was conducted at a different location with above ground drip irrigation and furrow irrigation using the cotton variety 'Deltapine 41'. The irrigation water was monitored to deliver 100% of consumptive use (CU), 85% of CU, 70% of CU, 100% of CU every other day and CU which was the grower's estimate of consumptive use. All open flowers were tagged using Kwik-Lok labels dated with the day the flower opened. Tagging of flowers started from first open flower and continued for 17 weeks in 1982 and 15 weeks in 1983. Plants were sampled twice in 1982 and three times in 1983 for LAI and partitioning studies. All open bolls were harvested on a weekly basis for a total of 12 harvests. In the 1982 experiments no statistical analyses were made because the five treatments were grown in different fields and could not be randomized together. A general comparison showed that Treatment 1, Deltapine 62 above ground drip system produced the highest number of flowers but it had the lowest percent of boll set. Treatment 5, Deltapine 90, furrow system, produced the lowest number of flowers but it had the highest percent of boll set. Treatment 3, Deltapine 62 below ground drip system, had the highest boll set and Treatment 5 retained the least number of bolls. Treatment 2, Deltapine 90 above ground dry system, had the highest seed cotton yield and Treatment 5 had the lowest yield. In the 1983 experiment, there were no significant differences in flowering, boll set, or yield among the five irrigated treatments. The furrow treatment could not be compared statistically with validity with the drip treatment because it was grown in a different field but a general comparison showed that it produced the lowest number of flowers and seed cotton yield although it had the highest percent of boll set.

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The yield, fruiting habits, plant nitrogen uptake, dry matter production, and boll and fiber properties were studied in relation to nitrogen fertilizer and water application rate interactions in trickle-irrigated cotton. Nitrogen applications did not significantly increase yields because of high initial soil nitrogen and nitrogen added with the irrigation water. Significant yield differences did exist between the 60% consumption use (CU) irrigation level and the 90% and 120% CU irrigation levels. The 60% CU level resulted in appreciable plant water stress and appreciably lower yields. Nitrogen additions significantly increased seasonal plant petiole NO₃-N values as well as the number of flowers produced during the growing season. Increased water application rate lengthened the periods of peak flowering and peak boll production but had no effect on petiole NO₃-N. Increased water application rate did not significantly increase boll production, but an increasing trend of increased boll production with increasing water application rates was evident. Nitrogen applications did not significantly increase boll production, but percent boll set for the control treatment was higher. Both nitrogen additions and increased water application rates increased plant nitrogen uptake, which ranged from 163 kg N/ha to 242 kg N/ha for the water levels and 188 kg N/ha to 220 kg N/ha for the nitrogen treatments. Increased water application rates significantly increased dry matter production during the growing season. Nitrogen and water application rates affected only a few boll and fiber properties. Average boll size was increased significantly with increasing water application rates. This partially accounted for the higher yield observed at the higher irrigation levels.

Crop coefficients (K(c)) are a useful means of predicting how much water is needed for irrigating a crop. The crop water stress index (CWSI), on the other hand, is a means of knowing when to irrigate. Two field experiments were conducted during the summers of 1990 and 1991 at Maricopa Agricultural Center and Marana Agricultural Center, respectively, to evaluate water use (evapotranspiration, ET) of different cotton varieties, to develop crop coefficients for cotton grown in the state of Arizona, and to evaluate empirical and theoretical crop water stress indices under field conditions. For the 1990 experiment, ET from the cotton variety DPL 77 was obtained using soil water balance (SWB) and steady state heat balance (SSHB) techniques. For the 1991 experiment, ET from two cotton varieties (DPL 20 and Pima S-6) was estimated using the Bowen ratio energy balance (BREB) method and the steady state heat balance method. Reference evapotranspiration (ETᵣ) was obtained from weather stations located close to the experimental plots. Average daily ET from the SSHB measurements ranged from 8.24 to 15.13 mm and from 10.34 to 12.12 mm for the 1990 and 1991 experiments, respectively. Total ET from the SWB was approximately 19% less than the total ET estimated by the SSHB. Total ET from individual plants was well correlated with average stem area over the evaluation periods. Daily ET from the two cotton varieties (DPL20 and Pima S-6) was approximately similar when irrigation conditions were the same, but differed later by as much as 48.4% as irrigation continued for the variety Pima S-6 only. Daily ET from the BREB measurements and ETᵣ were used to develop a crop coefficient curve for cotton grown at Marana, Arizona, which had a maximum smoothed value of 1.21. A critical value of CWSI equal to 0.3 was obtained by observing the pattern of the CWSI values over well-watered and drier conditions, and from previous research. Using the developed crop coefficient curve and the CWSI should provide a useful means of scheduling irrigation for cotton grown under climatic conditions similar to those at Marana, Arizona.

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The application of the crop water stress index (CWSI) method to irrigation scheduling is enhanced by knowledge of the relationship between CWSI and soil water potential (SWP) and how this relationship is affected by soil texture. A study using the same cultivar of cotton on three different soils was conducted in southern Arizona over a single growing season. Detailed data were collected of CWSI and soil moisture content for several treatments that scheduled irrigations at threshold CWSI values. CWSI was correlated with soil water potential values calculated from pressure plate determined moisture release curves. Spatial variability of soil characteristics necessitated use of average rather than plot specific moisture release curves. Analysis showed a linear CWSI-SWP relationship that varied greatly with soil depth and study site. The study concluded that soil profile average SWP alone does not normalize the CWSI between sites with different soil textures.

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This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project. / Preface by G. E. P. Smith