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The rapid decline in water availability for irrigation on the Texas High Plains (THP) is a significant problem affecting crop production and the viability of a large regional economy worth approximately USD 7 billion annually. This region is the largest continuous cotton-producing area in the United States, and the timely delivery and efficient use of irrigation water are critical to the sustainability and profitability of cotton production in this region. Current irrigation scheduling must be improved to reduce water consumption without compromising crop production. Presently, irrigation scheduling based on reference evapotranspiration (ETo) is limited due to the lack of reliable and readily available in-field weather data and updated crop coefficients. Additionally, in-field variability in crop water demand is often overlooked, leading to lower irrigation efficiency. To address these challenges, we explored the potential use of an unmanned aerial vehicle (UAV)-based crop monitoring system to support irrigation management decisions. This study was conducted in Lubbock, Texas, in 2022, where high temporal and spatial resolution images were acquired using a UAV from a cotton field experiment with four irrigation levels. Soil moisture and canopy temperature sensors were deployed to monitor crop response to irrigation and rainfall. The results indicated a significant effect of water stress on crop growth (revealed by UAV-based canopy cover (CC) measurements), yield, and fiber quality. Strong correlations between multi-temporal CC and lint yield (R2 = 0.68 to 0.88) emphasized a clear trend: rainfed treatments with lower yields exhibited reduced CC, while irrigated plots with higher CC displayed increased yields. Furthermore, irrigated plots produced more mature and uniform fibers. This study also explored various evapotranspiration calculation approaches indicating that site-specific CC measurements obtained from a UAV could significantly reduce irrigation application. A regression model linking evapotranspiration to canopy cover demonstrated promising potential for estimating water demand in crops with an R2 as high as 0.68. The findings highlight the efficacy of UAV-based canopy features in assessing drought effects and managing irrigation water in water-limited production regions like the THP. [ABSTRACT FROM AUTHOR]

Nitrogen recommendations for Upland cotton (Gossypium hirsutum L.) in the western USA are based on spring soil N[O.sup.-.sub.3]-N tests. In-season monitoring of plant N status is another approach. Our primary objective was to test spectral reflectance and chlorophyll meter measurements as in-season N decision aids for irrigated cotton, and to compare these with soil test-based N management. The secondary objective was to determine the fate of [sup.15]N as affected by N management and irrigation modes. Urea ammonium nitrate was applied with low energy precision (LEPA) center-pivot, surface drip, and subsurface drip irrigation. Microplots received 3 atom% [sup.15]N. Soil test N application was based on 0-to 60-cm soil N[O.sup.-.sub.3]-N and 1400 kg lint [ha.sup.-1] expected yield. Thirty-four kilograms of N per hectare was applied when green vegetative index (GVI) or chlorophyll meter readings relative to well-fertilized plots were

Pre-plant soil N[O.sup.-.sub.3]-N tests and petiole N[O.sup.-.sub.3]-N analysis are bases for Upland cotton (Gossypium hirsutum L.) N management in the western USA. Alternative approaches include proximal multispectral reflectance sensing and chlorophyll meter readings. Our objective was to determine if spectral reflectance and chlorophyll meter measurements correlate with cotton leaf N and biomass. Urea ammonium nitrate was applied after emergence and with low energy precision (LEPA) center-pivot, surface or subsurface drip irrigation water up to peak bloom. Multispectral reflectance readings 0.5 m above the canopy, chlorophyll meter readings, and biomass samplings were taken at early squaring, early bloom, and peak bloom for 3 site-years in Lubbock, TX and Ropesville, TX. Green vegetative indices (GVI) and green normalized difference vegetative indices (GNDVI) calculated from reflectance data generally correlated better with leaf N and leaf N accumulation than did red vegetative indices (RVI) and red normalized difference vegetative indices (RNDVI). Biomass and lint yield correlated more often with red-based indices than green-based indices. Chlorophyll meter readings correlated with leaf N as often as GVI and GNDVI did. Biomass, however was poorly related to chlorophyll meter readings. These results demonstrate the effectiveness of GVI, GNDVI, and chlorophyll meter readings in assessing leaf N, and RVI and RNDVI in assessing cotton biomass. However, we recommend converting vegetative indices or chlorophyll meter readings to sufficiency indices, which are calculated from indices or readings relative to well-fertilized plots. Sufficiency indices were able to successfully predict little or no need for in-season N fertilizer in the low-yielding 2000 crops (sufficiency index > 0.95), and predicted greater need of N fertilizer in the high-yielding 2001 crop (sufficiency index < 0.95).

Field heterogeneity in soil texture and site elevation (SE) may affect crop water and N use. A 2-yr (1998-1999) study was conducted on the Texas High Plains to determine the interdependence between management (irrigation and N fertilization), soil heterogeneity (texture, soil water, and N[O.sub.3]-N) and topography, and their impact on cotton (Gossypium hirsutum L.) lint yield and N uptake. Treatments were irrigation at 50 and 75% estimated cotton evapotranspiration (ET) and N rates at 0, 90, and 135 kg [ha.sup.-1]. Soil water content (SWC), lint yield, N uptake, and N fertilizer recovery, measured as a function of management and space, were higher on low positions. Mixed model analysis showed that irrigation was significant on SWC, lint yield, and N uptake (P < 0.05) each year. The N treatment had no effect on lint yield or N uptake in 1998 because of high soil residual N[O.sub.3]-N, and the model residual was significant for all measured variables (P < 0.0001). Sand, SWC, lint yield, and N uptake were negatively correlated with SE (r [greater than or equal to] -0.64). In 1998, lint yield, SWC, clay, sand, and SE were cross correlated within 60 to 80 m. Multivariate state-space analysis showed that lint yield at position i was weighted on lint yield, SWC, clay, sand, and SE at previous position i - 1. It is concluded that 75% ET and N rate at 90 kg [ha.sup.-1] would be the basis to consider variable water and N rates related to field conditions, and the state-space model quantified spatial interdependence between irrigation, fertilization, and field heterogeneity.

Pearl millet (Pennisetum glaucum (L.) R. Br.) production in the West African Sahel is constrained by low, erratic rainfall and low soil nutrient (particularly P) availability. Outdoor pot and growth chamber experiments tested the hypothesis that increasing soil P supply increases transpirational water-use efficiency (WUE(sub T)), under water-stressed and non-water-stressed conditions. Pearl millet was grown outdoors under semiarid conditions in covered pots containing 85 kg of acid, P-deficient Betis sand (sandy, siliceous, thermic Psammentic Paleustalf). Plants were treated with four P levels and two water treatments, and harvested at 14-d intervals. Significant main and interactive effects on WUE(sub T) due to P level, water treatment, and time of harvest were found. The slope of the curve relating DM to cumulative transpiration (T(sub cum)) increased with P level and water stress when data from all harvests were pooled. In the growth chamber, WUE(sub T) of non-water-stressed plants ranged with increasing P level from 3.22 to 9.12 g kg to the -1 at 29 days after sowing (DAS) in pots containing 6 kg soil, and from 0.84 to 9.24 g kg to the -1 at 49 DAS in pots containing 18 kg soil. The ratio of leaf net photosynthetic rate to transpiration (WUE(sub gas)) at 500 micron mol m to the -2 s to the -1 photosynthetic photon flux density (PPFD) ranged from 1.88 micron g mg to the -1 for plants receiving no P to 10.25 micron g mg to the -1 for those receiving 0.310 g P 6 kg to the -1 soil. Between PPFD levels of 500 and 2000 micron mol m to the -2 s to the -1, plants receiving no P increased WUE(sub gas) to only 3.60 micron g mg to the -1, whereas those receiving higher levels of P increased WUE(sub gas) to as much as 18.2 micron g mg to the -1. Our finding that increasing soil P availability increases WUE(sub T) under water-stressed and non-water-stressed conditions reinforces previous conclusions that water supply in the Sahel and similar semiarid environments cannot be effectively managed for improved crop production without addressing soil fertility constraints.