Thesis (M.S. - Agricultural Engineering)--University of Arizona, 1986. / Includes bibliographical references (leaves 96-99).

The CERES-Wheat Crop Growth and Development model treats temperature, nitrogen and water stresses as limiting factors. For each day the model calculates a stress index for temperature, N and water, compares the magnitude of the indices, and then adjusts the calculated daily potential growth using the index of the most severe stress, while ignoring the other stresses. Under the conditions in Oregon, however, mild N and water stress will often be present together in about equal degree of stress. Some published results suggest that both stresses affect growth and yield under those conditions. Therefore, this work was undertaken to evaluate the combined effects of N and water stress on growth and development of wheat (Triticum aestivum L.). To make such an evaluation one must be able to control both N and water supplies to the plant and the response of the plant to these two variables must be measured at different growth stages. A system of imposing controlled plant water stress developed by Snow and Tingey (1985) was adapted and evaluated for its potential to impose controlled levels of both N and water stress to single wheat plants. Using a 12 mmol N and 4 cm pathlength as optimum N and water supply treatment, 2 mmol N and 12 cm pathlength and a 1 bar standard ceramic disc in the floral foam column as N and water stress treatments during tillering resulted in 15 % reduction in tillers/plant for limits to the N supply alone, a 39 % reduction for limits to the water supply alone, and a 52 % reduction when both stresses were imposed simultaneously. There was no effect of N or water supply treatments on the leaf appearance rate on the main stem, a measure of the rate of progress toward flowering. Both N and water supplies had a strong effect on tillers/plant which, in turn, affected plant biomass and its constituent parts. The effects were independent, suggesting that, to accurately model the wheat canopy development when both mild N and water stresses are present, both stresses must be considered. A 'law of minimum' concept as currently used in the CERES-Wheat model would not be an accurate model for the process of tillering. In an experiment where stress was imposed during flowering and grain-filling, the grain yield/plant varied significantly with both N and water supplies. The interaction between N and water treatments on grain yield was also significant and was due primarily to their significant interaction on mean kernel weight. The major determinant of grain yield was tillers/plant at harvest. Both N and water supplies affected kernels/ear and N stress caused a reduction of 12% in both fertile spikelets/ear and kernels/fertile spikelet. At an optimum water supply, the difference between the effect of optimum and medium N supply on grain yield was not significant but yield in low N supply was reduced by 54%. In the medium water supply, the grain yield at both medium and low N treatments were significantly lower than at optimum N supply. There was no significant difference in yield between N supply treatments in the low water supply treatments. Thus, under severe water stress, a 'law of minimum' concept appeared to be valid, but under less severe stress both N and water supply affected grain yield. The data on leaf water potential and leaf temperature showed that plants in low water supply treatments maintained consistently lower leaf water potential and higher leaf temperature than in optimum water supply treatment. / Graduation date: 1993

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.

Thesis (Ph. D. - Plant Sciences)--University of Arizona, 1988. / Includes bibliographical references (leaves 72-81).

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.

A pearl millet hybrid (Pennisetum americanum (L.) Leeke) and its two parents were evaluated for their photosynthetic rates, diffusive resistance, canopy temperature, transpiration rates, stomatal aperture and frequency, and growth responses to various irrigation levels. The experiments were conducted on Brazito sandy loam soil at The University of Arizona Campus Agricultural Center, Tucson, AZ in 1985 and 1986. A sprinkler was used to create water treatments. Photosynthetic rates were not significantly different among genotypes at each water level. Transpiration rate, diffusive resistance, canopy minus ambient temperature, and photosynthetic rate were all significantly related to water treatments with correlation coefficients .ranging from 0.91 to 0.98. Under water stress, the female transpired more, exhibited lower diffusive resistance and had a cooler canopy compared to the hybrid and male parent. Based on these characteristics, the female seemed to expend more energy on heat dissipation than yield improvement. Water stress reduced stomatal aperture but increased stomata! frequency. The hybrid had significantly higher stomatal frequency at all water levels and smaller aperture between 63 and 125 mm irrigation levels. Dry matter, leaf area, leaf area index, and plant height were also reduced by water stress. In general, the hybrid and the male parent produced significantly more dry matter and were significantly taller than the female. The female parent had significantly higher leaf area and leaf area index late in 1986. Grain yield was reduced by water stress. Both years, the female grown under optimum moisture conditions had approximately half the grain yield as compared to the male and the hybrid. The major components contributing to the higher grain yield of the hybrid and male were larger panicles and more productive tillers.

Limited water supplies are available to satisfy the increasing demands of crop production. It is therefore very important to conserve the water, which comes as rainfall, and water, which is used in irrigation. A proper irrigation water management system requires accurate, simple, automated, non-destructive method to schedule irrigations. Utilization of infrared thermometry to assess plant water stress provides a rapid, nondestructive, reliable estimate of plant water status which would be amenable to larger scale applications and would over-reach some of the sampling problems associated with point measurements. Several indices have been developed to time irrigation. The most useful is the crop water stress index (CWSI), which normalizes canopy to aIr temperature differential measurements, to atmospheric water vapour pressure deficit. A field experiment was conducted at Cedara, KwaZulu-Natal, South Africa, to determine the non-water-stressed baselines, and CWSI of cereal rye (Secale cereale L.) from 22 July to 26 September 2002, and aImual (Italian) ryegrass (Lolium multiflorum Lam.) from October 8 to December 4, 2002, when the crops completely covered the soil. An accurate measurement of canopy to air temperature differential is crucial for the determination of CWSI using the empirical (Idso et al., 1981) and theoretical (Jackson et al., 1981) methods. Calibrations of infrared thermometers, a Vaisala CS500 air temperature and relative humidity sensor and thermocouples were performed, and the reliability of the measured weather data were analysed. The Everest and Apogee infrared thermometers require correction for temperatures less than 15 QC and greater than 35 QC. Although the calibration relationships were highly linearly significant the slopes and intercepts should be corrected for greater accuracy. Since the slopes of the thermocouples and Vaisala CS500 air temperature sensor were statistically different from 1, multipliers were used to correct the readings. The relative humidity sensor needs to be calibrated for RH values less than 25 % and greater than 75 %. The integrity of weather data showed that solar irradiance, net irradiance, wind speed and vapour pressure deficit were measured accurately. Calculated soil heat flux was underestimated and the calculated surface temperature was underestimated for most of the experimental period compared to measured canopy temperature. The CWSI was determined using the empirical and theoretical methods. An investigation was made to determine if the CWSI could be used to schedule irrigation in cereal rye and annual rye grass to prevent water stress. Both the empirical and theoretical methods require an estimate or measurement of the canopy to air temperature differential, the non-waterstressed baseline, and the non-transpiring canopy to air temperature differential. The upper (stressed) and lower (non- stressed) baselines were calculated to quantify and monitor crop water stress for cereal rye and annual ryegrass. The non-water-stressed baselines were described by the linear equations Te - Ta = 2.0404 - 2.0424 * VPD for cereal rye and Te - Ta = 2.7377 - 1.2524 * VP D for annual ryegrass. The theoretical CWSI was greater than the empirical CWSI for most of the experimental days for both cereal rye and annual ryegrass. Variability of empirical (CWSI)E and theoretical (CWSI)T values followed soil water content as would be expected. The CWSI values responded predictably to rainfall and irrigation. CWSI values of 0.24 for cereal rye and 0.29 for annual ryegrass were found from this study, which can be used for timing irrigations to alleviate water stress and avoid excess irrigation water. The non-water-stressed baseline can also be used alone if the aim of the irrigator is to obtain maximum yields. However the non-water-stressed baseline determined using the empirical method cannot be applied to another location and is only valid for clear sky conditions. And the non-water-stressed baseline determined using theoretical method requires computation of aerodynamic resistance and canopy resistances, as the knowledge of canopy resistance, however the values it can assume throughout the day is still scarce. The baseline was then determined using a new method by Alves and Pereira (2000), which overcomes these problems. This method evaluated the infrared surface temperature as a wet bulb temperature for cereal rye and annual ryegrass. From this study, it is concluded that the infrared surface temperature of fully irrigated cereal rye and annual ryegrass can be regarded as a surface wet bulb temperature. The value of infrared surface temperature can be computed from measured or estimated values of net irradiance, aerodynamic resistance and air temperature. The non-water-stressed baseline is a useful concept that can effectively guide the irrigator to obtain maximum yields and to schedule irrigation. Surface temperature can be used to monitor the crop water status at any time of the day even on cloudy days, which may greatly ease the task of the irrigator. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003

No description available.

Thesis (Ph.D.)--University of Adelaide, School of Earth and Environmental Sciences, Discipline of Soil and Land Systems, 2004. / "October, 2004" Includes bibliographical references (leaves 228-254). Also available in a print form.

Thesis (MScAgric)--University of Stellenbosch, 2001. / ENGLISH ABSTRACT: The application of water and nutrients via a drip irrigation system influences the water distribution in the soil, soil characteristics and root distribution beneath the dripper. To determine the water distribution pattern beneath a dripper in sandy soil, EnviroSCAN (Sentek) capacitance probes were installed directly below the dripper and at distances of 20, 40 and 60 cm from the dripper. The continuous monitoring of the soil water content (SWC) beneath the dripper provided a good indication of how the water applied through the dripper is distributed in the soil. In this study a semi-impermeable layer in the soil was detected through observing water accumulation patterns in the SWC. Water accumulated above the layer and SWC values increased to far above the upper level of easily available soil water (EAWupper),while the lower soil layers remained drier. The measurements also show that the horizontal water movement is restricted to 20 cm from the dripper. Specific parameters, such as the lower level of easily available soil water (EAWlower),can be used to determine optimal irrigation management. Together with the water distribution study, the root distribution beneath a dripper was also investigated. A high concentration of roots in the area beneath the dripper was found, which corresponds with the area wetted by irrigation. In another study, three irrigationlfertigation methods where investigated to ascertain the influence on soil characteristics and root distribution. These were: micro irrigation (MI) (micro-spinner irrigation with broadcast granular fertilization), conventional drip fertigation (CDF) (daily drip irrigation with daily or weekly fertigation with a unbalanced nutrient solution, containing macronutrients only) and daily drip fertigation (DDF) (daily fertigation of a balanced nutrient solution, containing macro- and micronutrients). The study was conducted in two locations, viz. in the Western Cape Province, on sandy soil, and in the Eastern Cape Province, on silt loam soil. Micro Irrigation: A wide and even root distribution in the entire wetted volume was found on the sandy and silt loam soil. On the sandy soil, the soil pH(KC1)directly beneath the spinner was significantly lower than the pH(KC1)at positions further away from the spinner. Conventional Drip Fertigation: Root studies on sandy soil indicate a poor root development beneath the dripper, with a high concentration of roots in the area between the drippers. The poor root development directly beneath the dipper may be due to oxygen deficiency and/or acidification beneath the dripper. The soil pH(KC1)values show a significant lower pH(KC1)value directly beneath the dripper than further away. In comparison to the sandy soil, the roots developed well beneath a dripper in a silt loam soil. It appears as if soil acidity and/or oxygen deficiency was not a problem on this soil type. The rest of the root system was also well developed. This may be due to this soil's higher water holding capacity which creates a bigger wetted zone. Daily Drip Fertigation: In the sandy soil it seems that the roots developed in a continuous column beneath the dripper line, with little root development further than 20 cm from the dripper line. Where over-irrigation occurred, it caused a poor root development directly beneath the dripper. The root density in this treatment was much higher than in the other two treatments. The use of a balanced nutrient solution and pulse irrigation may be reasons for the better root development. In a silt loam soil a very high concentration of roots was found beneath the dripper and the rest of the root system was also well developed. As with the CDF treatment, it appears as if oxygen deficiency was not a problem on this soil type. / AFRIKAANSE OPSOMMING: Die toediening van water en voedingstowwe deur 'n drip-besproeiings stelsel beïnvloed die waterverspreiding in die grond sowel as die grondeienskappe en wortelverspreiding onder die dripper. Die waterverspreiding onder 'n dripper in 'n sandgrond is bepaal deur EnviroSCAN kapasitansie meetpenne direk onder die dripper en 20, 40 en 60 cm van 'n dripper af te installeer. Die aaneenlopende monitering van die grondwaterinhoud het 'n goeie indikasie van waterverspreiding in die grond gegee. Die horisontale waterbeweging is grootliks beperk tot 'n 20 cm radius vanaf die dripper en die waterbeweging was hoofsaaklik in 'n vertikale rigting. Die teenwoordigheid van 'n semi-deurlaatbare grondlaag in die grondprofiel is opgemerk deur water-akkumulasie in die profiel waar te neem. Wortelverspreiding onder die dripper is ook ondersoek en 'n hoë konsentrasie wortels is in die benatte sone gevind. In 'n verdere studie is drie besproeiings/sproeibemestings behandelings gebruik om die invloed van besproeiing/sproeibemesting op grondeienskappe en wortelverspreiding te ondersoek. Die drie behandelings was: mikro-besproeiing (mikro-besproeiing met korrelbemesting), konvensionele-drip-sproeibemesting (daaglikse drip-besproeiing met daaglikse of weeklikse sproeibemesting van 'n ongebalanseerde, voedingsoplossing wat alleenlik uit makro-elemente bestaan) en daaglikse-drip-sproeibemesting (daaglikse drip-besproeiing met daaglikse sproeibemesting van 'n gebalanseerde voedingsoplossing wat mikro- en makro-elemente bevat). Die studie is in twee areas gedoen, een in die Wes-Kaap, op 'n sandgrond, en die ander in die Oos-Kaap, op 'n slik-leemgrond. Mikro-besproeiing: Die wortelverspreidings studies op die sand- en slik-leemgrond wys op 'n wye en eweredige wortelontwikkeling in die totale benatte volume. Op die sand grond is gevind dat die grond pR(KCl)direk onder die sproeiertjie betekenisvol laer was as die pR(KCl)waardes verder weg van die sproeiertjie. Konvensionele-drip-sproeibemesting: Die wortelverspreiding in die sandgrond wys op geringe wortelontwikkeling direk onder die dripper met die hoogste konsentrasie wortels tussen die drippers. Grondversuring en/of suurstoftekorte onder die dripper kan die oorsaak wees van die swak wortelontwikkeling direk onder die dripper. Die grond pR(KCl)direk onder die dripper was betekenisvol laer as die pR(KCl)verder weg van die dripper. In vergelyking met die sandgrond, het die wortels in die slik-leemgrond goed ontwikkelonder die dripper. Dit wil voorkom of versuring en suurstoftekorte onder die dripper nie 'n probleem was in die slik-leemgrond nie. Die res van die wortelstelsel was ook goed ontwikkel. Dit mag wees weens die grond se hoë waterhoudingsvermoë wat 'n groot benatte area tot gevolg het. Daaglikse-drip-sproeibemesting: In die sand grond wil dit voorkom asof die wortels in 'n aaneenlopende kolom onder die dripperlyn ontwikkel met weinig wortelontwikkeling verder as 20 cm van die dripperlyn. Waar oorbesproeiing 'n probleem was, was daar weinig wortelontwikkeling in 'n klein area direk onder die dripper. Die wortel-digtheid in die behandeling was baie hoër as in die ander behandelings. Die gebruik van 'n gebalanseerde voedingsoplossing en puls-besproeiing mag dalk redes wees vir die beter wortelontwikkeling. In die slik-leemgrond is 'n hoë konsentrasie wortels onder die dripper gevind en die res van die wortelstelsel was ook goed ontwikkel. Soos in die konvensionele-drip-sproeibemesting behandeling wil dit voorkom of suurstoftekort en versuring onder die dripper nie 'n probleem was in die grond nie.