Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis--University of Wisconsin. / Supported in part by Environmental Science Services Administration, grant E-287-68(G).

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Ground-based, remotely sensed reflectance and temperature data were collected over differentially irrigated, developing cotton and sorghum canopies in order to investigate interrelations of these parameters; to monitor their temporal changes ; to understand their spatial structure ; and to estimate crop coefficient (KO from canopy reflectance. Spectral reflectance and derived vegetation indices showed ability to significantly discriminate among differential irrigation levels of sorghum canopies, starting the fourth week of growth. All vegetation indices increased as a result of crop development, with the perpendicular vegetation index (PVI ) demonstrating the greatest potential for assessing water stress conditions, whereas, soil indices behaved independently of crop development and water stress. Canopy temperature and derived water stress indices, on the other hand, were in high concordance and were able to detect crop water stress with variable degrees of sensitivity. Experimental variograms revealed that cotton reflectance and temperature were not spatially dependent when all water treatments were included. For the moisture stress treatments, only canopy temperature exhibited spatial dependence early in the period of stress. Sorghum canopy reflectance and temperature demonstrated some spatial structure; however, a drift was suspected due to regularity in the data spatial distribution. Normalized difference (ND), normalized perpendicular vegetation index (NPVI) and normalized green vegetation index (NGVI), for fifty days covering the period from planting to heading, were fitted with a complementary error function equation with minor adjustment. Both NPVI and NGVI displayed a 1:1 relation with interpolated tabular values of basal Kc, whereas ND deviated from the 1:1 relation for the period beyond 30 days after planting. The model was also found to be valid for estimating K(c) for moderately deficit irrigation conditions.

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.

Water availability is a critical limiting element in semi-arid ecosystem productivity and presents particular challenges in Sahelian countries such as Senegal. The landscapes are characterized by the presence of two common semi-arid shrubs (Piliostigma reticulatum and Guiera senegalensis), both of which may be important hydraulic regulators in these water-limited ecosystems. Dry season observations revealed higher moisture levels in soil surrounding the shrub shallower roots relative to bare soil. This observation led us to hypothesize that these shrubs may participate in a natural irrigation phenomenon termed as "hydraulic redistribution" (HR). This dissertation reports on three studies performed to ascertain the existence of and investigate the characteristics of this hypothesized phenomenon. The first study investigated seasonal variability in soil water and shrub root patterns. Soil moisture content declined steadily in the 0.9-1.2 m depth range and increased in the 0.2-0.4 m depth range, which supports the HR hypothesis. The second study quantified plot scale water balance fluxes. Notably, shrub water uptake from the water table served as a crucial contribution to the system water balance. The third study investigated the magnitude and dynamics of HR during the dry season. Crop-shrub associations were evaluated from measurements of soil moisture and potential, root sap flow and plant physiological measurements. This study proved the existence of HR and quantified the HR magnitude (~0.1 mm d⁻¹). For shrubs and annual crops in close association with shrubs, HR clearly provides a mechanism for drought-stress avoidance and maintenance of plant physiological functions. At the landscape scale, the interplay between shrub root morphology and HR may play a vital role in ecosystem function with practical implications for nutrient cycling and water balance in arid ecosystems. / Graduation date: 2006

Graduate

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