Abstracts

Beringer, J. and Tapper, N.J. 2000, 'The Influence of Subtropical Cold Fronts on the Surface Energy Balance of a Semi-arid Site', Journal of Arid Environments, vol. 44, pp. 437-450.

The passage of subtropical cold fronts through central Australia produces the only significant mesoscale meteorological features in the region. The interaction of these cold fronts with the surface energy balance strongly affects the local weather and climate. The surface energy balance was measured at a semi-arid site in Alice Springs, central Australia, to determine how it was influenced by the passage of subtropical cold fronts. Both Bowen ratio and eddy correlation methods were used. The daytime energy balance of the site showed high net radiation that was partitioned into 75% sensible heat flux and 25% soil heat flux with little or no latent heat flux. At night there was a large net radiative loss that was balanced primarily by a loss of heat from the soil. The cold fronts predominately passed through Alice Springs at night and showed a strong surface signature. The fronts brought moister air resulting in higher water vapour pressures during their passage. The nocturnal boundary layer was often disturbed as the front passed, resulting in warm, moist air being mixed down toward the land surface. Mixing decreased the soil heat flux and increased latent heat fluxes toward the surface. Moisture that accumulated at the surface at these times was often evaporated after a return to drier conditions. During the daytime, surface signatures in soil and sensible heat fluxes were less distinct due to the strong convective mixing. Latent heat fluxes followed a similar trend to the nocturnal case.

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da Silva, V.P.R., de Azevedo, P.V., et al. 2007, 'Surface Energy Fluxes and Evapotranspiration of a Mango Orchard Grown in a Semiarid Environment', Agronomy Journal, vol. 99, pp. 1391-1396.

Data from field experiments conducted in the semiarid climatic conditions of northeast Brazil were used to investigate the energy flux relations and evapotranspiration (ET) of a mango (Mangifera indica L.) orchard. The Bowen ratio-energy balance method was applied during the 1998-1999 fruiting cycles to estimate the energy balance components of the mango orchard, while the FAO Penman-Monteith approach was used for determining the reference evapotranspiration (ET_o). Results indicated that latent heat flux density (λE) could be obtained, with reasonable precision, as a function of measured net radiation flux density (R_n). The percentage of R_n used as λE was higher for the fruit growth and fruit maturation phenological stages, and lower for the flowering and fruit fall stages. For both field campaigns, λE was found to be the major component of energy balance, comprising >70% of the available energy. Soil heat flux was always the smaller component, comprising <8%. Daily mean value of ET was higher during the 1998 fruiting cycle than that observed in 1999. Inversely, the ET increased approximately 6% from the 1998 to 1999 fruiting cycle. These results may be used for planning and management of irrigation for mangos grown in similar environmental conditions.

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Grant, R.F., Kimball, B.A., et al. 2001, 'Modeling Interactions among Carbon Dioxide, Nitrogen, and Climate on Energy Exchange of Wheat in a Free Air Carbon Dioxide Experiment', Agronomy Journal, vol. 93, pp. 638-649.

Changes in mass and energy exchange by crops under rising atmospheric CO₂ concentration (C_a) may be affected by N and weather; C_a interacts with weather on mass and energy exchange through limitations on latent heat flux imposed by stomatal conductance, which is affected by C_a, and aerodynamic conductance, which is affected by weather. We examined the bases for these interactions with the ecosystem model ecosys. Simulation results were tested with energy flux data from a Free Air CO₂ Enrichment (FACE) experiment in which wheat (Triticum aestivum L.) was grown under 548 vs. 363 μmol mol^-1 C_a and fertilized with 7 vs. 35 g N m^-2. Both model and experimental results indicated that raising C_a from 363 to 548 μmol mol^-1 reduced midday latent heat fluxes by ca. 50 W m^-2 for wheat fertilized with 35 g N m^-2, and by ca. 100 W m^-2 for wheat fertilized with only 7 g N m^-2 when N deficits developed later in the growing season. These reductions were smaller under low wind speeds (<5 km h^-1) and stable boundary conditions when aerodynamic conductance became the dominant constraint to transpiration. At a seasonal time scale, raising C_a from 363 to 548 μmol mol^-1 reduced simulated (measured) evapotranspiration of wheat by 9% (7%) when fertilized with 35 g N m^-2, and by 16% (19%) with 7 g N m^-2. Changes with C_a in mass and energy exchange used in climate change studies should therefore reflect the site-specific availability of N, as well as climate attributes such as wind speed.

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Grant, R.F., Kimball, B.A., et al. 2004, 'Modeling Elevated Carbon Dioxide Effects on Water Relations, Water Use, and Growth of Irrigated Sorghum', Agronomy Journal, vol. 96, pp. 1693-1705.

Elevated concentrations of atmospheric CO₂ (C_a) are believed to raise sorghum [Sorghum bicolor (L.) Moench] productivity by improving water relations. In ecosys, water relations are simulated by solving for the canopy water potential (Ψ_C) at which water uptake from a model of soil-root-canopy water transfer equilibrates with transpiration from the canopy energy balance. Simulated water relations were tested with Ψ_C, water uptake, and energy exchange measured under ambient (363 μmol mol^-1) and elevated (566 μmol mol^-1) C_a and high vs. low irrigation in a free air CO₂ enrichment experiment during 1998 and 1999. Model results, corroborated by field measurements, showed that elevated C_a raised Ψ_C and lowered latent heat fluxes under high irrigation and delayed water stress under low irrigation. Changes in Ψ_C modeled under ambient vs. elevated C_a varied diurnally, with lower Ψ_C causing earlier midafternoon stomatal closure under ambient C_a. Modeled changes in sorghum water status caused elevated C_a to raise seasonal water efficiency under high and low irrigation by 20 and 26% (vs. 20 and 13% measured) in 1998 and by 9 and 27% (vs. 6 and 26% measured) in 1999. Ecosys was used to generate an irrigation response function for sorghum yield, which indicated that yields would rise by ≈13% for a range of irrigation rates if air temperatures were to rise by 3°C and C_a by 50%. Current high sorghum yields could be achieved with ≈120 mm or ≈20% less irrigation water if these rises in temperature and C_a were to occur.

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Ibáñez, M. and Castellví, F. 2000, 'Simplifying Daily Evapotranspiration Estimates over Short Full-Canopy Crops', Agronomy Journal, vol. 92, pp. 628-632.

A knowledge of daily evapotranspiration (λE) from surface radiative temperature at a regional scale is of interest in agronomy. The aerodynamic formulation and the radiative Bowen ratio-energy balance method provide good estimates of instantaneous λE based on remote-sensed canopy temperatures. Instantaneous latent heat flux estimates for the daytime period can be integrated to obtain a value for daily λE when detailed crop knowledge and continuous meteorological measurements are available. We propose a different method for estimating daily evapotranspiration for short, unstressed crops with a leaf area index (LAI) > 3. The method is based on the radiative Bowen ratio-energy balance method and on the similarity between the diurnal course of λE and solar irradiance. This regression-based approach uses continuous measurements of air vapor pressure, air temperature, surface radiative temperature, and solar irradiance during daylight hours. Moreover, detailed crop knowledge is not required. The method was tested in a Mediterranean area with semicontinental climatic characteristics on grass (Festuca arundinacea Schreb.), wheat (Triticum aestivum L.), and alfalfa (Medicago sativa L.). Daily λE was estimated with an error <15% compared with estimates of lE made using a Bowen ratio-energy balance equipment. This new approach is promising for remote sensing applications at a regional scale, although it requires further verification for other climatological conditions.

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Jalali-Farahani, H.R., Slack, D.C., et al. 1994, 'Evaluation of Resistances for Bermudagrass Turf Crop Water Stress Index Models', Agronomy Journal, vol. 86, pp. 574-581.

Evaluation of the Penman-Monteith-based crop water stress index (CWSI), as a potential indicator of turf irrigation timing, requires reliable estimates of potential (minimum) canopy resistance, r_cp, and aerodynamic resistance, r_a. This paper compares various methods of estimating r_cp and r_a for evaluation of water stress of bermudagrass turfgrass (Cynodon dactylon cv. Midiron) in the desert Southwest of the USA. First, two empirical CWSI models were developed using field data collected from well-watered and severely water-stressed turf plots during the 1986 growing season in Tucson, AZ. Second, the regression constants in the empirical lower limits of CWSI (canopy temperature minus air temperature, T_c - T_a, of nonwater-stressed turf) and the parameters associated with similar terms (net radiation and vapor pressure deficit) in the Penman-Monteith equation were equated, thus solving for r_cp and r_a. This yielded mean r_cp and r_a values of 79 and 13 s m^-1, respectively. Using these estimates, the predicted lower limits of T_c - T_a from the Penman-Monteith equation were in good agreement with values measured over well-watered turf. However, the predicted T_c - T_a for severely water-stressed turf agreed very poorly with the measured values. The use of the log-law equation yielded independent estimates of r_a generally greater than 45 s m^-1 and resulted in erroneous estimates of T_c - T_a by the Penman-Monteith equation. Using a crop-based method and data from the turf plots, estimates of the resistances were made in which constant r_cp and r_a values of 62.5 and 20 s m^-1, respectively, were found. The crop-based method was found to be not only simple to use, but also performed satisfactorily in predicting T_c - T_a. A well-defined curvilinear relationship between estimates of midday canopy resistance (r_c) and CWSI was determined for a wide range of turf water stress, implying that these two estimates nearly equally reflect the state of turf water stress. Our findings also suggest threshold midday CWSI and rc values for short bermudagrass turf of 0.16 and 125 s m^-1, respectively, to indicate the need for irrigation.

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Kongoli, C.E. and Bland, W.L. 2002, 'Influence of Manure Application on Surface Energy and Snow Cover: Field Experiments', Journal of Environmental Quality, vol. 31, pp. 1166-1173.

Application of manure to frozen and/or snow-covered soils of high-latitude, continental climate regions is associated with enhanced P losses to surface water bodies, but the practice is an essential part of most animal farming systems in these regions. Field experiments of the fates of winter-applied manure P are so difficult as to make them essentially impractical, so a mechanistic, modeling approach is required. Central to a mechanistic understanding of manure P snowmelt runoff is knowledge of snowpack disappearance (ablation) as affected by manure application. The objective of this study was to learn how solid manure applied to snow-covered fields modulates the surface energy balance and thereby snow cover ablation. Manure landspreading experiments were conducted in Arlington, WI during the winters of 1998 and 1999. Solid dairy manure was applied on top of snow at a rate of 70 Mg ha^-1 in 1998, and at 45 and 100 Mg ha^-1 in 1999. Results showed that the manure retarded melt, in proportion to the rate applied. The low-albedo manure increased absorption of shortwave radiation compared with snow, but this extra energy was lost in longwave radiation and turbulent flux of sensible and latent heat. These losses result in significant attenuation of melt peaks, retarding snowmelt. Lower snowmelt rates beneath manure may allow more infiltration of meltwater compared with bare snow. This infiltration and attenuated snowmelt runoff may partially mitigate the enhanced likelihood of P runoff from unincorporated winter-spread manure.

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Qiu, G.Y., Ben-Asher, J., et al. 1999, 'Estimation of Soil Evaporation Using the Differential Temperature Method', Soil Science Society of America Journal, vol. 63, pp. 1608-1614.

Evaporation of soil water is a major water balance component during early growth stages of irrigated field crops, row crops with incomplete cover, and in soils with high water table. Quantification of soil evaporation can help in environmental and irrigation management. The objective is to develop a new method to estimate daily soil evaporation using differential measurements of temperature. A major advantage of this approach is that measurements of sensible heat flux can be replaced by those of surface temperature. An empirical coefficient was determined from integrated energy fluxes over daytime hours and mean daytime temperatures. It leads to a new soil evaporation transfer coefficient that can replace the aerodynamic resistance to calculate sensible heat flux. Experiments were conducted in a field with sandy soil that was irrigated with sprinklers and included a weighing lysimeter to measure actual evaporation. Measurements were of net radiation, soil heat flux over a wet and a reference air-dried soil, and air and surface temperatures of both soils. Regression between modeled and actual evaporation on a daily basis produced a slope of 1.05 and r² = 0.9. Thirty days of cumulative model evaporation exceeded the measurements by 5%. The proposed coefficient can theoretically vary from 0 for wet soil to 1 for dry soil and can thus provide limits between 0 and potential evaporation. Actually, the coefficient increased from 0.2 to 0.8 in wet soil and from 0.8 to 1.0 in dry soil. The soil evaporation transfer coefficient was easy to measure, and it was sufficiently stable to adequately estimate soil evaporation.

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Schwartz, R.C., Baumhardt, R.L., et al. 2008, 'Estimation of Soil Water Balance Components Using an Iterative Procedure', Vadose Zone Journal, vol. 7, pp. 115-123.

Quantifying the hydrologic balance at high temporal resolution is necessary to evaluate field-scale management effects on soil water storage. Our objective was to develop and evaluate a hybrid procedure to estimate drainage, infiltration, and evaporation based on changes in plot-scale soil water storage on a Pullman clay loam. Soil water contents were monitored in 2005 at 0.5-h intervals on 12 plots instrumented with time-domain reflectometry probes at 0.05, 0.1, 0.15, 0.2, and 0.3 m depths, and weekly using a neutron moisture meter to a depth of 2.3 m in 0.2-m increments. During periods in August 2005 when either a plane of zero flux existed or when a wetting front penetrated into an upper soil layer at ˜0.24 m, changes in soil water storage were used to iteratively fit hydraulic parameters to estimate soil water fluxes into and out of the control volume. Predicted hydraulic conductivities were not significantly different (p = 0.471) from hydraulic conductivities calculated using the iterative method during three other months in 2005 and yielded drainage rates that differed by less than 0.05 mm d^-1 as compared to calculated changes in storage below the plane of zero flux. By considering the delayed response of water content measurements to precipitation inputs, cumulative infiltration and evaporation throughout a month with 103-mm precipitation could be estimated from the measured changes in soil water storage with expected uncertainties of ± 5 mm. The proposed procedure permits the indirect estimation of soil water balance components useful for comparing plot-scale treatments and overcomes some of the difficulties associated with weighing lysimeter and meterological approaches.

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Tolk, J.A., Howell, T.A., et al. 2006, 'Nighttime Evapotranspiration from Alfalfa and Cotton in a Semiarid Climate', Agronomy Journal, vol. 98, pp. 730-736.

Nighttime evapotranspiration (ET_N) has typically been neglected in estimating water loss from land surfaces. Our objective was to quantify the contribution of ET_N to daily (24-h) ET (ET₂₄) of irrigated and dryland cotton (Gossypium hirsutum L.) and irrigated alfalfa (Medicago sativa L.) grown in a semiarid climate. The results were then examined using a Penman-Monteith ET model which separates control of ET into its radiation (equilibrium) and atmospheric demand (imposed) components. Nighttime ET was measured at Bushland, TX using weighing lysimeters containing monolithic soil cores of Pullman clay loam (fine, mixed, superactive thermic Torrertic Paleustoll) for alfalfa in 1998 and cotton in 2001. Measured ratios of ET_N to ET₂₄ ranged from an average of 3% for a dryland cotton crop to 7.2% for irrigated alfalfa over a season. In the largest events, ET_N was as much as 12% of ET₂₄ with single nighttime losses approaching 2 mm. Model calculations showed that virtually all ET_N was the result of imposed atmospheric conditions, primarily vapor pressure deficit. However, ET_N was also related to sensible heat transfer to the canopy. Nighttime ET can be an important part of total ET of irrigated crops in a semiarid environment.

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