Reference and Crop Evapotranspiration in South Central Nebraska. II: Measurement and Estimation of Actual Evapotranspiration for Corn

In planning, designing, and managing of surface and groundwater supply, it is essential to accurately quantify actual evapotranspiration (ETc) from various vegetation surfaces within the water supply areas to allow water management agencies to manipulate the land use pattern alternatives and scenarios to achieve a desired balance between water supply and demand. However, significant differences among water regulatory agencies and water users exist in terms of methods used to quantify ETc. It is essential to know the potential differences associated with using various empirical equations in quantifying ETc as compared with the measurements of this critical variable. We quantified and analyzed the differences associated with using 15 grass (ETo) and alfalfa-reference (ETr) combination, temperature and radiation-based reference ET (ETref) equations in quantifying grass-reference actual ET (ETco) and alfalfa-reference actual ET (ETcr) as compared with the Bowen ratio energy balance system (BREBS)-measured ETc (ETc-BREBS) for field corn (Zea mays L.). We analyzed the performance of the equations for their full season, irrigation season, peak ET month, and seasonal cumulative ETc estimates on a daily time step for 2005 and 2006. The step-wise Kc values instead of smoothed curves were used in the ETc calculations. The seasonal ETc-BREBS was measured as 572 and 561?mm in 2005 and 2006, respectively. The root-means-quare difference (RMSD) was higher for the full season than the irrigation season and peak ET month estimates for all equations. The standardized ASCE Penman-Monteith (PM) ETco had a RMSD of 1.37?mm?d?1 for the full growing season, 1.05?mm?d?1 for the irrigation season, and 0.76?mm?d?1 for the peak month ET. The ASCE-PM, 1963 and 1948 Penman ETc estimates were closest to the ETc-BREBS. The FAO-24 radiation and the HPRCC Penman ETc estimates also agreed well with the ETc-BREBS. Most combination equations performed best during the peak ET month except the temperature and radiation-based equations. There was an excellent correlation between the ASCE-PM ETco and ETcr with a high r2 of 0.99 and a low RMSD of 0.34?mm?d?1. The difference between the ETcr and ETco was found to be larger at the high ETc range (i.e., >8?mm), but overall, the ETcr and ETco values were within 3%. Significant differences were found between the cumulative ETco-METHOD and ETcr-METHOD versus ETc-BREBS. Most combination equations, including the standardized ASCE-PM ETco and ETcr underestimated ETc-BREBS during the early periods of the growing season where the soil evaporation was the dominant energy flux of the energy balance and in the late season near and after physiological maturity when the transpiration rates were less than the midseason. The underestimations early in the season can be attributed to the lack of ability of the physical structure of the ETref×crop coefficient approach to “fully” account for the soil surface conditions when complete canopy cover is not present. The results of this study can be used as a reference tool by the water resources regulatory agencies and water users and can provide practical information on which method to select based on the data availability for reliable estimates of daily ETc for corn.     

Measurement of Evaporation on Bare Soil and Estimating Surface Resistance

A new evaporation measuring device incorporating an evaporation chamber has been developed and checked for its accuracy. This device is unique that it uses a chamber that is completely open at one end and thereby minimizes the effect of the chamber on the natural profiles of temperature, humidity, and turbulence. It was used in estimating a newly formulated surface resistance to bare soil evaporation under dry topsoil conditions. A simple energy balance model incorporating the depth of evaporating surface, blended with a new approach for describing surface resistance was developed and successfully tested with a limited set of data obtained from a simple experiment, assuming ideal conditions. In addition to the newly formulated surface resistance, the depth of the dry soil layer was also estimated and was relatively comparable with measured value. The newly formulated surface resistance was found to be insignificant compared to the resistance imposed by the dry soil layer. The total surface resistance was modeled as a power function of soil moisture in the top 0–1 cm of soil, while the newly formulated resistance did not show any relation with the soil moisture.     

Dynamics of Nocturnal, Daytime, and Sum-of-Hourly Evapotranspiration and Other Surface Energy Fluxes over Nonstressed Maize Canopy

The magnitude and driving forces of nocturnal evaporative losses, ETcnight, and the interactions of other surface energy fluxes and microclimatic variables under various climatic, soil, and management conditions are not well understood. Such relationships are important for ecophysiological studies. This research attempts to investigate such relationships. Furthermore, ETcnight can be a sizable portion of the daily total evaporative losses. Most empirical equations, especially ones that use solar or net radiation to estimate daily evapotranspiration (ET), either ignore or poorly treat the contribution of ETcnight to the daily total ET. Neglecting ETcnight can lead to errors in determining the daily or the sum-of-hourly ETc (i.e., ETcSOH) and can also cause cumulative errors when making long-term water balance analyses. In this paper, the magnitudes, trends, and contribution to the nocturnal surface energy balance of various microclimatic variables (air temperature, Ta; vapor pressure deficit, VPD; relative humidity, RH; and wind speed at 3?m, u3) and surface energy fluxes (ETcnight; soil heat flux, G; sensible heat flux, H; and net radiation, Rn); were quantified and interpreted for a nonstressed and subsurface-drip-irrigated maize canopy. The effect of microclimatic variables and surface energy flux components on the Bowen ratio energy balance system (BREBS)-measured ETcnight and daytime evaporative loss, ETcday, were investigated in the growing season of 2005 (i.e., April 22–September 30) and 2006 (May 12–September 27). The nighttime evaporative losses were high early in the season during partial canopy closure because of increased surface soil evaporation and were also high later in the season during and after leaf aging, physiological maturity, and leaf senescence. The seasonal average nighttime evaporative losses for 2005 and 2006 were 0.19 and 0.11??mm/night, respectively. Losses of 0.50?mm or more occurred in 2005 and 2006 on eight and seven nights, respectively. The seasonal total ETcnight, ETcday, and ETcSOH in 2005 were 31, 612, and 642?mm, respectively. The ETc values in 2006 were 16, 533, and 547?mm, respectively. In both years, the percent ratio of ETcday to ETcSOH usually was more than 80–85%. ETcnight was affected primarily by u3, VPD, and Ta. A strong relationship between ETcnight and nighttime sensible heat was observed. Some of the largest ratios of ETcnight to ETcSOH occurred on rainy nights with strong winds. Because of strong winds, the ETcnight was high owing to the clear coupling among all energy exchanges within and above the canopy as a result of the mixing of the lower boundary layer of the microclimate. The results of this study showed that the ETcnight can be up to 5% of the ETcSOH, even for a subsurface-drip-irrigated maize canopy in which the soil surface is usually dry, thus, less evaporative losses potential compared with the surface or sprinkler-irrigated surfaces in which ETcnight would be expected to be considerably higher because of wetter surface conditions. ETcnight needs to be quantified for different vegetation surfaces and management practices, surface wetting, and climatic conditions to better account for nighttime water losses and better understand nighttime energy balance mechanisms.     

Comparison of Measured and FAO-56 Modeled Evaporation from Bare Soil

This paper evaluates how well the FAO-56 style soil water evaporation model simulates measurements of evaporation (E) from bare soil. Seven data sets were identified from the literature and in all but one case, the individuals who took the measurements were contacted and they provided the writers with specific weather and soils data for model input. Missing weather and soils data were obtained from online sources or from the National Climatic Data Center. Simulations for three possible variations of soil data were completed and compared. The measured and the FAO-56 simulated E/ETo and cumulative evaporation trends and values were similar. Specifically, the average evaporation weighted percent difference between the measured and the simulated cumulative evaporation was between ?7.5 and ?0.5%. This evaluation suggests model accuracy of about ±15% with the use of sound weather data and a fairly generalized understanding of soil properties in the location being evaluated.     

Crushed Glass-Dredged Material (CG-DM) Blends: Role of Organic Matter Content and DM Variability on Field Compaction

Trial embankments comprised of crushed glass-dredged material (CG-DM) blends and a 100% DM embankment were constructed to provide the necessary data sets to determine if a moisture content (MC) correction was required for the nuclear density (ND) gauge, as DM may contain a high organic matter content (OC). The MCs of thin-walled tube samples of CG-DM blends collected immediately below the ND gauge were compared to the corresponding ND gauge readings. A direct correlation between the MC data pairings from the tube samples and ND gauge readings showed that the ND gauge was greater than 97% accurate for MCs up to 55% and OCs up to 10% for the CG-DM blends evaluated in this study. However, the MC determined by the ND gauge was underpredicted (not overpredicted) by approximately 2.5%, contrary to theoretical expectations. A comparison of the average MC results per embankment indicated that the ND gauge was generally within 1% of the tube sample values, again on the low side. Interestingly, the rutting of the individual embankment lifts, often used as an informal metric for compaction compliance also was found to be contrary to expectations. The (re)constructed CG-DM embankments of this study were again shown to satisfy local Department of Transportation embankment construction criteria in most cases.     

Tb0.3Dy0.7(Fe1-xAlx)1.95合金显微组织及低磁场的磁致伸缩性能研究

研究了Al添加量变化对Tb0.3Dy0.7(Fe1-xAlx)1.95合金显微组织、磁致伸缩系数的影响。结果发现,随着Al添加量的增加,晶体显微组织中的析出物增加;在一定的磁场强度下,磁致伸缩系数随Al添加量而变化,当磁场强度低于40kA/m时,磁致伸缩系数随Al添加量的变化曲线出现一峰值,当磁场强度高于40kA/m时,磁致伸缩系数随Al添加量的增加而降低。     

Responses of pigeon (Columba livia) Wulst neurons during acquisition and reversal of a visual discrimination task.

The avian visual "Wulst" is a target of the ascending thalamofugal visual pathway. In pigeons (Columba livia), lesion damage to the Wulst has little effect on simple visual discriminations, but impairs performance on tasks such as reversal learning. We recorded the responses of single Wulst neurons as pigeons were trained on the acquisition and subsequent reversal of a visual discrimination. Of the 64 units recorded, 54 (84%) displayed a significant difference in firing rate between some component of the task and the intertrial interval that separated trials. More important, 14 units (22%) displayed a significant change in firing rate exclusively to the S+ and/or S- as learning progressed either during acquisition or reversal. The responses of these 14 neurons indicate that learning during initial acquisition was as likely to correlate with a change in firing rate as during reversal, and some neuronal responses could be characterized as representing reward properties together with visual stimulus features. As such, responses of pigeon Wulst neurons indicate a role in representing aspects of learning as much as the physical/perceptual properties of visual stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)