Reduction In Net Radiation Research Articles

Effects of local aerosol and transported dust pollution on the surface energy balance over farmland in eastern China during spring

Compared with the urban boundary layer, air pollution–surface energy budget interactions in farmland areas are relatively sparse. To investigate the roles of local aerosol and transported dust pollution on the surface energy balance over farmland, a field experiment was conducted during spring 2016 in eastern China. The heavy pollution episodes during the measurement period were typically accompanied by easterly winds, low wind speeds, and high relative humidity. On polluted days, relative to clean days, both downward and upward shortwave radiation decreased by 46%, while downward and upward longwave radiation increased by 4% and 1%. The combined effects of the four radiation components resulted in a 33% reduction in net radiation on polluted days, and thus, the surface energy budget changed. Polluted days had 15% (58%) less sensible (latent) heat flux than clean days. This resulted in a higher daytime Bowen ratio on polluted days (0.83) compared to clean days (0.44). In a case analysis of dust pollution, high wind speeds and continuous high pressure induced the outbreak of the dust pollution episodes. Compared to local pollution events, the latent (sensible) heat flux was reduced more (less) than that in transported dust pollution events. Generally, this work has important implications for understanding the surface energy balance across land–atmosphere interactions over farmland regions, which are experiencing frequent aerosol pollution from local emissions and transboundary dust events.

Assessing the potential for crop albedo enhancement in reducing heatwave frequency, duration, and intensity under future climate change

Adapting to the impacts of future warming, and in particular the impacts of heatwaves, is an increasingly important challenge. One proposed strategy is land-surface radiation management via crop albedo enhancement. This has been argued to be an effective method of reducing daily hot temperature extremes regionally. However, the influence of crop albedo enhancement on heatwave events, which last three or more days, is yet to be explored and this remains an important knowledge gap. Using a fully coupled earth system model with 10 ensemble members, we show that crop albedo enhancement by up to ＋0.1 reduces the frequency of heatwave days over Europe and North America by 10 to 20 days; with a larger reduction over Europe under a future climate driven by SSP2-4.5. The average temperature anomaly during heatwaves (the magnitude of the event), is reduced by 0.8 °C to 1.2 °C where the albedo was enhanced, but reductions in mean heatwave duration are limited. There was a marked reduction in the mean annual cumulative heatwave intensity across most of Eurasia and North America, ranging from 32 °C to as high as 80 °C in parts of southern Europe. These changes were largely driven by a reduction in net radiation, decreasing the sensible heat flux, which reduces the maximum temperature, and therefore, heatwave frequency and intensity. These changes were largely localised to where the albedo enhancement was applied with no significant changes in atmospheric circulation or precipitation, which presents advantages for implementation. While our albedo perturbation of up to ＋0.1 is large and represents the likely upper limit of what is possible with more reflective crops, and we assume that more reflective crops are grown everywhere and instantly, these results provide useful guidance to policy makers and farmers on the maximum possible benefits of using more reflective crops in limiting the impacts of heatwaves under future climate.

Vertical Gradient Variations in Radiation Budget and Heat Fluxes in the Urban Boundary Layer: A Comparison Study Between Polluted and Clean Air Episodes in Beijing During Winter

AbstractPrevious studies have reported air pollution‐radiation interactions in the urban boundary layer (UBL), but vertical gradient variations in the radiation budget and heat flux under air pollution conditions are relatively sparse. In this study, based on gradient observations from the Beijing 325 m meteorological tower in December 2015, the characteristics of near‐surface radiation balance and energy budget at three levels under different pollution conditions were comparatively investigated. Relative to clean days, both downward and upward shortwave radiation (DSR and USR) dropped during daytime, while downward and upward longwave radiation (DLR and ULR) enhanced during nighttime on heavily polluted days, showing that with evaluated height, the drop magnitudes of DSR and USR decreased, while the enhancement magnitude of DLR (ULR) decreased (increased). The combined effects of four radiation components significantly induced the reduction in net radiation (Rn) on polluted days, leading to the near‐surface energy budget change. In addition, the monthly averaged anthropogenic heat flux (Qf) was estimated to quantitatively calculate the heat storage (G) term in the surface energy budget. During daytime, compared to the clean episodes, the sensible heat flux (H) was reduced more than Rn in the whole near‐surface UBL during heavy polluted episodes, resulting in smaller H/(Rn + Qf) and larger G/(Rn + Qf). Finally, we revealed that weak thermal forcing effects caused by insufficient availability of net radiation energy at the surface and weak dynamic motion associated with weak winds were both responsible for the larger reduction (increase) in H (G) during pollution episodes in the whole near‐surface UBL.

An assessment of potential climate impact during 1948–2010 using historical land use land cover change maps

AbstractEarlier studies of land use land cover change (LULCC) normally used only a specified LULCC map with no interannual variations. In this study, using an Atmospheric General Circulation Model (AGCM) coupled with a land surface model, biophysical impacts of LULCC on global and regional climate are investigated by using a LULCC map which covers 63 years from 1948 to 2010 with interannual variation. A methodology has been developed to convert a recently developed LULCC fraction map with 1° × 1° resolution to the AGCM grid points in which only one dominant type is allowed. Comprehensive evaluations are conducted to ensure consistency of the trend of the original LULCC fraction change and the trend of the fraction of grid point changes over different regions. The model was integrated with a potential vegetation map (CTL) and the map with LULCC, in which a set of surface parameters such as leaf area index, albedo and other soil and vegetation parameters were accordingly changed with interannual variation. The results indicate that the interannual LULCC map simulation is able to reproduce better interannual variability of surface temperature and rainfall when compared to the control simulation. LULCC causes negative effect on global precipitation, with the strongest significant signals over degraded regions such as East Asia, West Africa and South America, and some of these changes are consistent with observed regional anomalies for certain time periods. LULCC causes reduction in net radiation and evapotranspiration which leads to changes in monsoon circulation and variation in magnitude and pattern of moisture flux convergence and subsequent reduction in precipitation. Meanwhile, LULCC enhances surface warming during the summer in the LULCC regions due to greatly reduced evapotranspiration. In contradiction to the surface, upper troposphere temperatures are cool because of less latent heat released into the upper troposphere, which leads to weaker circulation in LULCC regions.

Temporal analysis of net radiation in the metropolitan area of San Luis Potosí - México using landsat 5 and 8 satellite images

The San Luis Potosí Valley is a basin situated in the municipalities of San Luis Potosí and Soledad de Graciano Sánchez belonging to the San Luis Potosí State and which make up the metropolitan area. This state is located in the north-central zone of México; it has a territorial extension of 64,165 m2 that represents 3.12% of the national territory. It is bounded to the north by the states of Coahuila, Nuevo Leon and Tamaulipas; to the south by Guanajuato, Queretaro and Hidalgo; to the east by Veracruz and to the west by Zacatecas and Jalisco. The analysis of the radiation capture phenomenon was carried out in the San Luis Potosí Metropolitan Area (ZMSLP). The study area is located between latitudes 22° 20′ 00″ and 22° 02′ 00″ north and longitudes 101° 12′ 00″ and 100° 44′ 00″ west. Net radiation is the phenomenon in which radiation fluxes interact between atmosphere and the earth’s surface. This phenomenon leads to processes such as evapotranspiration and photosynthesis. The net radiation analysis can be performed by three methods: (a) by using radiation monitoring stations, (b) by statistical models using data from meteorological stations such as temperature and relative humidity, and (c) by using satellite imagery. Since satellite images offer the possibility of analyzing large tracts of land, this method has been chosen to carry out the study using two images per year, one for the dry season and one for the rainy season between 1990 and 2017. The results show a considerable reduction in net radiation between the study periods, possibly due to the metropolitan area growth that impacts on the environment and therefore to the processes mentioned above.

Impact of land use and land cover transitions and climate on evapotranspiration in the Lake Naivasha Basin, Kenya

The Lake Naivasha Basin in Kenya has experienced significant land use cover changes (LUCC) that has been hypothesized to have altered the hydrological regime in recent decades. While it is generally recognized that LUCC will impact evapotranspiration (ET), the precise nature of such impact is not very well understood. This paper describes how land use conversions among grassland and croplands have influenced ET in the Lake Naivasha Basin for the period 2003 to 2012. MODIS data products were used in combination with the European Centre for Medium-Range Weather Forecasts (ECMWF) data sets to model ET using the Surface Energy Balance System (SEBS). The results indicate that conversions from grassland to cropland accounted for increases in ET of up to 12% while conversion from cropland back to grasslands (abandonment) reduced ET by ~4%. This suggests that recently cultivated agricultural lands could increase local water demands, while abandonment of the farms could decrease the water loss and eventually increase the water availability. Also, recovery of ET following re-conversion from cropland to grassland might be impeded due to delayed recovery of soil properties since parts of the catchment are continuously being transformed with no ample time given for soil recovery. The annual ET over the 10 years shows an estimated decline from 724 mm to 650 mm (~10%). This decline is largely explained by a reduction in net radiation, an increase in actual vapour pressure whose net effect also led to decrease in the surface-air temperature difference. These findings suggest that in order to better understand LUCC effects on water resources of Lake Naivasha, it is important to take into account the effect of LUCC and climate because large scale changes of vegetation type from grassland to cropland substantially will increase evapotranspiration with implications on the water balance.

Aerosol-induced perturbation of surface fluxes over different landscapes in a tropical region

ABSTRACTGlobal picture of modification of land surface fluxes due to aerosol loading, especially over different landscapes, is not yet clear. The impact of aerosols on surface fluxes over Indo-Gangetic Basin, a highly polluted region, is studied by selecting four stations in the north Indian plains that are agriculturally heterogeneous in nature, viz., short crop area, industrial area, seasonal agricultural area, and permanent dense canopy area. Data from satellite (Moderate Resolution Imaging Spectroradiometer (MODIS)), model (Global Land Data Assimilation (GLDAS-NOAH), National Centre for Environmental Prediction/Oregon State University/Airforce/Hydrology Research Laboratory, and reanalysis (MERRA-Modern-Era Retrospective analysis for Research and Applications) as well as ground observations (AERONET (AErosol RObotic NETwork) and Dibrugarh University measurements) on daily and monthly scales are used to validate and assess the aerosol-induced modification of surface fluxes. To ensure that satellite products are reasonably accurate, aerosol optical depth (AOD) and evapotranspiration (ET) from MODIS are validated against ground truth of AOD and ET from GLDAS-NOAH and found to have correlations of 0.90 and 0.76, respectively. Aerosol radiative forcing (ARF) was calculated as the difference between ‘clear sky’ and ‘clean clear sky’ of net radiation (R) at surface obtained from MERRA. Since surface fluxes are perturbed directly by ARF on a daily scale with maximum effect during ‘sunlit’ hours, change in fluxes is calculated on the daily scale. The impact of aerosols alone on latent heat flux (LE) is quantified by using the linear relation between R and LE along with the aerosol-induced reduction of net radiation. Latent heat flux and R from GLDAS-NOAH showed an excellent correlation between 0.75 and 0.97. The impact of aerosols on surface fluxes, represented as perturbed latent and sensible heat fluxes normalized by ARF, is found to be followed an asymptotic curve against Bowen ratio (B) which indicates aridity of the land surface. This study shows that the aerosols reduce LE by 60% of ARF over the permanent dense canopy with considerable soil moisture (B = 0.5), while the same amount of ARF reduces LE by 25% only over the semi-arid region (B = 2) over the north Indian plains for the period from October 2008 to May 2009.

The mark of vegetation change on Earth\u2019s surface energy balance

Changing vegetation cover alters the radiative and non-radiative properties of the surface. The result of competing biophysical processes on Earth’s surface energy balance varies spatially and seasonally, and can lead to warming or cooling depending on the specific vegetation change and background climate. Here we provide the first data-driven assessment of the potential effect on the full surface energy balance of multiple vegetation transitions at global scale. For this purpose we developed a novel methodology that is optimized to disentangle the effect of mixed vegetation cover on the surface climate. We show that perturbations in the surface energy balance generated by vegetation change from 2000 to 2015 have led to an average increase of 0.23 ± 0.03 °C in local surface temperature where those vegetation changes occurred. Vegetation transitions behind this warming effect mainly relate to agricultural expansion in the tropics, where surface brightening and consequent reduction of net radiation does not counter-balance the increase in temperature associated with reduction in transpiration. This assessment will help the evaluation of land-based climate change mitigation plans.

Variation of actual evapotranspiration and its impact on regional water resources in the Upper Reaches of the Yangtze River

Changes on water resources volume of 1960–2007 in the Upper Reaches of the Yangtze River are discussed in this paper. A time series (1960–2007) and its spatial patterns of actual evapotranspiration (ET a) estimation are analyzed based on Advection-Aridity model (A–A model) with selected parameters for 72 meteorological stations in the upper reaches of the Yangtze River Basin. The result indicates that average ET a of the basin changed in stages: slight increases in the first decade; a drop followed by fluctuations and lower increases in the next ten years; and then a dramatic decline in the latest decade, with three abrupt change years detected in the early 1970s, 1990s, and 2000s. ET a increases from mountainous northwest areas to plains in eastern areas, with significant decreases in plain regions and increases in the mountain region. ET a in the Sichuan Basin is found to decrease at the most rapid rate, and ET a in the sub-area to the south of the upland mountain region is increasing significantly. Such patterns of spatial trend in ET a result in reductions in regional variation, both annually and in all seasons. The change in ET a of this area is believed to be related to local water resource changes, and is impacted by the changes in climate factors. Temperature of this area has increased, and sunshine duration hours have shortened, leading to a reduction in net radiation. Relative humidity also has decreased. Comprehensively, reduction in both energy and moisture supply for evapotranspiration result in an increasing ET a. Moreover, a sharp drop of ET a occurred in the early 2000s, and meteorological factors including wind speed also show abrupt change to different extents. The impact of ET a on regional water balance at both inter-annual and intra-annual scales is also discussed in this study.

A Case Study of the Impacts of Dust Aerosols on Surface Atmospheric Variables and Energy Budgets in a Semi-Arid Region of China

The authors present a case study investigating the impacts of dust aerosols on surface atmospheric variables and energy budgets in a semi-arid region of China. Enhanced observational meteorological data, radiative fluxes, near-surface heat fluxes, and concentrations of dust aerosols were collected from Tongyu station, one of the reference sites of the International Coordinated Energy and Water Cycle Observations Project (CEOP), during a typical dust storm event in June 2006. A comprehensive analysis of these data show that in this semi-arid area, higher wind velocities and a continuously reduced air pressure were identified during the dust storm period. Dust storm events are usually associated with low relative humidity weather conditions, which result in low latent heat flux values. Dust aerosols suspended in the air decrease the net radiation, mainly by reducing the direct solar radiation reaching the land surface. This reduction in net radiation results in a decrease in soil temperatures at a depth of 2 cm. The combination of increased air temperature and decreased soil temperature strengthens the energy exchange of the atmosphere-earth system, increasing the surface sensible heat flux. After the dust storm event, the atmosphere was dominated by higher pressures and was relatively wet and cold. Net radiation and latent heat flux show an evident increase, while the surface sensible heat flux shows a clear decrease.

Plant canopy effects on soil thermal and hydrological properties and soil respiration

A soil–plant–air continuum multilayer model was developed to simulate soil respiration. The model can be used to investigate the effects of three canopy characteristics on soil thermal and hydrological properties and soil respiration: leaf area index, the maximum rate of carboxylation, and the vertical profile of leaf area density. The model did not consider physiological effects of canopy characteristics, such as the effect of photosynthesis on root respiration. Rather, it examined how changes in temperature and moisture in the soil layers, caused by changes in the plant canopy, affect soil respiration. Model validation and numerical experiments were performed using above-canopy hydrometeorological variables, including seasonal changes related to a dry season with strong evaporative demand (high solar radiation and high temperature) and a rainy season with weaker evaporative demand. The results suggest that the model can successfully reproduce seasonal changes in soil profiles of moisture, temperature, CO 2 gas concentration, and respiration, and that canopy characteristics can limit soil respiration because of thermal and hydrological effects. An increase in leaf area decreased soil respiration in two ways: by decreasing soil temperature through a reduction in net radiation to the soil surface and by decreasing soil moisture through an increase in canopy transpiration. An increase in the maximum rate of carboxylation decreased soil respiration because it decreased soil moisture by increasing canopy transpiration; there was little thermal effect on soil respiration. A canopy with a denser leaf area in the upper layers created a faster wind velocity in the lower portion and on the ground than did a canopy with the same leaf area index, but with a denser leaf area in the lower layers. The faster wind velocity significantly decreased soil moisture at the soil surface and decreased soil temperature through an increase in soil evaporation; these effects decreased soil respiration. The hydrological effects of the three parameters were most evident in the dry season, while the thermal effects were evident year-round.

The role of clouds in the surface energy balance over the Amazon forest

Deforestation in the Amazon region will initially impact the energy balance at the land surface through changes in land cover and surface hydrology. However, continuation of this human activity will eventually lead to atmospheric feedbacks, including changes in cloudiness which may play an important role in the final equilibrium of solar and terrestrial radiation at the surface. In this study, the different components of surface radiation over an undisturbed forest in the Amazon region are computed using data from the Amazon region micrometerological experiment (ARME). Several measures of cloudiness are defined: two estimated from the terrestrial radiation measurements, and one from the solar radiation measurements. The sensitivity of the surface fluxes of solar and terrestrial radiation to natural variability in cloudiness is investigated to infer the potential role of the cloudiness feedback in the surface energy balance. The results of this analysis indicate that a 1% decrease in cloudiness would increase net solar radiation by ca. 1.6 W/m2. However, the overall magnitude of this feedback, due to total deforestation of the Amazon forest, is likely to be of the same order as the magnitude of the decrease in net solar radiation due to the observed increase in surface albedo following deforestation. Hence, the total change in net solar radiation is likely to have a negligible magnitude. In contrast to this conclusion, we find that terrestrial radiation is likely to be more strongly affected; reduced cloudiness will decrease net terrestrial radiation; a 1% decrease in cloudiness induces a reduction in net terrestrial radiation of ca. 0.7 W/m2; this process augments the similar effects of the predicted warming and drying in the boundary layer. Due to the cloudiness feedback, the most significant effect of large-scale deforestation on the surface energy balance is likely to be in the modification of the terrestrial radiation field rather than the classical albedo effect on solar radiation fields. The net effect of clouds is to reduce net radiation; a 1% increase in cloudiness induces a reduction in net radiation of ca. 1 W/m2. The implications of this negative feedback on large-scale land-atmosphere interactions over rainforests are discussed. © 1998 Royal Meteorological Society

The role of clouds in the surface energy balance over the Amazon forest

Deforestation in the Amazon region will initially impact the energy balance at the land surface through changes in land cover and surface hydrology. However, continuation of this human activity will eventually lead to atmospheric feedbacks, including changes in cloudiness which may play an important role in the final equilibrium of solar and terrestrial radiation at the surface. In this study, the different components of surface radiation over an undisturbed forest in the Amazon region are computed using data from the Amazon region micrometerological experiment (ARME). Several measures of cloudiness are defined: two estimated from the terrestrial radiation measurements, and one from the solar radiation measurements. The sensitivity of the surface fluxes of solar and terrestrial radiation to natural variability in cloudiness is investigated to infer the potential role of the cloudiness feedback in the surface energy balance. The results of this analysis indicate that a 1% decrease in cloudiness would increase net solar radiation by ca. 1.6 W/m2. However, the overall magnitude of this feedback, due to total deforestation of the Amazon forest, is likely to be of the same order as the magnitude of the decrease in net solar radiation due to the observed increase in surface albedo following deforestation. Hence, the total change in net solar radiation is likely to have a negligible magnitude. In contrast to this conclusion, we find that terrestrial radiation is likely to be more strongly affected; reduced cloudiness will decrease net terrestrial radiation; a 1% decrease in cloudiness induces a reduction in net terrestrial radiation of ca. 0.7 W/m2; this process augments the similar effects of the predicted warming and drying in the boundary layer. Due to the cloudiness feedback, the most significant effect of large-scale deforestation on the surface energy balance is likely to be in the modification of the terrestrial radiation field rather than the classical albedo effect on solar radiation fields. The net effect of clouds is to reduce net radiation; a 1% increase in cloudiness induces a reduction in net radiation of ca. 1 W/m2. The implications of this negative feedback on large-scale land-atmosphere interactions over rainforests are discussed. © 1998 Royal Meteorological Society

Study of the sensitivity of a regional model in response to land cover change over northern China

Changing land surface types may have important consequences for the climate system. Predicting even the local, immediate effects of changing land surface types on the local energy and water balance has been difficult, because the land surface parameterization schemes used previously in climate models have been inadequate and the resolution of GCMs (global circulation models) has been too coarse to describe mesoscale forcings such as vegetation gradient adequately and to yield accurate regional climate detail. Grassland is a major ecosystem in northern China, playing a crucial role in the surface energy and water budget. Inclusion of a biosphere and atmosphere transfer scheme (BATS) into the National Centre for Atmospheric Research (NCAR) regional climate model (RegCM) permits an exploratory study of the possible effects of land cover change. Based on the observation of desertification processes in this area, an experiment with seven-day integration was designed in which all of the grassland in northern China is replaced by desert. The results show that, with the conversion of grassland into the desert, the surface hydrological cycle over northern China is weakened, with less precipitation and, in particular, a reduction in evaporation; the surface heat budget also changes, especially with the reduction in latent heat flux being much more than the reduction of net radiation, which results in an increase in surface temperature. These results are relevant to changes of parameters in BATS owing to changing land surface. Some detailed patterns of the effects of changing land surface on local energy and water balance have been simulated. Copyright © 1998 John Wiley & Sons, Ltd.

Response of citrus trees to modified radiation regime in semi-arid conditions

Citrus trees are characterized by a large canopy and low hydraulic conductivity. In Israel's semi-arid summer climate this could cause transpiration to exceed water uptake and cause temporary excessive water deficits. It was hypothesized that reduction of radiative load would reduce transpiration and thus reduce deficits. Net radiation of lemon trees in the hottest season was reduced by shading hedgerows with reflective nets for approximately one month in both 1994 and 1995. Stem sap flow and climate variables were measured continuously. Daily courses of leaf conductance and leaf water potentials were measured on selected days. Midday net radiation below the dense and sparse shade net treatments was 47% and 73% of that above the control trees. Midday 'sunlit' leaf temperatures below the nets were reduced by 2.7 and 1.6°C, respectively. The reduction in net radiation caused large changes in leaf conductance. Average midday sunlit leaf conductance measured in 1995 under the dense and sparse treatments and control were 4.1, 2.9 and 1.8 mm s -1 , respectively (significantly different at P <0.01). Similar differences in sunlit leaf conductance were found in 1994. Shade leaf conductance was not affected by the treatments. Daily total and midday sap flow under the dense net were reduced by 6-7% and 10-11%, respectively. Sap flow under the sparse net did not change significantly in 1994, but in 1995 daily and midday sap flows were reduced by 6% and 7%, respectively. Midday leaf water potentials increased by 0.2 and 0.1 MPa under dense shade in 1994 and 1995, respectively. Under sparse shade midday leaf water potentials increased by 0.1 MPa in 1994, but did not change significantly in 1995. A modified Penman-Monteith model evaluated transpiration if leaf conductance were constant in the different radiation environments. At leaf conductance levels found in the unshaded trees, dense shade was estimated to cause a 25% reduction in transpiration, while leaf conductance values found in trees under the dense shade would lead to an increase in transpiration of more than 35% in unshaded trees. The ability of the tree to maintain almost constant transpiration in different radiation environments and thus avoid water deficit by adjusting the conductance of sunlit leaves is discussed in terms of environmental influences and significance to the plant's water balance.

Energy Relationships in the Design of Floating Covers for Evaporation Reduction

A theoretical analysis of the energy balance equation for a partially covered body of water indicates that surface reflectance for solar radiation and infrared emittance are important properties to consider in designing covers for maximum efficiency in reducing evaporation. White materials satisfy the requirement that both these parameters be as large as possible. Experiments were conducted on insulated evaporation tanks partially covered with foamed wax, lightweight concrete, white butyl rubber, and styrofoam. Several shapes and sizes were tested. The radiative properties of the cover were again noted to be important, and thin covers proved to be slightly more efficient than thick insulated covers. Evaporation reduction was proportional to the percent of surface area covered, the constant of proportionality depending upon the color and type of materials used. For the white, impermeable materials tested, the constant of proportionality was near unity. The reduction in evaporation was also highly correlated with the reduction in net radiation.

Microclimate modification by manipulation of net radiation

An argument assuming the shade to be flush with the canopy and based on the alteration of albedo, predicts the reduction in net radiation with increasing shade, both above and below the shade, to be a linear function of the horizontal area shaded. Observations of the effect of a louvered shade on net radiation by traversing net radiometers beneath the shade, on evaporation by atmometer sampling, on evapotranspiration by a neutron scattering technique and on leaf temperature extremes by thermocouples, were made over and within a soybean canopy. The reduction in net radiation was observed to be directly proportional to the horizontal area shaded. For the case in which two-thirds of the area was shaded, the reduction in atmometer evaporation was over three times as large as the reduction in evapotranspiration and the moderation of average maximum and minimum leaf temperatures was on the order of 5°F. Graphical representations of the variations of net radiation with time for the geometrically repeated patterns beneath the louvers suggest a modification of stomatal activity which might explain the great difference between the effect of shade on evaporation and evapotranspiration.

The effect of albedo on the evapotranspiration rate

The problem was to evaluate the possibility of reducing the evapotranspiration of an irrigated field by increasing its reflectivity to incoming radiation. A numerical computation approach was used to solve the differential equations governing the transfer phenomena in a horizontally homogeneous atmospheric boundary layer and underlying soil. A few preliminary tests showed that although wind velocities were not accurately predicted, the effect of such inaccuracies on the energy budget results was not significant. Comparisons of calculated results for fields with natural albedo (0.25) and fields with albedo artificially increased to 0.40, indicated the following: 1. ( 1) Under a clear sky, the relative reduction in net radiation due to the treatment, was about 30%, as compared with only 20% reduction in the solar radiation absorbed at the surface. 2. ( 2) The major factors affecting the saving of water were the albedo and the wetness of the surface. Wind, surface roughness and thermal stratification were of only secondary importance. 3. ( 3) The savings in water due to the treatment were calculated to be roughly 30% for all wetness conditions.

Thermal Energy Exchange of a Birch Tree and a Spruce Tree at Night

Measurements of thermal radiation from a birch tree (Betula papyrifera) and a spruce tree (Picea glauca) were made on four clear summer nights at five distances from the base of each tree. Slightly higher net radiation values were observed under the spruce crown. A rapid reduction in net radiation was observed on the periphery of the crowns. Ecological relationships dependent on the distribution of infrared radiation are discussed.