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• Soil albedos were reduced by 50 % during rain events. • The roughness length for momentum,(z0) over bare ground was 0.02 m. • The monsoon rains (July - Sept.) were 62 % of the total annual rainfall. • The roughness length increased to 0.12 m after 3 years. • NDVI was a key indicator to assess vegetation regrowth. Just before the installation of a long-term energy/carbon flux tower site at the 10,000-acre Appleton-Whittle Research Ranch in southern Arizona, a wildfire burned nearly 90 % of this semi-arid grassland area in the region and there was little vegetation remaining on the surface. Wildfires in this region occur roughly every 7 - 15 years. The establishment of the site allowed for the determination of recovery time for this semi-arid grassland ecosystem. We examined how the wildfire altered the land surface characteristics such as broadband and photosynthetically active radiation (PAR) albedo, surface roughness, and the seasonal net ecosystem exchange. This information was also used to determine the amount of time it took this ecosystem to recover back to a quasi-baseline state. We found that it took approximately 3 years for the surface roughness to increase from 0.04 to 0.12 m, and then stay near that level in the following years. The monsoon season (July - September) net ecosystem exchange (NEE) was positive (source) for two years following the wildfire and did not become a significant net carbon sink until 2005. The ecosystem recovery from this wildfire event may have occurred sooner but the accumulated rainfall for the 2 years following the fire were both well below the 20-year average of 254 mm. We were also able to determine that immediately after the wildfire, the soil shortwave and PAR albedo was 0.20 and 0.13, respectively. However, with the onset of the monsoon and rainfall, both the shortwave and PAR albedos decreased to 0.10 and 0.07, respectively. The albedos reverted to the original values after several days of no additional rainfall. [ABSTRACT FROM AUTHOR]

Abstract: The seasonal and interannual variability in surface energy exchange and evapotranspiration (E) of two temperate semi-arid grasslands in southeastern Arizona, USA, were investigated using multi-year (2004–2007) eddy covariance measurements. The study sites, a post-fire site (AG) and an unburned site (KG), received 43–87% of the annual precipitation (P) during the North American Monsoon season (July–September), with the lowest values in the drought years of 2004 and 2005. Irrespective of the differences in temperature, surface albedo, vegetation cover and soil characteristics both sites responded similarly to changes in environmental conditions. The seasonal and interannual variations in the partitioning of net radiation to turbulent fluxes were mainly controlled by P through the changes in soil water content (θ) and vegetation growth. Drastic changes in albedo, vegetation growth and energy fluxes occurred following the onset of the monsoon season in July. During the dry or cool periods of autumn, winter and spring, the sensible heat flux was the largest component of the energy balance, whereas latent heat flux dominated during the warm and wet periods of summer. The dry-foliage Priestley–Taylor coefficient (α) declined when θ in the 0–15cm soil layer dropped below 0.08m3 m−3 at AG, and 0.09m3 m−3 at KG, respectively. The July–September average of dry-foliage surface conductance, α and E, reached their lowest values in 2004 at AG and in 2005 at KG. During July–September, monthly E was linearly correlated to the monthly mean θ and the broadband normalized difference vegetation index (NDVI), whereas during May–June the relationship between NDVI and E was not significant. Annual E varied from 264 to 322mm at AG and from 196 to 284mm at KG with the lowest value during the severe drought year at the site. July–September E had positive correlation with total P, the mean NDVI and the number of growing season days during that period. Annual P explained more than 80% of the variance in annual E. [Copyright &y& Elsevier]

Discrepancies in closure of the surface energy balance is often an issue for many land surface types. The role of canopy storage terms from canopy water content and photosynthesis is usually neglected in the surface energy balance of crops. Data from a research flux tower in central Illinois were used to evaluate these storage terms and their impact on the closure of the surface energy balance. When considered separately, the storage terms are generally a small fraction (<5%) of the net radiation. However, the combination of soil and canopy heat storage and the stored energy in the carbohydrate bonds from photosynthesis are shown to comprise roughly 15% of the total net radiation for maize and 7% for soybean during the morning hours from 06:00 to 12:00 h when the canopy is fully developed. When all of the storage terms were considered, the slopes of the 1:1 line between net radiation and the partitioned fluxes (latent, sensible, ground, and storage) increased by 10% and the scatter about the 1:1 line decreased for both maize and soybean with the r2 increasing by 0.05. [Copyright &y& Elsevier]

• HOckey-Stick Transition hypothesis evaluated using canopy turbulence measurements. • Nonlinear relationship between turbulence and wind speed within forest canopy. • Strongly linear relationship between turbulence and wind speed at canopy top. • HOckey-Stick Transition hypothesis most applicable under stable atmospheric regimes. Turbulence governs many atmospheric processes including mixing, transport, and energy transfer. Consequently, there is a strong need for the examination and validation of existing turbulence theories. The HOckey-Stick Transition (HOST) hypothesis was proposed to challenge traditional understanding of near-surface turbulence processes derived from Monin-Obukhov Similarity Theory (MOST). Within the MOST framework, the momentum flux entirely depends upon ∂ U ¯ / ∂ z (i.e., the change in mean wind speed (U ¯) with height (z)), but this relationship is not as straightforward under HOST. Because HOST was developed using observations over relatively uniform, homogeneous terrain, questions arise regarding HOST's applicability within and above heterogeneous forest canopies where multi-level turbulence measurements are somewhat rare but are essential for developing a unified similarity scaling applicable over complex surfaces. To this end, we used one year (1 January 2016 through 31 December 2016) of turbulence measurements sampled at eight heights along a 60-m tower within and above a mixed deciduous forest at Chestnut Ridge in eastern Tennessee in the southeastern U.S. We examined the diurnal and seasonal variability of selected turbulence parameters (i.e., friction velocity (u *) and turbulence velocity scale (V T K E)) to detail the micrometeorological characteristics of the site during the study period. We then used these turbulence measurements to evaluate HOST by determining their relationship with U ¯ and to assess the dependencies of this relationship on time of day, season, wind direction, and atmospheric stability. We found that HOST is most applicable under very stable regimes, whereas the relationships between u * and U ¯ , and between V T K E and U ¯ , were more linear above the forest canopy than within the forest canopy and when the canopy was not foliated. Overall, this work builds upon previous studies that have described limitations in MOST and identifies scenarios when the HOST hypothesis may be more applicable than MOST for representing near-surface turbulence processes. [ABSTRACT FROM AUTHOR]

Abstract: Continuous measurements of carbon flux from 1997 to 2002 by eddy-covariance were used to evaluate the carbon budget for a maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) rotation agricultural ecosystem that has been in no-till cultivation for over 14 years. These measurements were used to determine the net ecosystem exchange of carbon (NEE) at the local scale and for the North Central Region of the U.S. (NCR). Results show that at the local scale, the no-till ecosystem is a carbon sink when planted with maize (576gCm−2 per year) and soybean (33gCm−2 per year). On a regional scale, the carbon sink is proportionally lower than that of the local scale. This is attributed to regional consumption of the grain (over 85% of all NCR grain is consumed elsewhere in the world) combined with the carbon emissions associated with agricultural practices. Since nearly 100% of both maize and soybean yields are consumed annually, e.g. all carbon stored in grain is consumed somewhere in the world, the long-term carbon-sequestration potential of this system is lower than revealed with the local and regional analysis. Accounting for 100% grain consumption, maize still acts as a C-sink of 184gCm−2 per year while soybean becomes a C-source of 94gCm−2 per year. As these two crops are grown in rotation, the system when accounting for all emissions to the atmosphere is a net sink of 90gCm−2 per year. Overall, however, the potential exists for long-term carbon sequestration. [Copyright &y& Elsevier]