Michael L. Roderick

The Cause of Decreased Pan Evaporation over the Past 50 Years

We measured acoustic emission energy during antigorite dehydration in a multianvil press from 1.5 to 8.5 gigapascals and 300° to 900°C. There was a strong acoustic emission signal on dehydration, and analysis of recovered samples revealed brittle deformation features associated with high pore-fluid pressures. These results demonstrate that intermediate depth (50 to 200 kilometers) seismicity can be generated by dehydration reactions in the subducting slab.

Moving beyond heterogeneity and process complexity: A new vision for watershed hydrology

Field studies in watershed hydrology continue to characterize and catalogue the enormous heterogeneity and complexity of rainfall runoff processes in more and more watersheds, in different hydroclimatic regimes, and at different scales. Nevertheless, the ability to generalize these findings to ungauged regions remains out of reach. In spite of their apparent physical basis and complexity, the current generation of detailed models is process weak. Their representations of the internal states and process dynamics are still at odds with many experimental findings. In order to make continued progress in watershed hydrology and to bring greater coherence to the science, we need to move beyond the status quo of having to explicitly characterize or prescribe landscape heterogeneity in our (highly calibrated) models and in this way reproduce process complexity and instead explore the set of organizing principles that might underlie the heterogeneity and complexity. This commentary addresses a number of related new avenues for research in watershed science, including the use of comparative analysis, classification, optimality principles, and network theory, all with the intent of defining, understanding, and predicting watershed function and enunciating important watershed functional traits.

On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation

The volume of shade within vegetation canopies is reduced by more than an order of magnitude on cloudy and/or very hazy days compared to clear sunny days because of an increase in the diffuse fraction of the solar radiance. Here we show that vegetation is directly sensitive to changes in the diffuse fraction and we conclude that the productivity and structure of vegetation is strongly influenced by clouds and other atmospheric particles. We also propose that the unexpected decline in atmospheric [CO2] which was observed following the Mt. Pinatubo eruption was in part caused by increased vegetation uptake following an anomalous enhancement of the diffuse fraction by volcanic aerosols that would have reduced the volume of shade within vegetation canopies. These results have important implications for both understanding and modelling the productivity and structure of terrestrial vegetation as well as the global carbon cycle and the climate system.

Wind speed climatology and trends for Australia, 1975–2006: Capturing the stilling phenomenon and comparison with near‐surface reanalysis output

[1] Near-surface wind speeds (u) measured by terrestrial anemometers show declines (a ‘stilling’) at a range of midlatitude sites, but two gridded u datasets (a NCEP/NCAR reanalysis output and a surface-pressure-based u model) have not reproduced the stilling observed at Australian stations. We developed Australia-wide 0.01 resolution daily u grids by interpolating measurements from an expanded anemometer network for 1975–2006. These new grids represented the magnitude and spatialvariability of observed u trends, whereas grids from reanalysis systems (NCEP/NCAR, NCEP/DOE and ERA40) essentially did not, even when minimising the sea-breeze impact. For these new grids, the Australianaveraged u trend for 1975–2006 was 0.009 m s 1 a 1 (agreeing with earlier site-based studies) with stilling over 88% of the land-surface. This new dataset can be used in numerous environmental applications, including benchmarking general circulation models to improve the representation of key parameters that govern u estimation. The methodology implemented here can be applied globally. Citation: McVicar, T. R., T. G. Van Niel, L. T. Li, M. L. Roderick, D. P. Rayner, L. Ricciardulli, and R. J. Donohue (2008), Wind speed climatology and trends for Australia, 1975 – 2006: Capturing the stilling phenomenon and comparison with near-surface reanalysis output, Geophys. Res. Lett., 35, L20403,

The use of time-integrated NOAA NDVI data and rainfall to assess landscape degradation in the arid shrubland of Western Australia

Abstract Ground-based data on total phytomass were collected in 1998 and 1999 from four sampling domains, each nearly 1000 km 2 , within the arid shrubland of Western Australia. These data were used in models relating rainfall and landscape characteristics to total phytomass to provide landscape-scale estimates of total phytomass and rainfall-use efficiency for 1992–1999 (referred to as RUE P ). These modelled estimates were compared with remotely sensed estimates of total phytomass (I-NDVI) and rainfall-use efficiency (I-NDVI/annual rainfall; referred to as RUE N ) using data from NOAA satellites. There was good agreement between ground-based and remotely sensed estimates of total phytomass but less agreement between estimates of rainfall-use efficiency. Partitioning the landscape on the basis of landscape resilience did not improve the amount of variance accounted for in RUE P or RUE N and there were conflicting relationships between landscape-scale indicators of landscape function and RUE N . There was, however, evidence to suggest that temporal change in RUE N may provide a useful broad-scale indicator of landscape degradation or recovery over decadal time intervals. Recommendations are made for broad-scale application of this indicator based on temporal trends in RUE N .

Observational evidence from two mountainous regions that near- surface wind speeds are declining more rapidly at higher elevations than lower elevations: 1960-2006

Coupling recent observed declines of terrestrial mid-latitude near-surface wind speed (u) with knowledge that high-elevation sites rapidly experience climate change led to an assessment of the regional near-surface elevation dependence of u (u(Z)) at two mountainous regions (central China and Switzerland). The monthly u(Z) were calculated from 1960-2006. In both regions u(Z) were higher in winter (similar to 2.25 m s(-1) km(-1)) compared to summer (similar to 1.25 m s(-1) km(-1)). For the first time u(Z) trends were calculated, the results were strongly seasonal, ranging from similar to-0.025 m s(-1) km(-1) a(-1) in winter to similar to-0.005 m s(-1) km(-1) a(-1) in summer. For both regions u(Z) trend results showed that u has declined more rapidly at higher than lower elevations, even though different u dynamics were observed. The u(Z) trends have important implications for climatic changes of coupled land-surface/boundary-layer processes (such as evapotranspiration) at high-elevation regions where much of the globes fresh water is derived. Citation: McVicar, T. R., T. G. Van Niel, M. L. Roderick, L. T. Li, X. G. Mo, N. E. Zimmermann, and D. R. Schmatz (2010), Observational evidence from two mountainous regions that near-surface wind speeds are declining more rapidly at higher elevations than lower elevations: 1960-2006, Geophys. Res. Lett., 37, L06402, doi:10.1029/2009GL042255.

The contribution of reduction in evaporative cooling to higher surface air temperatures during drought

This research was supported by the Australian Research Council (CE11E0098), the National Natural Science Foundation of China (91125018), and the China Scholarship Council (201306210089).

Long‐term CO2 fertilization increases vegetation productivity and has little effect on hydrological partitioning in tropical rainforests

Understanding how tropical rainforests respond to elevated atmospheric CO2 concentration (eCO2) is essential for predicting Earths carbon, water, and energy budgets under future climate change. Here we use long-term (1982–2010) precipitation (P) and runoff (Q) measurements to infer runoff coefficient (Q/P) and evapotranspiration (E) trends across 18 unimpaired tropical rainforest catchments. We complement that analysis by using satellite observations coupled with ecosystem process modeling (using both “top-down” and “bottom-up” perspectives) to examine trends in carbon uptake and relate that to the observed changes in Q/P and E. Our results show there have been only minor changes in the satellite-observed canopy leaf area over 1982–2010, suggesting that eCO2 has not increased vegetation leaf area in tropical rainforests and therefore any plant response to eCO2 occurs at the leaf level. Meanwhile, observed Q/P and E also remained relatively constant in the 18 catchments, implying an unchanged hydrological partitioning and thus approximately conserved transpiration under eCO2. For the same period, using a top-down model based on gas exchange theory, we predict increases in plant assimilation (A) and light use efficiency (e) at the leaf level under eCO2, the magnitude of which is essentially that of eCO2 (i.e., ~12% over 1982–2010). Simulations from 10 state-of-the-art bottom-up ecosystem models over the same catchments also show that the direct effect of eCO2 is to mostly increase A and e with little impact on E. Our findings add to the current limited pool of knowledge regarding the long-term eCO2 impacts in tropical rainforests.

Changing Australian vegetation from 1788 to 1988: effects of CO2 and land-use change

We present a tractable and transparent approach (the TMSC model) to estimating the total stock of carbon (roots, stems and leaves) in living vegetation (C living), from gross primary productivity (GPP) estimates. The TMSC model utilises the TMS scheme of canopy functional types and a generic allometric scheme to derive these estimates. Model estimates are presented for the Australian continent under the following three vegetation–[CO2] scenarios: the present (1988) vegetation and a hypothetical natural (1988) vegetation cover with atmospheric CO2 concentration ([CO2]) of 350 µmol mol–1 (pveg350 and nveg350), and the natural vegetation (1788) having [CO2] of 280 µmol mol–1 (nveg280). The change between the nveg280 and pveg350 scenarios represents the combined effects of changes in land use and CO2. The change resulting from CO2 alone is the difference between the nveg280 and nveg350 scenarios. The estimated C living for the continent is 21 Gt for pveg350, 23 Gt for nveg350 and 10 Gt for nveg280. This translates to an averaged rate of increase in C living (CSI) of about 50 Tg C year–1 over the last 200 years for the continent. Where wooded areas have been extensively cleared for agriculture, the CSI is negative (down to –4 g C m–2 year–1). Elsewhere, the CSI over the last 200 years ranges from ~55 g C m–2 year–1 in the tropical and subtropical forests to ~0 g C m–2 year–1 in the most arid regions.

Air embolisms exsolving in the transpiration water - : the effect of constrictions in the xylem pipes

When water flows through a constriction, air can come out of solution (i.e. it can exsolve). This phenomenon is manifested in the transpiration stream of plants. Observations of gas in functioning xylem prompted a hypothesis predicting the daily balance between air and water in wood: a sudden fall in water content at sunrise, followed by an increase in water content during the day. An extended record by time domain reflectometry of volumetric water content (VWC) every 2 h throughout a summer shows the detailed pattern of change of VWC during 25 individual days, giving good agreement with the hypothesis. This hypothesis has wide-ranging consequences for experiments using cut plant parts. Perfusing aqueous solutions through excised xylem also can exsolve air from the water, causing declines in flow. The location of such air was investigated in cryo-fixed perfused vine stems by cryo-scanning electron microscopy. Bubbles formed at residual walls of perforation plates in small vessels, and filled many large vessels. The input surface is revealed as a major source of exsolved air. Precautions to reduce this effect are outlined and discussed.