Cave ventilation is influenced by variations in the CO2-dependent virtual temperature

Dynamics and drivers of ventilation in caves are of growing interest for different fields of science. Accumulated CO2 in caves can be exchanged with the atmosphere, modifying the internal CO2 content, affecting stalagmite growth rates, deteriorating rupestrian paintings or creating new minerals. Current estimates of cave ventilation neglect the role of high CO2 concentrations in determining air density – approximated via the virtual temperature (Tv) –, affecting buoyancy and therefore the release or storage of CO2. Here we try to improve knowledge and understanding of cave ventilation through the use of Tv in CO2-rich air to explain buoyancy for different values of temperature (T) and CO2 content. Also, we show differences between T and Tv for 14 different experimental sites in the vadose zone, demonstrating the importance of using the correct definition of Tv to determine air buoyancy in caves. The calculation of Tv (including CO2 effects) is currently available via internet using an excel template, requiring the input of CO2 (%), air temperature (ºC) and relative humidity (%).This research was funded by the Andalusian regional government project GEOCARBO (P08- RNM-3721) and GLOCHARID, including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects CarboredII (CGL2010-22193-C04-02), SOILPROF (CGL2011- 15276-E) and CARBORAD (CGL2011-27493), as well as the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 244122

Analysis of effective resistance calculation methods and their effect on modelling evapotranspiration in two different patches of vegetation in semi-arid SE Spain

Effective parameters are of major importance in modelling surface fluxes at different scales of spatial heterogeneity. Different ways to obtain these effective parameters for their use in meso-scale and GCM models have been studied. This paper deals with patch-scale heterogeneity, where effective resistances were calculated in two patches with different vegetation (Retama sphaerocarpa (L.) Boiss shrubs, and herbaceous plants) using different methods: aggregating soil and plant resistances in parallel, in series or by an average of both. Effective aerodynamic resistance was also calculated directly from patch fluxes. To assess the validity of the different methods used, the Penman-Monteith equation was used with effective resistances to estimate the total λE for each patch. The λE estimates found for each patch were compared to Eddy Covariance system measurements. Results showed that for effective surface resistances, parallel aggregation of soil and plant resistances led to λE estimates closer to the measured λE in both patches (differences of around 10%). Results for effective aerodynamic resistances differed depending on the patch considered and the method used to calculate them. The use of effective aerodynamic resistances calculated from fluxes provided less accurate estimates of λE compared to the measured values, than the use of effective aerodynamic resistances aggregated from soil and plant resistances. The results reported in this paper show that the best way of aggregating soil and plant resistances depends on the type of resistance, and the type of vegetation in the patch.This work received financial support from several different research projects: the PROBASE (ref.: CGL2006-11619/HID) and CANOA (ref.: CGL2004-04919-C02-01/HID) projects funded by the Spanish Ministry of Education and Science; and the BACAEMA (“Balance de carbono y de agua en ecosistemas de matorral mediterráneo en Andalucía: Efecto del cambio climático”, RNM-332) and CAMBIO (“Efectos del cambio global sobre la biodiversidad y el funcionamiento ecosistémico mediante la identificación de áreas sensibles y de referencia en el SE ibérico”, RNM 1280) projects funded by the regional government Junta de Andalucía. The first author enjoyed a pre-doctoral grant from the Spanish Ministry of Science and Technology

Hot-Moments of Soil CO2 Efflux in a Water-Limited Grassland

The metabolic activity of water-limited ecosystems is strongly linked to the timing and magnitude of precipitation pulses that can trigger disproportionately high (i.e., hot-moments) ecosystem CO2 fluxes. We analyzed over 2-years of continuous measurements of soil CO2 efflux (Fs) under vegetation (Fsveg) and at bare soil (Fsbare) in a water-limited grassland. The continuous wavelet transform was used to: (a) describe the temporal variability of Fs; (b) test the performance of empirical models ranging in complexity; and (c) identify hot-moments of Fs. We used partial wavelet coherence (PWC) analysis to test the temporal correlation between Fs with temperature and soil moisture. The PWC analysis provided evidence that soil moisture overshadows the influence of soil temperature for Fs in this water limited ecosystem. Precipitation pulses triggered hot-moments that increased Fsveg (up to 9000%) and Fsbare (up to 17,000%) with respect to pre-pulse rates. Highly parameterized empirical models (using support vector machine (SVM) or an 8-day moving window) are good approaches for representing the daily temporal variability of Fs, but SVM is a promising approach to represent high temporal variability of Fs (i.e., hourly estimates). Our results have implications for the representation of hot-moments of ecosystem CO2 fluxes in these globally distributed ecosystems

Commonalities of carbon dioxide exchange in semiarid regions with monsoon and Mediterranean climates

Comparing biosphere–atmosphere carbon exchange across monsoon (warm-season rainfall) and Mediterranean (cool-season rainfall) regimes can yield information about the interaction between energy and water limitation. Using data collected from eddy covariance towers over grass and shrub ecosystems in Arizona, USA and Almeria, Spain, we used net ecosystem carbon dioxide exchange (NEE), gross ecosystem production (GEP), and other meteorological variables to examine the effects of the different precipitation seasonality. Considerable crossover behavior occurred between the two rainfall regimes. As expected in these usually water-limited ecosystems, precipitation magnitude and timing were the dominant drivers of carbon exchange, but temperature and/or light also played an important role in regulating GEP and NEE at all sites. If significant rainfall occurred in the winter at the Arizona sites, their behavior was characteristically Mediterranean whereby the carbon flux responses were delayed till springtime. Likewise, the Spanish Mediterranean sites showed immediate pulse-like responses to rainfall events in non-winter periods. The observed site differences were likely due to differences in vegetation, soils, and climatology. Together, these results support a more unified conceptual model for which processes governing carbon cycling in semiarid ecosystems need not differ between warm-season and cool-season rainfall regimes.This paper is the result of a fellowship funded by the OECD Co-operative Research Programme: Biological Resource Management for Sustainable Agricultural Systems to R.L. Scott. This paper has been supported in part by the Andalusian regional government project GEOCARBO and GLOCHARID (P08-RNM-3721), European Union Funds (ERDF and ESF), the Spanish flux-tower network CARBORED-ES (Science Ministry project CGL2010-22193-C04-02), and the European Commission collaborative project GHG Europe (FP7/2007-2013; grant agreement 244122)

Deep CO 2 soil inhalation / exhalation induced by synoptic pressure changes and atmospheric tides in a carbonated semiarid steppe

Knowledge of all the mechanisms and processes involved in soil CO2 emissions is essential to close the global carbon cycle. Apart from molecular diffusion, the main physical component of such CO2 exchange is soil ventilation. Advective CO2 transport, through soil or snow, has been correlated with the wind speed, friction velocity or pressure (p). Here we examine variations in subterranean CO2 molar fractions (χc) over two years within a vertical profile (1.5 m) in a semiarid ecosystem, as influenced by short-timescale p changes. Analyses to determine the factors involved in the variations in subterranean χc were differentiated between the growing period and the dry period. In both periods it was found that variations in deep χc (0.5–1.5 m) were due predominantly to static p variations and not to wind or biological influences. Within a few hours, the deep χc can vary by fourfold, showing a pattern with two cycles per day, due to p oscillations caused by atmospheric tides. By contrast, shallow χc (0.15 m) generally has one cycle per day as influenced by biological factors like soil water content and temperature in both periods, while the wind was an important factor in shallow χc variations only during the dry period. Evidence of emissions was registered in the atmospheric boundary layer by eddy covariance during synoptic pressure changes when subterranean CO2 was released; days with rising barometric pressure – when air accumulated belowground, including soil-respired CO2 – showed greater ecosystem uptake than days with falling pressure. Future assessments of the net ecosystem carbon balance should not rely exclusively on Fick's law to calculate soil CO2 effluxes from profile data.This research was funded by the Andalusian regional government project GEOCARBO (P08-RNM-3721) and GLOCHARID, including European Union ERDF funds, with support from Spanish Ministry of Science and Innovation projects Carbored-II (CGL2010-22193-C04-02), SOILPROF (CGL2011-15276-E) and CARBORAD (CGL2011-27493), as well as the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 244122

Application of Sliding Mode Control to the design of a Buck-based sinusoidal Generator

This paper is devoted to the design of a sliding-mode control scheme for a buck-based inverter, with programmable amplitude, frequency, and dc offset, with no external sinusoidal reference required. A general procedure for obtaining an autonomous (time independent) switching surface from a time-dependent one is presented. For this surface, the system exhibits a zeroth-order dynamics in sliding motion. On the other hand, from the sliding-domain analysis, a set of design restrictions is established in terms of the inverter output filter Bode diagram and the output signal parameters (amplitude, frequency and dc offset), facilitating the subsequent design procedure. The control scheme is robust with respect to both power-stage parameter variations and external disturbances and can be implemented by means of conventional electronic circuitry. Simulations and experimental results for both reactive and nonlinear loads are presented.Peer Reviewe

Application of sliding-mode control to the design of a buck-based sinusoidal generator

This paper is devoted to the design of a sliding-mode control scheme for a buck-based inverter, with programmable amplitude, frequency, and DC offset, with no external sinusoidal reference required. A general procedure for obtaining an autonomous (time independent) switching surface from a time-dependent one is presented. For this surface, the system exhibits a zeroth-order dynamics in sliding motion. On the other hand, from the sliding-domain analysis, a set of design restrictions is established in terms of the inverter output filter Bode diagram and the output signal parameters (amplitude, frequency and DC offset), facilitating the subsequent design procedure. The control scheme is robust with respect to both power-stage parameter variations and external disturbances and can be implemented by means of conventional electronic circuitry. Simulations and experimental results for both reactive and nonlinear loads are presented.Peer ReviewedPostprint (published version

Evaluación comparativa entre el uso de MOSFETs de carburo de silicio (SiC) y de silicio (Si) en un emulador fotovoltaico

Postprint (published version

Caracterización automatizada de sistemas fotovoltaicos conectados a red

Postprint (published version

Aplicación de MOSFETs de carburo de silicio (SiC) en inversores fotovoltaicos conectados a red

Postprint (published version