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This paper studies the influence of metal precursors in the CVD´s catalyst synthesis of carbon nanotubes (CNTs) used as fillers in a polypropylene (PP) matrix (∼0.3 wt%). Two catalytic schemes, Fe/Al2O3 (50:50) and Ni–Fe/Al2O3 (40:10:50), were prepared to determine the influence of the reduction temperature over the characteristics and mechanical properties of CNT as PP fillers. The conversion temperature was varied to see the dependance of the CNT structure to this variable (700 °C–750 °C–800 °C). CNTs products were characterized by SEM and Raman spectroscopy. The SEM micrographs showed a sharper fiber type CNTs for the bimetallic catalyst and the Raman confirmed that better crystallites are obtain over the Fe catalyst. The Fe–PP composite presented enhanced mechanical properties when compare with Fe–Ni–PP, with tensile strength, hardness, and impact properties are higher in 16%, 9%, and 9% respectively. Other carbonaceous materials, as CNF, with less crystallinity presented poorer mechanical properties. Finally, can be stated that for the use of CNF as fillers in PP composites a Fe/Al2O3 catalyst, and a reaction temperature 700 °C–750 °C will produce a CNF with 60 nm mean diameter, is better than the use of Fe-Ni based catalysts.

The recent paper by Morillas et al. [Morillas, L. et al. Using radiometric surface temperature for surface energy flux estimation in Mediterranean drylands from a two-source perspective, Remote Sens. Environ. 136, 234–246, 2013] evaluates the two-source model (TSM) of Norman et al. (1995) with revisions by Kustas and Norman (1999) over a semiarid tussock grassland site in southeastern Spain. The TSM - in its current incarnation, the two-source energy balance model (TSEB) - was applied to this landscape using ground-based infrared radiometer sensors to estimate both the composite surface radiometric temperature and component soil and canopy temperatures. Morillas et al. (2013) found the TSEB model substantially underestimated the sensible H (and overestimated the latent heat LE) fluxes. Using the same data set from Morillas et al. (2013), we were able to confirm their results. We also found energy transport and exchange behavior derived from primarily the observations themselves to differ significantly from a number of prior studies using land surface temperature for estimating heat fluxes with one-source modeling approaches in semi-arid landscapes. However, revisions to key vegetation inputs to TSEB and the soil resistance formulation resulted in a significant reduction in the bias and root mean square error (RMSE) between model output of H and LE and the measurements compared to the prior results from Morillas et al. (2013). These included more representative ground-based vegetation greenness and local leaf area index values as well as modifications to the coefficients of the soil resistance formulation to account for the very rough (rocky) soil surface conditions with a clumped canopy. This indicates that both limitations in remote estimates of biophysical indicators of the canopy at the site and the lack of adjustment in soil resistance formulation to account for site specific characteristics, contributed to the earlier findings of Morillas et al. (2013). This suggests further studies need to be conducted to reduce the uncertainties in the vegetation and land surface temperature input data in order to more accurately assess the effects of the transport exchange processes of this Mediterranean landscape on TSEB formulations.

In arid and semiarid environments, where precipitation is scarce and mainly limited to the wet season of the year, the water contribution by non-rainfall water inputs (NRWI) may play a significant role in the water balance. Natural ecosystems are heterogeneous with a great variety of surface covers, such as stones, biological soil crusts (BSC), bare soil, trees, shrubs and other plants. To be able to understand the role that NRWI may have in a system, all the surface types involved and all the NRWI sources (fog, dew and water vapour adsorption) should be differentiated, analyzed and studied separately. This manuscript study NRWI on different cover surfaces of the soil in a natural coastal-steppe ecosystem. Automated microlysimeters were located in the field containing small Macrochloa tenacissima plants, bare soil, stones and biological soil crusts. Daily changes in the water content of the samples were registered. The different sources of NRWI were differentiated and their partial contributions to the total NRWI and to the daily evaporation were analyzed. Each cover type showed a different response in the presence of NRWI and these responses were also dependent on the NRWI source. In turn, the surface cover influenced the subsequent evaporation the day after. The number of dew events varied with the surface cover type and water vapour adsorption occurred all days in all the covers, alone or preceding a fog or a dew event. Dew represented the main NRWI source in plants and stones, while water vapour adsorption was the main input in bare soil and BSC. Fog was a minor component of the NRWI during the study period and its partial contribution to the total input was similar for all the cover types. NRWI satisfied a great part of the evaporation in the sample, especially in plants and stones.

Differences in vegetation pattern between slope aspects in semiarid environments are well known, with shaded aspects presenting a higher biomass. The micrometeorological and soil conditions involved in non-rainfall water inputs (NRWI), comprising dew and water vapour adsorption (WVA) were compared between two contrasted slopes and different environmental conditions (wet and dry periods). Changes in natural soil surfaces were measured using automated microlysimeters, and the partial contributions of dew and WVA to the total NRWI were clarified. Dew amounts were higher on the northeast facing slope and were directly related to dew durations. Differences in dew deposition between slopes were mainly driven by insolation patterns, which controlled the surface temperatures, the soil water content and, in turn, dew duration. Apart from spatial variation in microclimate, WVA deposition was higher in the southwest facing slope due to its higher clay content and electric conductivity and because of its lower soil water content. Water vapour adsorption was directly governed by the relative humidity amplitude in summer (with dry soil) but not in winter. A significant amount of water evaporation was satisfied by NRWI, reaching 100% in dry periods and being WVA the main input.

The temperature-based two-source model (TSM) of Norman et al. (1995) has not been properly evaluated under the water stress conditions that are typical in natural Mediterranean drylands. In such areas, the asynchrony between precipitation and energy supply strongly reduces evapotranspiration, E (or latent heat flux, LE , if expressed in energy terms), making sensible heat flux ( H ) the dominant turbulent heat flux. In this study, we present a detailed analysis of the main environmental factors affecting the TSM effectiveness under such challenging conditions. The accuracy of the TSM, evaluated via errors in 15-min H estimates, was shown to have a diurnal variation. Accuracy was clearly reduced for solar elevation angles lower than 25° and during marginal hours of daytime, before 10 am and after 3 pm. The surface to air temperature difference ( T R − T a ) and the wind speed were the two environmental factors showing the strongest effect on the TSM accuracy. In contrast with results observed in other ecosystems, in this Mediterranean tussock grassland the TSM accuracy was not clearly reduced by cloudiness and it was improved under highly stressed vegetation conditions. The parallel resistances scheme of the TSM ( TSM P ) showed overall lower errors and a lower tendency to underestimate at high H values, but the series resistances scheme of the TSM ( TSM S ) increased the model accuracy under some specific circumstances such as low energy supply and atmospheric neutral conditions. Finally, two extrapolation methods to obtain daytime ( Rn > 55 W m −2 ) turbulent fluxes from the 15-min estimates of TSM were compared: (i) assuming the self-preservation of the evaporative and the non-evaporative fraction ( EF and NEF method ) and (ii) averaging the total daytime instantaneous fluxes ( Averaging method ). Despite the assumption of daytime self-preservation of EF and NEF was showed consistent, this method retrieved less accurate daytime estimates of H , and E than the Averaging method as a result of inaccuracies affecting estimates of EF and NEF from the TSM at our site. Moreover, better daytime estimates of H and E were obtained when using instantaneous fluxes from the TSM P than from the TSM S . Thus, reliable daytime estimates of H were obtained from the TSM P in a Mediterranean dryland, with mean errors of 20% and high correlations ( R 2 = 0.85). However, daytime E was strongly overestimated (125%) using the TSM by both methods, although a good correlation with eddy covariance measurements was found ( R 2 = 0.84).

In the present climate change context it is important to understand the carbon balance seasonality of Mediterranean areas, that will suffer important changes in precipitation according to the last climate change predictions. This work analyzed the seasonality of carbon exchanges of three Mediterranean ecosystems according to a variety of water and temperature regimes due to differences in altitude (alpine, subalpine and lowland). Results show that the timing and duration of the growing season depended on temperature at the alpine site, while the dependence on water availability increased as altitude decreased. Thus, maximum values of net carbon uptake occurred in late spring for the alpine and subalpine sites (up to 60 and 30 gC m −2 month −1 respectively) whereas the lowland site absorbed carbon throughout winter (up to 30 gC m −2 month −1 ). Similarly increases in aridity conditions resulted in monthly increases in carbon emissions during dry periods. Thus from May to October, the lowland emitted up to 60°gC°m −2 °month −1 , the subalpine emitted half that with a delay of two months, whereas the alpine site continued with slight uptake sequestration. Finally, the EVI could be used to provide reasonably accurate estimates of photosynthesis ( R 2 around 0.6) but this relation varies depending on the site.

Non-rainfall atmospheric water input (NRWI), which is comprised of fog, dew and soil water vapour adsorption (WVA), has been proven to be an important water source in arid and semiarid environments. Its minor contribution to the water balance and the difficulty in measuring it have resulted in a wide variety of measurement methods (duration and quantification), especially for dew. Microlysimeters seem to be the most realistic method for dew measurement on natural surfaces and they can also detect WVA. This paper presents an automated microlysimeter that enables accurate studies of NRWI and evaporation on soil and small plants. Furthermore, we have developed a field strategy for their long term placement and installation which prevents damage from rainfall, soil movement or other field conditions, keeping the microlysimeters balanced and dry. This design allows the measurement of evaporation and NRWI on different cover types, including small plants. By monitoring the surface temperatures, dewfall and water vapour adsorption can be distinguished and the relative contribution of dew and WVA on the NRWI can also be found. Our automated microlysimeter design, construction and field installation have proven to be an useful and effective tool in an NRWI study.

[1] An adaptation of a simple model for evapotranspiration (E) estimations in drylands based on remotely sensed leaf area index and the Penman-Monteith equation (PML model) (Leuning et al., 2008) is presented. Three methods for improving the consideration of soil evaporation influence in total evapotranspiration estimates for these ecosystems are proposed. The original PML model considered evaporation as a constant fraction (f) of soil equilibrium evaporation. We propose an adaptation that considers f as a variable primarily related to soil water availability. In order to estimate daily f values, the first proposed method (fSWC) uses rescaled soil water content measurements, the second (fZhang) uses the ratio of 16 days antecedent precipitation and soil equilibrium evaporation, and the third (fdrying), includes a soil drying simulation factor for periods after a rainfall event. E estimates were validated using E measurements from eddy covariance systems located in two functionally different sparsely vegetated drylands sites: a littoral Mediterranean semiarid steppe and a dry-subhumid Mediterranean montane site. The method providing the best results in both areas was fdrying (mean absolute error of 0.17 mm day−1) which was capable of reproducing the pulse-behavior characteristic of soil evaporation in drylands strongly linked to water availability. This proposed model adaptation, fdrying, improved the PML model performance in sparsely vegetated drylands where a more accurate consideration of soil evaporation is necessary.

This work received financial support from several different research projects: the PROBASE (CGL2006-11619/HID), BACARCOS (CGL2011-29429) and CARBORAD (CGL2011-27493), funded by the Ministerio de Ciencia e Innovacion; the GEOCARBO(RNM3721), GLOCHARID and COSTRAS (RMN-3614) projects funded by Consejeria de Innovacion, Ciencia y Empresa (Andalusian Regional Ministry of Innovation, Science and Business) and European Union funds (ERDF and ESF). OU received a JAE Ph.D. research grant fromthe CSIC.

The need for a better understanding of factors controlling the variability of soil water content (θ) in space and time to adequately predict the movement of water in the soil and in the interphase soil-atmosphere is widely recognised. In this paper, we analyse how soil properties, surface cover and topography influence soil moisture (θ) over karstic lithology in a sub-humid Mediterranean mountain environment. For this analysis we have used 17 months of θ measurements with a high temporal resolution from different positions on a hillslope at the main recharge area of the Campo de Dalías aquifer, in Sierra de Gádor (Almería, SE Spain). Soil properties and surface cover vary depending on the position at the hillslope, and this variability has an important effect on θ. The higher clay content towards the lower position of the hillslope explains the increase of θ downslope at the subsurface horizon throughout the entire period studied. In the surface horizon (0-0.1 m), θ patterns coincide with those found at the subsurface horizon (0.1-0.35 m) during dry periods when the main control is also exerted by the higher percentage of clay that increases downslope and limits water depletion through evaporation. However, in wet periods, the wettest regime is found in the surface horizon at the upper position of the hillslope where plant cover, soil organic matter content, available water, unsaturated hydraulic conductivity (Kunsat) and infiltration rates are higher than in the lower positions. The presence of rock outcrops upslope the θ sampling area, acts as runoff sources, and subsurface flow generation between surface and subsurface horizons also may increase the differences between the upper and the lower positions of the hillslope during wet periods. Both rock and soil cracks and fissures act disconnecting surface water fluxes and reducing run-on to the lower position of the hillslope and thus they affect θ pattern as well as groundwater recharge. Understanding how terrain attributes, ground cover and soil factors interact for controlling θ pattern on karst hillslope is crucial to understand water fluxes in the vadose zone and dominant percolation mechanisms which also contribute to estimate groundwater recharge rates. Therefore, understanding of soil moisture dynamics provides very valuable information for designing rational strategies for the use and management of water resources, which is especially urgent in regions where groundwater supports human consume or key economic activities.

The changing climate is affecting biological and physical processes occurring in the biosphere, including the biogeochemical cycles of water and carbon. The response of ecosystem carbon exchange to changes in temperature and the water balance is still very uncertain, a fact that highlights the need for research in order to understand the role of the biosphere in the future global carbon budget. South east Spain is on the borderline between the tropical and mid-latitude climate zones, with climates ranging from sub-humid to arid, and the majority representing the driest area in Europe. Initial predictions on climate change for this region point to decreases in total precipitation and the number of precipitation events, meaning a decrease in water availability underlining the vulnerability of the region to desertification. This region, due to these special climatic conditions, has been the subject of experimental carbon and water field research in recent years. This paper defines the state of the art of carbon and water balance measurement and modelling studies in this region, analyses the different processes involved in aggregate exchanges (vegetation and soil; net ecosystem carbon balance–evapotranspiration, photosynthesis–transpiration, soil respiration–evaporation), and identifies needs for future research.

This paper explores the sensitivity of a three-source (vegetation, P, bare soil, BS, and soil under plant, S) evapotranspiration clumped model (CM) developed for sparsely vegetated areas, to the parameterisation of the surface resistances in semi-arid climate. We analysed the sensitivity of the CM to: (i) the location and depth of the soil water content (θ) measurements, (ii) the accuracy in the measurements of the variables involved in the parameterisation of the surface resistances, with special attention to the in-canopy water vapour saturation deficit (D0) and to the average plant leaf area index (L), and (iii) the simplification of the parametric equations of the soil surface resistances. The sensitivity of the CM was tested in a stand of Anthyllis cytisoides located in a semi-arid area of the Southeast of Spain. In this stand, θ was measured at three different depths (0.02 m, 0.05 m and 0.15 m), D0 was calculated through iterations of the CM, taking into account the water vapour fluxes of the different evaporating sources, and L was measured with a destructive direct method. These variables were included in parametric equations for estimating the surface resistances of P ( g s p ), BS and S ( r s p , r s bs and r s s , respectively). Evapotranspiration estimates were compared to Eddy Covariance system measurements. Results showed that for estimating r s bs and r s s the θ should be measured as superficially as possible, whereas when estimating r s p the θ should be measured at a depth where the effect of the extremely low values of the superficial layer of soil is excluded or attenuated (in our study area this depth was 0.15 m or the integrated θ of the first 0.15 m). Moreover, the CM was more sensitive to the accuracy in the estimation of D0 than to any other variable, while errors in the measurement of L and θ had similar effects in the CM evapotranspiration estimates (a 50% underestimation of D0 gave rise to an underestimation of λE of 70%, whereas an overestimation of 50% in D0 led to an overestimation of λE of 16%; errors of ±50% in L and θ gave rise to errors of λE of ±16%). Finally, the CM showed a higher sensitivity to the parameterisation of each surface separately, with a specific parametric equation for each soil surface resistance, than to the use of specific values of θ measured in each soil surface.

This paper analyses the temporal dynamics of soil water balance components in a representative recharge area of the Sierra de Gaacutedor (Almeria, southeastern Spain) in two hydrological years. Two approaches are used to estimate daily potential recharge (PR): Approach 1 based on deriving PR from the water balance as the difference between measurements of rainfall (P) and actual evapotranspiration (E) obtained by eddy covariance; and Approach 2 with PR obtained from the dynamic pattern of the soil moisture (θ) recorded at two depths in the site's thin soil (average 0.35 m thickess). For the hydrological year 2003/04, which was slightly drier than the 30-year average, E accounted for 64% of rainfall and occurred mainly in late spring and early summer. The PR estimated by Approach 1 was 181 ± 18 mm year-1 (36% of rainfall), suggesting an effective groundwater recharge in the study area. In the unusually dry hydrological year 2004/05, E was about 215 mm year-1, close to the annual rainfall input, and allowing very little (8 ± 12 mm year-1) PR according to Approach 1. Estimation of PR based on Approach 2 resulted in PR rates lower than those found by Approach 1, because Approach 2 does not take into account the recharge that occurs through preferential flow pathways (cracks, joints and fissures) which were not monitored with the θ probes. Moreover, using Approach 2, the PR estimates differed widely depending on the time scale considered: with daily mean θ data, PR estimation was lower, especially in late spring, while θ data at 30 min resolution yielded a more reliable prediction of the fraction of total PR resulting from the downward movement of soil water by gravity.

Measurements of CO2 fluxes with Eddy Covariance (EC) systems are ongoing over different ecosystems around the world, through different measuring networks, in order to assess the carbon balance of these ecosystems. In carbonate ecosystems, characterized by the presence of subterranean pores and cavities, ventilation of the CO2 accumulated in these cavities and pores can act as an extra source of CO2 exchange between the ecosystem and the atmosphere. In this work we analyse the effect of the subterranean heterogeneity of a carbonate ecosystem on measurements of CO2 fluxes by comparing measurements from two EC systems with distinct footprints. Results showed that both EC systems agreed for measurements of evapotranspiration and of CO2 in periods when respiratory and photosynthetic processes were dominant (biological periods), with a regression slope of 0.99 and 0.97, respectively. However, in periods when the main source of CO2 comes from the ventilation of subterranean pores and cavities (abiotic periods) agreement is not good, with a regression slope of 0.6. Ground-penetrating radar measurements of the sub-surface confirmed the existence of high sub-surface heterogeneity that, combined with different footprints, lead to differences in the measurements of the two EC systems. These results show that measurements of CO2 fluxes with Eddy covariance systems over carbonate ecosystems must be taken carefully, as they may not be representative of the ecosystem under consideration.

17 pages, 10 figures.-- Printed version published on Sep 15, 2008., Author's version available at: http://www.eeza.csic.es/eeza/documentos/Elsevier_Garcia_2008.pdf, There is a need to develop operational land degradation indicators for large regions to prevent losses of biological and economic productivity. Disturbance events press ecosystems beyond resilience and modify the associated hydrological and surface energy balance. Therefore, new indicators for water-limited ecosystems can be based on the partition of the surface energy into latent (λE) and sensible heat flux (H)., In this study, a new methodology for monitoring land degradation risk for regional scale application is evaluated in a semiarid area of SE Spain. Input data include ASTER surface temperature and reflectance products, and other ancillary data. The methodology employs two land degradation indicators, one related to ecosystem water use derived from the non-evaporative fraction (NEF = H / (λE + H)), and another related to vegetation greenness derived from the NDVI. The surface energy modeling approach used to estimate the NEF showed errors within the range of similar studies (R^2 = 0.88; RMSE = 0.18 (22%))., To create quantitative indicators suitable for regional analysis, the NEF and NDVI were standardized between two possible extremes of ecosystem status: extremely disturbed and undisturbed in each climatic region to define the NEFS (NEF Standardized) and NDVIS (NDVI Standardized). The procedure was successful, as it statistically identified ecosystem status extremes for both indicators without supervision. Evaluation of the indicators at disturbed and undisturbed (control) sites, and intermediate surface variables such as albedo or surface temperature, provided insights on the main surface energy status controls following disturbance events. These results suggest that ecosystem functional indicators, such as the NEFS, can provide information related to the surface water deficit, including the role of soil properties., This study received financial support from several different research projects: the integrated EU project, DeSurvey (A Surveillance System for Assessing and Monitoring of Desertification) (ref.: FP6-00.950, contract no. 003950), 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 Junta de Andalucía (Andalusian Regional Government).

In this work we tested techniques suitable for a future validation of the RATP model to simulate transpiration and photosynthesis of mature olive trees under field conditions. Canopy architecture was characterised with an electromagnetic 3D digitiser and the software 3A. Although the capability of the software to deal with big data sets has to be improved, the system seems to meet the RATP requirements. An array of radiation sensors mounted in an aluminium bar and located at different positions within the canopy showed to be an useful tool for monitoring radiation distribution; these data can be used to validate the RATP predictions. Leaves intercepting more radiation showed greater values both of area based nitrogen content and photosynthetic capacity, and the increment of nitrogen was found to be related to the increment in leaf mass area. A method was tested for assessing plant leaf area, which could allow us to determine the leaf area of the biggest trees in the orchard, with reduced time and labour. Ministerio de Ciencia y Tecnología AGL2002- 04048-CO3-01

In semi-arid areas, where vegetation is sparse and clumped, models used to estimate evapotranspiration (λE) consider soil and plants as different sources of evaporation. When working at higher scales of heterogeneity, the modelling of surface fluxes introduces effective parameters that enclose the sub-grid heterogeneity. In this work we used both approaches to estimate the λE of a sparse-vegetation patch of Retama sphaerocarpa (L.) Boiss in a semi-arid area in southeast Spain. Firstly, we used a multi-layer model considering plant, soil under plant and bare soil, each with its own surface and aerodynamic resistances and available energy, interacting at a within canopy height. Secondly, we used a single-layer model that uses the effective surface and aerodynamic resistances of the patch, calculated by different aggregations of the soil and plant resistances considered in the multi-layer model. The estimates of λE were compared with measured values obtained by an Eddy covariance system. Results show that the use of effective surface resistances aggregated in parallel and effective aerodynamic resistances aggregated in series in a single-layer model produced similar estimates of λE as a multi-layer model. When compared to the measured values, the estimates of the single-layer model were even more accurate than the estimates of the multi-layer model. The results of this paper show that, in areas with low vegetation cover, a simple model, where patch-scale heterogeneity is solved by the use of effective aggregated resistances, is adequate to estimate the patch-scale evapotranspiration. © 2007 Elsevier B.V. All rights reserved.

Canopy transpiration ( E c ) and soil evaporation ( E s ) in a Mediterranean Scots pine stand were simulated using a two-layer model, with a Jarvis-type submodel of canopy stomatal conductance ( G s ) and a soil resistance to evaporation expressed as a function of superficial soil moisture. Sap flow measurements and soil evaporation data, together with meteorological and soil moisture variables were used to calibrate the model. G s was calibrated using the generalized likelihood uncertainty estimation (GLUE) methodology, first with data from the year 2004, a year characterised by mild meteorological conditions. Then, data from the year 2003, which included an intense summer drought, was used to update the results from the previous calibration. The discrepancy between the diurnal courses of modelled and measured E c using best-fit parameters was not related to any particular situation of meteorology or soil moisture. Model performance improved at the daily scale, but the model failed to simulate E c adequately during the year 2005. Maximum modelled E s rates were 0.7 mm day −1 with the ratio E s / E c being typically under 0.3 during the growing season. The GLUE analysis revealed that parameters representing reference stomatal aperture at a vapour pressure deficit ( D ) value equal to 1 kPa ( G s,ref ), and sensitivity to D ( m ) were the most relevant, and were consistent with the hydraulic theory of stomatal regulation. Parameters controlling the response to superficial soil moisture deficit only appeared sensitive in the calibration with data from the year 2003, suggesting that response to deeper soil layers should also be considered in the model. Updating the original calibration reduced predictive uncertainty and constrained the value of some parameters. Nevertheless, it seems that representations of variable plant and soil hydraulic resistances, are required to simulate long-term E c in seasonally-dry Mediterranean forest stands.

Summary The aim of this work was first, to compare two methods for obtaining soil boundary-layer resistance, one based on the turbulent diffusion theory (TD) and another based on the energy balance of heated and unheated sensors (EB), and secondly, to analyse how the results obtained from both methods are affected by canopy structure parameters in typical sparse vegetation. The study was carried out in stands of three different species characteristic of the semiarid South East of Spain ( Retama sphaerocarpa , Anthyllis cytisoides and Stipa tenacissima ). Comparison of the two methods showed that EB boundary-layer resistances, unlike TD, implicitly considers the peculiarities of the plant architecture and structure, as well as its distribution in the field and the presence or absence of substrate under the canopies. In contrast, for the TD method, quantitative attributes of the vegetation (average vegetation height, h , leaf area index, L , fractional vegetative cover, f , drag coefficient, c d and the eddy diffusivity decay constant, n ) are necessary, which are not only sometimes difficult to obtain in the field, but also seem to be insufficient to explain the development of the soil boundary layer in sparse semiarid vegetation. The advantages of using the EB method are both practical and theoretical. Positioning of the sensors and data storage with automatic data loggers is easy, and does not require knowledge of the aerodynamic attributes of the vegetated area. Furthermore, it allows qualitative interpretation of the effect of non-transpirable vegetative elements, as well as the architecture of the vegetation and its distribution in the field, neither of which are considered in the turbulent diffusion theory.

24 pages, 4 figures, 4 tables.-- Special Issue "Remote Sensing of Natural Resources and the Environment"., Three operative models with minimum input data requirements for estimating the partition of available surface energy into sensible and latent heat flux using ASTER data have been evaluated in a semiarid area in SE Spain. The non-evaporative fraction (NEF) is proposed as an indicator of the surface water deficit. The best results were achieved with NEF estimated using the "Simplified relationship" for unstable conditions (NEF_Seguin) and with the S-SEBI (Simplified Surface Energy Balance Index) model corrected for atmospheric conditions (NEF_S-SEBIt,) which both produced equivalent results. However, results with a third model, NEF_Carlson, that estimates the exchange coefficient for sensible heat transfer from NDVI, were unrealistic for sites with scarce vegetation cover. These results are very promising for an operative monitoring of the surface water deficit, as validation with field data shows reasonable errors, within those reported in the literature (RMSE were 0.18 and 0.11 for the NEF, and 29.12 Wm-2 and 25.97 Wm-2 for sensible heat flux, with the Seguin and S-SEBIt models, respectively)., This study received financial support from several different research projects: the integrated EU project, DeSurvey (A Surveillance System for Assessing and Monitoring of Desertification) (ref.: FP6- 00.950, Contract nº. 003950), 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 Junta de Andalucía (Andalusian Regional Government).

Summary Information regarding dew precipitation is scarce, both because it is considered a minor component of the water balance and because it is difficult to measure. Nevertheless, although yielding relatively low amounts of water, dew can be of great importance for the local water balance in semiarid and arid environments. The aim of this study was to explore the adequacy of the Eddy Covariance technique, combined with qualitative methods (such as wetness sensors), as a tool to measure actual dew in semiarid conditions. This technique was used to assess the relative contribution of dew to the local water balance in a dry river bed representative of semiarid environments (Rambla Honda, Tabernas) located in the Almeria province (SE Spain). Results show that an accurate filtering and management of data from Eddy Covariance (EC) combined with the records of the wetness sensors (WS), allowed us to generate a data base which can be used to develop and parameterize theoretical or empirical dew deposition models. We found that a simple equation for potential dew predicted adequately actual dew for the study area, and permitted us to estimate a dew contribution of 13 mm from February to June 2003, which accounts for 12% of the rainfall for the same period. Dew deposition was higher in late winter and early spring, when vegetation starts to grow actively. In consequence, this additional input of water can be of great relevance to the local water balance especially during dry years.

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almeria) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F P >10 µmol m−2 s−1) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO2 fluxes were ca. −1.75 and −0.60 µmol m−2 s−1, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m−2 s−1, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (

Data for water vapor adsorption and evaporation are presented for a bare soil (sandy loam, clay content 15%) in a southern Spanish olive grove. Water losses and gains were measured using eight high-precision minilysimeters, placed around an olive tree, which had been irrigated until the soil reached field capacity (∼0.22 m3 m-3). They were subsequently left to dry for 10 days. A pair of lysimeters was situated at each of the main points of the compass (N, E, S, W), at a distance of 1 m. (the inner set of lysimeters; ILS) and 2 m (the outer set of lysimeters; OLS), respectively, from the tree trunk. Distinct periods o f moisture loss (evaporation) and moisture gain (vapor adsorption) could be distinguished for each day. Vapor adsorption often started just after noon and generally lasted until the (early) evening. Values of up to 0.7 mm of adsorbed water per day were measured. Adsorption was generally largest for the OLS (up to 100% more on a daily basis), and increased during the dry down. This was mainly the result of lower OLS surface soil moisture contents (period-average absolute difference ∼0.005 m3 m-3), as illustrated using various analyses employing a set of micrometeorological equations describing the exchange of water vapor between bare soil and the atmosphere. These analyses also showed that the amount of water vapor adsorbed by soils is very sensitive to changes in atmospheric forcing and surface variables. The use of empirical equations to estimate vapor adsorption is therefore not recommended., The experiment used equipment funded by Natural Environment Research Council (NERC) grants (NER/M/S/2000/00268 and NER/T/S/ 1999/00108). This research was supported by a Marie Curie European Reintegration Grant within the Sixth European Community RTD Framework Programme, MERG-CT-2004-510524.

Water deficit indices based on the spatial relationship between surface temperature (Ts) and NDVI, known as triangle approaches, are widely used for drought monitoring. However, their application has been recently questioned when the main factor limiting evapotranspiration is energy. Even though water is the main control in dryland ecosystems, these can also undergo periods of energy and temperature limitation. In this paper we aimed to: (i) evaluate the TVDI (Temperature–Vegetation Dryness Index) to estimate water deficits (e.g. ratio between actual and potential evapotranspiration), and heat surface fluxes using MODIS data; and (ii) provide insights about the factors most affecting the accuracy of results. Factors considered included the type of climatic control on evapotranspiration, λE, (i.e. water-limited vs. energy-limited), the quality of Tair estimates, the heterogeneity of land cover types and climatic variables in the region, or the algorithm to extract hydrological boundaries from the images. The TVDI was compared with eddy covariance (EC) data from two shrublands with different climatic controls for λE in South Spain. Evaluations showed that it could be used to estimate the water deficit when water was the main limiting factor (R = 0.81–0.88; Mean Average Error, MAE = 0.16–0.17) but not in energy-limited situations (R < 0.2; MAE = 0.10–0.2). Spatial heterogeneity in climatic variables also had a different impact on accuracy depending on limiting factors. Relative humidity was significant at the water-limited site while solar irradiance and air temperature were more important at the energy-limited site. The skill of the TVDI to estimate surface fluxes at the water-limited site was confirmed for the dominant sensible heat flux, H (R2 = 0.93; Mean Absolute Percentage Error, MAPE = 12.85%) but not for λE (R2 = 0.01, MAPE = 115.22%) as λE fluxes at this site are just slightly above the error of the eddy covariance system. At the energy-limited site, λE (R2 = 0.74; MAPE = 31.83%) and H estimates (R2 = 0.80; MAPE = 26.85%) were better than those from the SEBAL (Surface Energy Balance Algorithm for Land) or the PML (Penman–Monteith–Leuning) models. However, the skill to predict surface fluxes in this case was due to the net radiation inputs and not by the TVDI input. Our analyses also suggest: (1) to apply the TVDI excluding energy-limited sites/periods based on climatic knowledge of limiting factors on λE; (2) the best conditions for TVDI performance correspond to the situation when the controlling factors are less limiting, e.g. during the growing season (higher SWC and lower VPD); and (3) to account better for the role of vegetation controls on transpiration.

The difference between long-term actual evapotranspiration (AET) and precipitation (P) provides a useful indication of the extent to which a site retains or loses water resources, and therefore of the likely occurrence of specific land degradation processes. Sink areas OAET q PU receive lateral water inputs from other parts of the catchment. In arid and semi-arid environments these areas are frequently found in, or next to, the stream beds of ephemeral rivers and are often characterised by intensive land use or high conservation values. For both types of land use it is important to know if, and how much, AET exceeds P, and where the lateral water inputs come from. Thick sedimentary fills in the stream bed, variable climate conditions and ephemeral flow conditions pose specific difficulties to the evaluation of the water balance of these sites. The objective of this study was to develop an approach to explore the relative importance of lateral water inputs to shrub stands growing in thick sedimentary fills of semi-arid ephemeral rivers. The approach is based on (i) estimating long-term AET2 P balances in the channel sediments and (ii) assessing whether these inflows originate mainly from surrounding hillslopes or from the upstream part of the catchment. A physically based evapotranspiration model for sparse vegetation was used to estimate the long-term AET rates. The relative importance of hillslope runoff and channel flow was evaluated in a semi-quantitative fashion from a combination of surface area estimates and mostly published values of soil hydrological parameters. The approach was developed and tested in a selected stand of Retama sphaerocarpashrubs in a stream bed at the Rambla Honda field site (Tabernas

The accurate quantification of the energy available for sensible and latent heat transfer from plant canopies is essential for the prediction of impacts of climate on vegetation water use and growth. Unlike agricultural fields and extensive forests of more humid zones, vegetation growing in semi-arid climates is usually sparse creating a heterogeneous surface of shrubs, annuals and bare soil. Under these conditions many of the assumptions of the basic equations used in microclimatology, which assume a uniform vegetated surface, may be violated. It is proposed here that heterogeneous canopies may require a formulation of their energy balance that includes a measure of the canopy complexity in order to both interpret field measurements and be used in predictive models. This paper explores the need for a more complex formulation of the vegetation energy balance through a series of experiments on a sparse clumped shrubland of Retama sphaerocarpa in the Tabernas Desert, Almeria, south-east Spain. These experiments investigated the importance of the radiative properties of each surface on energy balance of soil, annuals and shrubs individually, and the surface as a whole. The study evaluated the use of the fractional vegetative cover (f) and the radiative characteristics of each surface (reflection coefficients and emissivities) for calculating net radiation partitioning between shrubs and bare soil. Results indicated that partitioning of net radiation between components could be accurately calculated from values of fractional vegetative cover, reflection coefficients and emissivities for both bare soil and plant surfaces. A sensitivity analysis showed the importance of specific radiation properties of each surface. Measurements of horizontal long-wave fluxes between components showed that the location of a plant with respect to other plants made little difference to its long-wave energy balance. The results also cmphasized the importance of soil water content on the energy balance, through its influence on albedo and soil heat storage. This was particularly true when measurements of soil heat flux were used to measure the available energy for soil under shrubs and bare soil because of strong hysteresis cycles. These cycles were larger in the bare soil than in the substrate under shrubs.

A model was developed that predicted evaporation based on combined information on the physiology of overstorey and substrate, micrometeorological conditions and spatial distribution of plants. Predicted evapotranspiration was verified at stand level for Retama sphaerocarpa shrubs, and model parameters were tested to determine their importance in controlling evaporation. Model predictions were compared with evapotranspiration, measured by a Bowen Ratio Energy Balance system (BREB), and transpiration, measured by sap flow of the stems of the shrubs in south eastern Spain. Modifications made to the original two source clumped model in the tested model significantly improved agreement between predicted surface evapotranspiration rates and rates measured by the Bowen Ratio method. The modifications made to the model were improved parameterisation of soil surface conductance, a more detailed description of the radiation balance, and improved parameterisation of the soil aerodynamic conductance terms. Improvements in the soil surface conductance estimates made the most significant change to model predictions, the second two modifications showed no significant improvement in prediction of evapotranspiration. A sensitivity analysis indicated that relatively large variations of leaf area index or albedo caused little variation in evapotranspiration during the period measured, whereas variations in soil water content caused large changes in predicted evapotranspiration. Transpiration rates of shrubs (measured and modelled) indicated an independence from surface soil moisture (0–25 cm) supporting the view that R. sphaerocarpa had access to reserves of water deep in the soil which enabled it to survive and grow vigorously in this type of semiarid environment. Thus, it was concluded that land use changes which affect redistribution of water resources (overland and subsurface flow) may threaten the stability survival of R. sphaerocarpa stands.

A two-source model (TSM) for surface energy balance, considering explicitly soil and vegetation components, was tested under water stress conditions. The TSM evaluated estimates the sensible heat flux (H) using the surface-air thermal gradient and the latent heat flux (LE) as a residual from the surface energy balance equation. The analysis was performed in a semiarid Mediterranean tussock grassland in southeast Spain, where H is the dominant flux and LE rates are low, challenging conditions under which the TSM has not been validated before. We evaluated two different resistance schemes: series and parallel; as well as the iterative algorithm included in the TSM to disaggregate the soil-surface composite temperature into its separate components. Continuous field measurements of composite soil-vegetation surface temperature (TR) and bare soil temperature (Ts) from thermal infrared sensors were used for model testing along with canopy temperature estimates (T'c), derived from TR and Ts.Comparisons with Eddy covariance and field data showed that the TSM produced reliable estimates of net radiation (Rn) and H fluxes, with errors of ~30% and ~10%, respectively, but not for LE, with errors ~90%. Despite of lower errors (~10%) in estimating H using parallel resistance, the series scheme was more robust showing slightly higher correlations (r2=0.78-0.80 vs. r2=0.75-0.77) and allowing a better disaggregation of soil and canopy fluxes. Differences between model runs using the iterative algorithm to disaggregate TR and the simplified version that uses separate inputs of Ts and T'c were minor. This demonstrates the robustness of the iterative procedure to disaggregate a composite soil-vegetation temperature into separate soil and vegetation components in semiarid environments with good prospects for image applications. © 2013 Elsevier Inc., This research was funded by Andalusia Regional Government projects AQUASEM (P06-RNM-01732), GEOCARBO (P08-RNM-3721), and GLOCHARID, European Union ERDF funds, with support from the Spanish Ministry of Science and Innovation CARBORAD Projects (CGL2011-27493), the Danish Council for Independent Research and Technology and Production Sciences (FTP) Grant 09-070382, and the research project RNM-6685 financed by the Regional Andalucian Government (Spain). L. Morillas received a Ph.D. research grant and funding from the Andalusia Regional Government for visiting the Institute of Geography and Geology, University of Copenhagen, in Denmark.

Measurements of CO2 fluxes with Eddy Covariance (EC) systems are ongoing over different ecosystems around the world, through different measuring networks, in order to assess the carbon balance of these ecosystems. In carbonaceous ecosystems, characterized by the presence of subterranean pores and cavities, ventilation of the CO2 accumulated in these cavities and pores can act as an extra source of CO2 exchange between the ecosystem and the atmosphere. In this work we analyse the effect of the subterranean heterogeneity of a carbonaceous ecosystem on measurements of CO2 fluxes by comparing measurements from two EC systems with distinct footprints. Results showed that both EC systems agreed for measurements of evapotranspiration and of CO2 in periods when respiratory and photosynthetic processes were dominant (biological periods), with a regression slope of 0.99 and 0.97, respectively. However, in periods when the main source of CO2 comes from the ventilation of subterranean pores and caves (abiotic periods) agreement is not good, with a regression slope of 0.6. Ground-penetrating radar measurements of the sub-surface confirmed the existence of high sub-surface heterogeneity that, combined with different footprints, lead to differences in the measurements of the two EC systems. These results show that measurements of CO2 fluxes with Eddy covariance systems over carbonaceous ecosystems must be taken carefully, as they may not be representative of the ecosystem under consideration.

This study presents the interannual CO(2) exchange of a semiarid shrubland overlying a carbonaceous substrate where abiotic processes may interact with biological processes in the ecosystem carbon cycle. Estimated by the eddy covariance technique and two different gap-filling models (RBF neural network and MDS ``Marginal Distribution Sampling''), the mean annual net ecosystem exchange (NEE) of CO(2) varied from -63 to 29 g C m(-2), averaging -2 +/- 23 g C m(-2) from May 2004 to December 2007. In addition, CO(2) exchange was divided into periods likely dominated by clear signals of biological activity (high daytime uptake, low Bowen ratio) and periods of apparent plant senescence (dry conditions) to better understand the relative contributions of ``biological versus abiotic processes'' to the annual NEE and the interannual variability of these processes. During biological periods, CO(2) exchange appears to be related to changes in the ecosystem leaf area index, whereas abiotic fluxes are most closely related to evapotranspiration. The timing of rainfall events seems to be a main determinant of both activation and duration of biological processes. During apparent abiotic periods, CO(2) exchange is characterized by daytime emissions averaging 1.5 mu mol m(-2) s(-1), whereas biological periods are characterized by daytime CO(2) uptake with mean values of around 1.4 mu mol m(-2) s(-1). As a preliminary result, the abiotic period characterized from 19% of 2004 to 45% of 2007 with a mean duration of approximately 2 months. However, future research is needed in order to achieve a more reliable division between determinant processes involved in CO(2) exchange over carbonaceous substrates.

In this work, daily latent heat (lambdaEd) or evapotranspiration and sensible heat (Hd) fluxes were estimated from the TVDI (Temperature Vegetation Dryness Index) modified to account for climatic gradients (TVDIt) using MODIS data in a Spanish region (Andalusia) with strong bioclimatic gradients. The TVDIt was correlated (R=0.90) with field measured AWC (available water content). When reinterpreting TVDIt as the ratio between actual and potential evapotranspiration to estimate surface energy fluxes, model vs. eddy covariance data from 2 semiarid sites were reasonable for Hd (R=0.94: RMSE=25.89 Wm-2). For lambdaEd RMSE was low (19.46 Wm-2) but correlations were only significant after excluding summer dates (R=0.71) when transpiration is very low. Model accuracy is currently limited by the accuracy in Rnd estimates (

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.

19 páginas, 9 figuras, 2 tablas, 58 referencias., It is widely believed that partial root drying (PRD) reduces water losses by transpiration without affecting yield. However, experimental work carried out to date does not always support this hypothesis. In many cases a PRD treatment has been compared to a full irrigated treatment, so doubt remains on whether the observed benefits correspond to the switching of irrigation or just to PRD being a deficit irrigation treatment. In addition, not always a PRD treatment has been found advantageous as compared to a companion regulated deficit irrigation (RDI) treatment. In this work we have compared the response of mature ‘Manzanilla’ olive trees to a PRD and an RDI treatment in which about 50%of the crop evapotranspiration (ETc) was supplied daily by localised irrigation. We alternated irrigation in the PRD treatment every 2 weeks in 2003 and every 3 weeks in 2004. Measurements of stem water potential (Ystem), stomatal conductance (gs) and net CO2 assimilation rate (A) were made in trees of both treatments, as well as in trees irrigated to 100% of ETc (Control trees) and in Rain-fed trees. Sap flow was also measured in different conductive organs of trees under both PRD and RDI treatments, to evaluate the influence of alternating irrigation on root water uptake and tree water consumption. We found small and random differences in Ystem, gs and A, which gave no evidence of PRD causing a positive effect on the olive tree performance, as compared to RDI. Stomatal conductance decreased in PRD trees as compared to Control trees, but a similar decrease in gs was also recorded in the RDI trees. Sap flow measurements, which reflected water use throughout the irrigation period, also showed no evidence of gs being more reduced in PRD than in RDI trees. Daily water consumption was also similar in the trees of the deficit irrigation treatments, for most days, throughout the irrigation period. Alternating irrigation in PRD trees did not cause a change in either water taken up by main roots at each side of the trees, or in the sap flow of both trunk locations and main branches of each side. Results from this work, and from previous work conducted in this orchard, suggest that transpiration is restricted in trees under deficit irrigation, in which roots are left in drying soil when water is applied by localised irrigation, and that there is no need to alternate irrigation for achieving this effect., This work was funded by the Dirección General de Investigación of the Spanish Ministerio de Ciencia y Tecnología (research project AGL2002-04048-CO3-01).