Evaporation Through a Dry Soil Layer: Column Experiments

Modeling of water vapor transport through a dry soil layer (DSL), typically formed in the top soil during dry seasons in arid and semi-arid areas, is still problematic. Previous laboratory experiments in controlled environments showed that the only vapor transport process through the DSL is by Fick's law of diffusion. However, field experiments exhibited consistently higher evaporation rates than predicted by diffusion flow only. Some proposed reasons for the mismatch were: (a) daily cycles of condensation and evaporation in the DSL due to changes in solar radiation; (b) wind effects on air movement in the DSL; (c) atmospheric pressure fluctuations; (d) nonlinear influence of the DSL thickness on the evaporation process. To link the laboratory experiments with field observations, we performed soil column experiments in the laboratory with thick (>50 cm) DSL, and with different wind speeds, two radiative lamp schedules (continuous and 12 h daily cycles) and different thicknesses of DSL. Atmospheric pressure, air temperature and humidity were measured continuously. The results show that the evaporation rates observed are larger than those predicted by diffusion flow only. We found that it was possible to model the evaporation rates as a function of atmospheric pressure fluctuations. In conclusion, atmospheric pressure fluctuations can induce evaporation rates in DSL larger than estimated by diffusion flow only, possibly explaining the discrepancy between laboratory and field evaporation rates

Integrating MRS data with hydrologic model - Carrizal Catchment (Spain)

Magnetic resonance sounding (MRS) provides quantitative hydrogeological information on hydrostratigraphy and hydraulic parameters of subsurface (e.g. flow and storage property of aquifers) that can be integrated in distributed hydrologic models. The hydraulic parameters are typically obtained by pumping tests. In this study, we propose an MRS integration method based on optimizing MRS estimates of aquifer hydraulic parameters through hydrologic model calibration. The proposed MRS integration method was applied in the 73 km2 Carrizal Catchment in Spain, characterized by a shallow unconfined aquifer with an unknown aquifer bottom. 12 MRS survey results were inverted with Samovar 11.3, schematized and integrated in the transient, distributed, coupled, hydrologic, MARMITES-MODFLOW model. As the aquifer bottom was unknown, the aquifer was schematized into one unconfined layer of uniform thickness. For that layer, MRS estimators of specific yield and transmissivity/hydraulic conductivity were calculated as weighted averages of the inverted MRS layers. The MRS integration with hydrologic model was carried out by introducing multipliers of specific yield and transmissivity/hydraulic conductivity that were optimized during transient model calibration using 11 time-series piezometric observation points. The optimized multipliers were 1.0 for specific yield and 3.5*10-9 for hydraulic conductivity. These multipliers were used, and can be used in future MRS investigations in the Carrizal Catchment (and/or adjacent area with similar hydrogeological conditions), to convert MRS survey results into aquifer hydraulic parameters. The proposed method of MRS data integration in the hydrologic model of Carrizal Catchment not only allowed us to calibrate the model but also to confirm the functional capability of MRS in quantitative groundwater assessment. Most importantly however, it demonstrated that if pumping tests are not available, the use of MRS integrated in distributed coupled hydrological models, or even in standalone groundwater models, provides a valuable aquifer parameterization alternative

Hydrogeophysics and remote sensing for the design of hydrogeological conceptual models in hard rocks - Sardón catchment (Spain)

Hard rock aquifers are highly heterogeneous and hydrogeologically complex. To contribute to the design of hydrogeological conceptual models of hard rock aquifers, we propose a multi-techniques methodology based on a downward approach that combines remote sensing (RS), non-invasive hydrogeophysics and hydrogeological field data acquisition. The proposed methodology is particularly suitable for data scarce areas. It was applied in the pilot research area of Sardón catchment (80 km2) located west of Salamanca (Spain). The area was selected because of hard-rock hydrogeology, semi-arid climate and scarcity of groundwater resources. The proposed methodology consisted of three main steps. First, we detected the main hydrogeological features at the catchment scale by processing: (i) a high resolution digital terrain model to map lineaments and to outline fault zones; and (ii) high-resolution, multispectral satellite QuickBird and WorldView-2 images to map the outcropping granite. Second, we characterized at the local scale the hydrogeological features identified at step one with: i) ground penetrating radar (GPR) to assess groundwater table depth complementing the available monitoring network data; ii) 2D electric resistivity tomography (ERT) and frequency domain electromagnetic (FDEM) to retrieve the hydrostratigraphy along selected survey transects; iii) magnetic resonance soundings (MRS) to retrieve the hydrostratigraphy and aquifer parameters at the selected survey sites. In the third step, we drilled 5 boreholes (25 to 48 m deep) and performed slug tests to verify the hydrogeophysical interpretation and to calibrate the MRS parameters. Finally, we compiled and integrated all acquired data to define the geometry and parameters of the Sardón aquifer at the catchment scale. In line with a general conceptual model of hard rock aquifers, we identified two main hydrostratigraphic layers: a saprolite layer and a fissured layer. Both layers were intersected and drained by fault zones that control the hydrogeology of the catchment. The spatial discontinuities of the saprolite layer were well defined by RS techniques while subsurface geometry and aquifer parameters by hydrogeophysics. The GPR method was able to detect shallow water table at depth between 1 and 3 m b.g.s. The hydrostratigraphy and parameterization of the fissured layer remained uncertain because ERT and FDEM geophysical methods were quantitatively not conclusive while MRS detectability was restricted by low volumetric water content. The proposed multi-technique methodology integrating cost efficient RS, hydrogeophysics and hydrogeological field investigations allowed us to characterize geometrically and parametrically the Sardón hard rock aquifer system, facilitating the design of hydrogeological conceptual model of the area

Partitioning and sourcing of evapotranspiration using coupled MARMITES-MODFLOW model, La Mata catchment (Spain)

The new, two-way coupled, distributed and transient MARMITES-MODFLOW (MM-MF) model, coupling land surface and soil zone domains with groundwater, is presented. It implements model-based partitioning and sourcing of subsurface evapotranspiration (ETss) as part of spatio-temporal water balance (WB). The partitioning of ETss involves its separation into evaporation (E) and transpiration (T), while the sourcing of E and T involves separation of each of the two into soil zone (Esoil and Tsoil) and groundwater (Eg and Tg) components. The objective of that development was to understand the system dynamics of a catchment with shallow water table, through spatio-temporal quantification of water fluxes and evaluation of their importance in water balances, focusing on the Eg and Tg components of ETss. While the Eg is computed using formulation from published study, the Tg is obtained through a novel phenomenological function, based on soil moisture availability and transpiration demand driven by climatic conditions. The MM-MF model was applied in the small La Mata catchment (~4.8 km2, Salamanca Province, Spain), characterized by semi-arid climate, granitic bedrock, shallow water table and sparse oak woodland. The main catchment characteristics were obtained using remote sensing, non-invasive hydrogeophysics and classical field data acquisition. The MM-MF model was calibrated in transient, using daily data of five hydrological years, between 1st October 2008 and 30th September 2013. The WB confirmed dependence of groundwater exfiltration on gross recharge. These two water fluxes, together with infiltration and Esoil, constituted the largest subsurface water fluxes. The Eg was higher than the Tg, which is explained by low tree coverage (~7%). Considering seasonal variability, Eg and Tg were larger in dry seasons than in wet season, when solar radiation was the largest and soil moisture the most depleted. A relevant observation with respect to tree transpiration was that during dry seasons, the decline of Tsoil, associated with the decline of soil moisture, was compensated by increase of Tg, despite continuously declining water table. However, in dry seasons, T was far below the atmospheric evaporative demand, indicating that the groundwater uptake by the tree species of this study constituted a survival strategy and not a mechanism for continued plant growth. The presented MM-MF model allowed to analyze catchment water dynamics and water balance in detail, accounting separately for impacts of evaporation and transpiration processes on groundwater resources. With its unique capability of partitioning and sourcing of ETss, the MM-MF model is particularly suitable for mapping groundwater dependent ecosystems, but also for analyzing impacts of climate and land cover changes on groundwater resources

Thermal activation between Landau levels in the organic superconductor β′′\beta''-(BEDT-TTF)2_{2}SF5_{5}CH2_{2}CF2_{2}SO3_{3}

We show that Shubnikov-de Haas oscillations in the interlayer resistivity of the organic superconductor $\beta''$-(BEDT-TTF)$_{2}$SF$_{5}$ CH$_{2}$CF$_{2}$SO$_{3}$ become very pronounced in magnetic fields $\sim$~60~T. The conductivity minima exhibit thermally-activated behaviour that can be explained simply by the presence of a Landau gap, with the quasi-one-dimensional Fermi surface sheets contributing negligibly to the conductivity. This observation, together with complete suppression of chemical potential oscillations, is consistent with an incommensurate nesting instability of the quasi-one-dimensional sheets.Comment: 6 pages, 4 figure

Superconductivity mediated by charge fluctuations in layered molecular crystals

We consider the competition between superconducting, charge ordered, and metallic phases in layered molecular crystals with the theta and beta" structures. Applying slave-boson theory to the relevant extended Hubbard model, we show that the superconductivity is mediated by charge fluctuations and the Cooper pairs have d(xy) symmetry. This is in contrast to the kappa-(BEDT-TTF)(2)X family, for which theoretical calculations give superconductivity mediated by spin fluctuations and with d(x)2(-y)2 symmetry. We predict several materials that should become superconducting under pressure

Validation of satellite daily rainfall estimates in complex terrain of Bali Island, Indonesia

Satellite rainfall products have different performances in different geographic regions under different physical and climatological conditions. In this study, the objective was to select the most reliable and accurate satellite rainfall products for specific, environmental conditions of Bali Island. The performances of four spatio-temporal satellite rainfall products, i.e., CMORPH25, CMORPH8, TRMM, and PERSIANN, were evaluated at the island, zonation (applying elevation and climatology as constraints), and pixel scales, using (i) descriptive statistics and (ii) categorical statistics, including bias decomposition. The results showed that all the satellite products had low accuracy because of spatial scale effect, daily resolution and the island complexity. That accuracy was relatively lower in (i) dry seasons and dry climatic zones than in wet seasons and wet climatic zones; (ii) pixels jointly covered by sea and mountainous land than in pixels covered by land or by sea only; and (iii) topographically diverse than uniform terrains. CMORPH25, CMORPH8, and TRMM underestimated and PERSIANN overestimated rainfall when comparing them to gauged rain. The CMORPH25 had relatively the best performance and the PERSIANN had the worst performance in the Bali Island. The CMORPH25 had the lowest statistical errors, the lowest miss, and the highest hit rainfall events; it also had the lowest miss rainfall bias and was relatively the most accurate in detecting, frequent in Bali, ≤ 20 mm day−1 rain events. Lastly, the CMORPH25 coarse grid better represented rainfall events from coastal to inlands areas than other satellite products, including finer grid CMORPH8

Hydrogeological conceptual model of large and complex sedimentary aquifer systems – Central Kalahari Basin (Botswana)

Successful groundwater resources evaluation and management is nowadays typically undertaken using distributed numerical groundwater flow models. Such models largely rely on hydrogeological conceptual models. The conceptual models summarize hydrogeological knowledge of an area to be modelled and thereby providing a framework for numerical model design. In this study, an efficient data integration method for developing hydrogeological conceptual model of the large and hydrogeologically-complex, Central Kalahari Basin (CKB) aquifer system, was undertaken. In that process, suitability of 3-D geological modelling with RockWorks code in iterative combination with standard GIS (ArcGIS) was tested. As a result, six hydrostratigraphic units were identified, their heads and related flow system interdependencies evaluated and hydraulic properties attached. A characteristic feature of the CKB is a thick unsaturated Kalahari Sand Unit (KSU), that restricts the erratic recharge input to <1 mm yr −1 in the centre to about 5–10 mm yr −1 in the eastern fringe. The analysis of the spatial distribution of topological surfaces of the hydrostratigraphic units and hydraulic heads of the aquifers, allowed to identify three flow systems of the three aquifers, Lebung, Ecca and Ghanzi, all three having similar radially-concentric regional groundwater flow patterns directed towards discharge area of Makgadikgadi Pans. That pattern similarity is likely due to various hydraulic interconnections, direct or through aquitard leakages, and also due to the presence of the overlying unconfined, surficial KSU, hydraulically connected with all the three aquifers, redistributing recharge into them. The proposed 3-D geological modelling with RockWorks, turned to be vital and efficient in developing hydrogeological conceptual model of a large and complex multi-layered aquifer systems. Its strength is in simplicity of operation, in conjunctive, iterative use with other software such as standard GIS and in flexibility to interface with numerical groundwater model. As a result of conceptual modelling, fully 3-d, 6 layer numerical model, with shallow, variably-saturated, unconfined layer is finally recommended as a transition from conceptual into numerical model of the CKB

Evaporation through a Dry Soil Layer: Column Experiments

Modeling of water vapor transport through a dry soil layer (DSL), typically formed in the top soil during dry seasons in arid and semi-arid areas, is still problematic. Previous laboratory experiments in controlled environments showed that the only vapor transport process through the DSL is by Fick's law of diffusion. However, field experiments exhibited consistently higher evaporation rates than predicted by diffusion flow only. Some proposed reasons for the mismatch were: (a) daily cycles of condensation and evaporation in the DSL due to changes in solar radiation; (b) wind effects on air movement in the DSL; (c) atmospheric pressure fluctuations; (d) nonlinear influence of the DSL thickness on the evaporation process. To link the laboratory experiments with field observations, we performed soil column experiments in the laboratory with thick (>50 cm) DSL, and with different wind speeds, two radiative lamp schedules (continuous and 12 h daily cycles) and different thicknesses of DSL. Atmospheric pressure, air temperature and humidity were measured continuously. The results show that the evaporation rates observed are larger than those predicted by diffusion flow only. We found that it was possible to model the evaporation rates as a function of atmospheric pressure fluctuations. In conclusion, atmospheric pressure fluctuations can induce evaporation rates in DSL larger than estimated by diffusion flow only, possibly explaining the discrepancy between laboratory and field evaporation rates

Mapping dry-season tree transpiration of an oak woodland at the catchment scale, using object-attributes derived from satellite imagery and sap flow measurements

Tree transpiration is an important plant-physiological process that influences the water cycle, thereby influencing ecosystems and even the quantity of available water resources. However, direct tree-transpiration measurements, particularly at large spatial scales, are still rare, due to the complexities associated with natural ecosystems. In this study we describe a scaling-up method for quantifying dry-season tree transpiration (Tc) of the Sardón catchment, located in Central-Western Spain, 50 km west of Salamanca. The method is applied to an oak woodland in that catchment which is dominated by two tree species: evergreen Quercus ilex and deciduous Quercus pyrenaica (Q.i. and Q.p., respectively). The method comprises five complementary steps: (a) sap-flux density (Jp) measurement; (b) definition of biometric-upscaling functions; (c) spatial scaling-up of tree transpiration using high-resolution remote-sensing-derived object attributes (50–60 cm per pixel); (d) modelling of dry-season temporal sap-flow variability; and (e) an assessment of the robustness/uncertainty of the method. The proposed method was applied to assess dry-season tree transpiration, however if there is appropriate sap flow data available, it can also be applied during any season of the year. The application of the proposed method to map tree transpiration in the Sardón catchment resulted in a mean Jp = 58.4 cm3 cm−2 day−1 for Q.p., characterized by tree density of 19 trees ha−1, and a mean Jp = 37.1 cm3 cm−2 day−1 for Q.i., characterized by tree density of 4 trees ha−1. The Jp data varied as dependent on the weather conditions and not on the change of soil moisture. The mean normalised tree transpiration (Tt) estimated per species, was also higher for Q.p. (1.19 mm day−1 ±8%) than for Q.i. (0.83 mm day−1 ±25%) trees. The remote-sensing scaling up of tree transpiration for the Sardón catchment resulted in a low mean dry-season Tc = 0.045 mm day−1 (±1%) typical for water limited environments with sparse tree coverage (average ≈7%). In that Tc, Q.p. represented 79%, whereas Q.i. 21%. The dry-season Tc varied spatially at 1 ha resolution but barely temporally throughout the dry season. The obtained Tc maps can be readily used as input for distributed models in water resources and land management decision-making