Climate, Not Conflict, Explains Extreme Middle East Dust Storm

The recent dust storm in the Middle East (September 2015) was publicized in the media as a sign of an impending ‘Dust Bowl.’ Its severity, demonstrated by extreme aerosol optical depth in the atmosphere in the 99th percentile compared to historical data, was attributed to the ongoing regional conflict. However, surface meteorological and remote sensing data, as well as regional climate model simulations, support an alternative hypothesis: the historically unprecedented aridity played a more prominent role, as evidenced by unusual climatic and meteorological conditions prior to and during the storm. Remotely sensed normalized difference vegetation index demonstrates that vegetation cover was high in 2015 relative to the prior drought and conflict periods, suggesting that agricultural activity was not diminished during that year, thus negating the media narrative. Instead, meteorological simulations using the Weather Research and Forecasting (WRF) model show that the storm was associated with a cyclone and ‘Shamal’ winds, typical for dust storm generation in this region, that were immediately followed by an unusual wind reversal at low levels that spread dust west to the Mediterranean Coast. These unusual meteorological conditions were aided by a significant reduction in the critical shear stress due to extreme dry and hot conditions, thereby enhancing dust availability for erosion during this storm. Concluding, unusual aridity, combined with unique synoptic weather patterns, enhanced dust emission and westward long-range transport across the region, thus generating the extreme storm

Analysis of the impact of surface layer properties on evaporation from porous systems using column experiments and modified definition of characteristic length

The hydraulic properties of the layer at the vicinity of the soil surface have significant impact on evaporation, and could be harnessed to reduce water losses. The effect of the properties of the upper layer on the evolution of phase distribution during the evaporation process is first illustrated from three-dimensional pore network simulations. This effect is then studied from experiments carried out on soil columns under laboratory conditions. Comparisons between homogeneous columns packed with coarse (sand) and fine (sandy loam) materials, and heterogeneous columns packed with layers of fine overlying coarse material and coarse overlying fine material of different thicknesses are performed to assess the impact of upper layer properties on evaporation. Experiments are analyzed using the classical approach based on the numerical solution of Richards’s equation and semi-analytical theoretical predictions. The theoretical analysis is based on the clear distinction between two drying regimes, namely the capillary regime and the gravity-capillary regime, which are the prevailing regimes in our experiments. Simple relationships enabling to estimate the duration of stage-1 evaporation (S1) for both regimes are proposed. In particular, this led to defining the characteristic length for the gravity-capillary regime from the consideration of viscous effects at low water content differently from available expressions. The duration of S1, during which most of the water losses occur, for both the homogeneous and two-layer columns are presented and discussed. Finally, the impact of liquid films and it consequences on the soil hydraulic conductivity function are briefly discussed

T-26. Exploring Root Uptake Under High Frequency Irrigation Using Electrical Resistivity Tomography

Root uptake and its relation to environmental factors, and primarily soil water content, are perhaps the least understood component in terrestrial water balance and is of high importance for water resources management, ecology and agriculture. In this research we explore the spatial and temporal distribution of soil water in high resolution using electrical resistivity tomography (ERT). Bell peppers were planted in a chamber and irrigated in two different schemes, differing only in irrigation frequency (daily and eight-daily irrigation, where the daily dose is equal for both treatments). This irrigation difference results in very different spatio-temporal distribution of the soil water in the root zone, which in turn derives spatio-temporal differences in root uptake. Experiment was conducted under a screen-house in Mediterranean summer conditions, i.e. very high evapotranspiration. Resistivity surveys, using 96 electrodes placed around the growth chamber and at soil surface (Figure 1) were taken over 10 times daily. Plants subjected to high frequency irrigation generally were faster in growth and matured about a week earlier. This is primarily attributed to the higher water content that exists in the root zone, and primarily during the climatically stressing noon hours. Inverted images (e.g. Figure 2) provide an interesting insight into the spatio-temporal distribution of the root uptake. This in turn can now be correlated to the spatial location of the roots, and to the soil induced water content dynamics

Infiltration from the pedon to global grid scales: an overview and outlook for land surface modelling

Infiltration in soils is a key process that partitions precipitation at the land surface in surface runoff and water that enters the soil profile. We reviewed the basic principles of water infiltration in soils and we analyzed approaches commonly used in Land Surface Models (LSMs) to quantify infiltration as well as its numerical implementation and sensitivity to model parameters. We reviewed methods to upscale infiltration from the point to the field, hill slope, and grid cell scale of LSMs. Despite the progress that has been made, upscaling of local scale infiltration processes to the grid scale used in LSMs is still far from being treated rigorously. We still lack a consistent theoretical framework to predict effective fluxes and parameters that control infiltration in LSMs. Our analysis shows, that there is a large variety in approaches used to estimate soil hydraulic properties. Novel, highly resolved soil information at higher resolutions than the grid scale of LSMs may help in better quantifying subgrid variability of key infiltration parameters. Currently, only a few land surface models consider the impact of soil structure on soil hydraulic properties. Finally, we identified several processes not yet considered in LSMs that are known to strongly influence infiltration. Especially, the impact of soil structure on infiltration requires further research. In order to tackle the above challenges and integrate current knowledge on soil processes affecting infiltration processes on land surface models, we advocate a stronger exchange and scientific interaction between the soil and the land surface modelling communities

G1. Infiltration into Soils

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Plant Water Use Efficiency over Geological Time – Evolution of Leaf Stomata Configurations Affecting Plant Gas Exchange

<div><p>Plant gas exchange is a key process shaping global hydrological and carbon cycles and is often characterized by plant water use efficiency (WUE - the ratio of CO₂ gain to water vapor loss). Plant fossil record suggests that plant adaptation to changing atmospheric CO₂ involved correlated evolution of stomata density (<i>d</i>) and size (<i>s</i>), and related maximal aperture, <i>a_max</i>. We interpreted the fossil record of <i>s</i> and <i>d</i> correlated evolution during the Phanerozoic to quantify impacts on gas conductance affecting plant transpiration, <i>E</i>, and CO₂ uptake, <i>A,</i> independently, and consequently, on plant WUE. A shift in stomata configuration from large <i>s-</i>low <i>d</i> to small <i>s-</i>high <i>d</i> in response to decreasing atmospheric CO₂ resulted in large changes in plant gas exchange characteristics. The relationships between gas conductance, <i>g_ws</i>, <i>A</i> and <i>E</i> and maximal relative transpiring leaf area, (<i>a_max</i>⋅<i>d</i>), exhibited hysteretic-like behavior. The new WUE trend derived from independent estimates of <i>A</i> and <i>E</i> differs from established WUE-CO₂ trends for atmospheric CO₂ concentrations exceeding 1,200 ppm. In contrast with a nearly-linear decrease in WUE with decreasing CO₂ obtained by standard methods, the newly estimated WUE trend exhibits remarkably stable values for an extended geologic period during which atmospheric CO₂ dropped from 3,500 to 1,200 ppm. Pending additional tests, the findings may affect projected impacts of increased atmospheric CO₂ on components of the global hydrological cycle.</p></div

Characteristic lengths affecting evaporative drying of porous media

Evaporation from porous media involves mass and energy transport including phase change, vapor diffusion, and liquid flow, resulting in complex displacement patterns affecting drying rates. Force balance considering media properties yields characteristic lengths affecting the transition in the evaporation rate from a liquid-flow-based first stage limited only by vapor exchange with air to a second stage controlled by vapor diffusion through the medium. The characteristic lengths determine the extent of the hydraulically connected region between the receding drying front and evaporating surface (film region) and the onset of flow rate limitations through this film region. Water is displaced from large pores at the receding drying front to supply evaporation from hydraulically connected finer pore at the surface. Liquid flow is driven by a capillary pressure gradient spanned by the width of the pore size distribution and is sustained as long as the capillary gradient remains larger than gravitational forces and viscous dissipation. The maximum extent of the film region sustaining liquid flow is determined by a characteristic length L-C combining the gravity characteristic length L-G and viscous dissipation characteristic length L-V. We used two sands with particle sizes 0.1-0.5 mm ("fine") and 0.3-0.9 mm ("coarse") to measure the evaporation from columns of different lengths under various atmospheric evaporative demands. The value of LG determined from capillary pressure-saturation relationships was 90 mm for the coarse sand and 140 mm. for the fine sand. A significant decrease in drying rate occurred when the drying front reached the predicted LG value (viscous dissipation was negligibly small in sand and LC LG). The approach enables a prediction of the duration of first-stage evaporation with the highest water losses from soil to the atmosphere