ALBERT

Description: Soil surface sealing is a widespread natural process occurring frequently in bare soil areas between vegetation patches. The low hydraulic conductivity that characterizes the seal layer reduces both infiltration and evaporation fluxes from the soil, and thus has the potential to affect local vegetation water uptake (VWU). This effect is investigated here using experimental data, 2D physically based modelling and a long-term climatic dataset from three dry sites presenting a climatic gradient in the Negev Desert, Israel. The Feddes VWU parameters for the dominant shrub at the study site ( Sarcopoterium spinosum ) were acquired using lysimeter experiments. The results indicate that during the season surface sealing could either increase or decrease VWU depending on initial soil water content, rainfall intensity, and the duration of the subsequent drying intervals. These factors have a marked effect on inter-annual variability of the seal layer effect on VWU, which on average was found to be 26% higher under sealed conditions than in the case of unsealed soil surfaces. The seal layer was found to reduce the period where the vegetation was under water stress by 31% compared with unsealed conditions. This effect was more pronounced for seasons with total rainfall depth higher than 10 cm/y, and was affected by interseasonal climatic variability. These results shed light on the importance of surface sealing in dry environments and its contribution to the resilience of woody vegetation. This article is protected by copyright. All rights reserved.

Description: A class of capillary flows in unsaturated porous media is characterized by quasi-steady viscous flow confined behind curved air-water interfaces and within liquid bodies held by capillary forces along crevices and grain contacts. The geometry of the connected capillary liquid network within the pore space resembles channels that form between adjacent bubbles in foam (Plateau borders) with solid grains representing gas bubbles in foam. For simplified channel geometry we combine expressions for viscous flow with continuity considerations to describe the evolution of the channels cross-sectional area during gravity drainage. This formulation enables modeling of unsaturated flow without invoking the Richards equation and associated hydraulic functions. We adapt a formalism originally developed for foam “free drainage” (drainage under gravity) or “forced drainage” (infiltration front motion) to a class of unsaturated flows in porous media that require a few input parameters only (mean channel corner angle, air entry value and porosity) for certain initial and boundary conditions. We demonstrate that the reduction in capillary channel cross section yields a consistent description of self-regulating internal fluxes towards attainment of the so-called “field capacity” in soil and provides an alternative method for interpretation of outflow experiments for prescribed pressure boundary conditions. Additionally, the geometrically-explicit formulation provides a more intuitive picture of capillary flows across textural boundaries (changes in channel cross-section and number of channels). The foam drainage methodology expands the range of tools available for analyses of unsaturated flow processes and offers more realistic links between liquid configuration and flow dynamics in unsaturated porous media.

Description: The rapid expansion of remotely-sensed spatial information and enhanced computational capabilities fuel increasing scientific and public expectations for reliable hydrologic predictions across time and spatial scales. Process-based hydrologic models often rely on the Richards equation (RE) formalism to represent unsaturated flow processes at different scales which raise the much debated question: does the underlying physics in the RE formulation apply at large scales of practical interest? The study analyses recent findings from various unsaturated flow processes (soil evaporation, internal redistribution, and capillary flow from point sources) revealing inherent characteristic length scales that delineate the range of applicability of the RE. These length scales reflect the role of intrinsic porous medium properties that shape liquid phase continuity and interplay of forces that drive and resist unsaturated flow. The study revisits some of the key assumptions in the RE and their ramifications for numerical discretization. An intrinsic length scale for hydraulic continuity deduced from pore size distribution has been shown to control soil evaporation dynamics (i.e., stage 1 to stage 2 transition), to provide upper bounds for regional evaporative losses, and governs the dynamics of internal redistribution towards field capacity . For large scale hydrologic applications, we show that the extent of lateral flow interactions under most natural capillary gradients rarely exceed a few meters. The study provides a framework for guiding numerical and mathematical models for capillary flows across different scales considering the conditions for coexistence of stationarity, hydraulic continuity and capillary gradients - essential ingredients for physically-consistent application of the RE. This article is protected by copyright. All rights reserved.

Description: The challenge of meeting the projected doubling of global demand for food by 2050 is monumental. It is further exacerbated by the limited prospects for land expansion and rapidly dwindling water resources. A promising strategy for increasing crop yields per unit land requires the expansion of irrigated agriculture and the harnessing of water sources previously considered “marginal” (saline, treated effluent, and desalinated water). Such an expansion, however, must carefully consider potential long-term risks on soil hydro-ecological functioning. The study provides critical analyses of use of marginal water and management approaches to map out potential risks. Long-term application of treated effluent ( TE ) for irrigation has shown adverse impacts on soil transport properties, and introduces certain health risks due to the persistent exposure of soil biota to anthropogenic compounds (e.g., promoting antibiotic resistance). The availability of desalinated water ( DS ) for irrigation expands management options, and improves yields while reducing irrigation amounts and salt loading into the soil. Quantitative models are used to delineate trends associated with long-term use of TE and DS considering agricultural, hydrological and environmental aspects. The primary challenges to the sustainability of agro-ecosystems lies with the hazards of saline and sodic conditions, and the unintended consequences on soil hydro-ecological functioning. Multidisciplinary approaches that combine new scientific knowhow with legislative, economic and societal tools are required to ensure safe and sustainable use of water resources of different qualities. The new scientific knowhow should provide quantitative models for integrating key biophysical processes with ecological interactions at appropriate spatial and temporal scales. This article is protected by copyright. All rights reserved.

Description: Infiltration is a key process in aspects of hydrology, agricultural and civil engineering, irrigation design, and soil and water conservation. It is complex, depending on soil and rainfall properties, and initial and boundary conditions within the flow domain. During the last century, a great deal of effort has been invested to understand the physics of infiltration and to develop quantitative predictors of infiltration dynamics. Jean-Yves Parlange and Wilfried Brutsaert have made seminal contributions, especially in the area of infiltration theory and related analytical solutions to the flow equations. This review retraces the landmark discoveries and the evolution of the conceptual approaches and the mathematical solutions applied to the problem of infiltration into porous media, highlighting the pivotal contributions of Parlange and Brutsaert. A historical retrospective of physical models of infiltration is followed by the presentation of mathematical methods leading to analytical solutions of the flow equations. The review then addresses the Time Compression Approximation (TCA) developed to estimate infiltration at the transition between pre- to post-ponding conditions. Finally, the effects of special conditions, such as the presence of air and heterogeneity in soil properties, on infiltration are considered.

Description: Deviations in the Priestley-Taylor (PT) coefficient α PT from its accepted 1.26 value are analyzed over large lakes, reservoirs, and wetlands where stomatal or soil controls are minimal or absent. The datasets feature wide variations in water body sizes and climatic conditions. Neither surface temperature nor sensible heat flux variations alone, which proved successful in characterizing α PT variations over some crops, explain measured deviations in α PT over water. It is shown that the relative transport efficiency of turbulent heat and water vapor is key to explaining variations in α PT over water surfaces, thereby offering a new perspective over the concept of minimal advection or entrainment introduced by PT. Methods that allow the determination of α PT based on low frequency sampling (i.e. 0.1 Hz) are then developed and tested, which are usable with standard meteorological sensors that filter some but not all turbulent fluctuations. Using approximations to the Gram determinant inequality, the relative transport efficiency is derived as a function of the correlation coefficient between temperature and water vapor concentration fluctuations ( R Tq ). The proposed approach reasonably explains the measured deviations from the conventional α PT =1.26 value even when R Tq is determined from air temperature and water vapor concentration time series that are Gaussian-filtered and sub-sampled to a cutoff frequency of 0.1 Hz. Because over water bodies, R Tq deviations from unity are often associated with advection and/or entrainment, linkages between α PT and R Tq offer both a diagnostic approach to assess their significance and a prognostic approach to correct the 1.26 value when using routine meteorological measurements of temperature and humidity. This article is protected by copyright. All rights reserved.

Description: Abstract Irrigated agriculture will have to increase production to meet the demand for food of the population of the world. A simple physically‐based method is presented that allows to determine appropriate irrigation rate and duration to avoid runoff, thus contributing to the design of efficient irrigation. The method relies on the infiltration capacity curve of the soil under interest. This curve allows determination of two important relationships: (a) maximal irrigation rate vs. irrigation dose, and (b) irrigation duration vs. rate. Two case studies illustrate the application of the method to adapt irrigation to the reduction of soil infiltration capacity resulting from the quality of the irrigation water (treated wastewater) or soil surface sealing. The range of irrigation rates for which duration can be estimated as the ratio between the required dose and the application rate is defined.

Description: Abstract Water vapor is a key element of the water regime in unsaturated profiles above deep aquifers in hyper‐arid regions. However, the interactions between water phases and the resulting evaporation and condensation are poorly understood under such conditions. The main driver for vapor condensation in deep vadose zone profiles is the geothermal gradient, displaying a decrease in temperatures toward the soil surface, thereby promoting condensation. We have analyzed the water regime in deep unsaturated profiles, with and without the geothermal gradient, and considered two types of hydrological scenarios: (1) assuming hydraulic continuity of liquid water over the entire profile and (2) assuming the presence of an evaporative front in the profile above which water flows to the surface in the vapor phase. We considered homogeneous profiles of two soil types, investigating the distribution with depth of the different state variables: temperature, relative humidity, water potential, and vapor pressure and concentration. We found that during evaporation, only extreme conditions of high relative humidity near the surface could lead to condensation. In addition, even when hydraulic continuity of liquid water is assumed over the entire soil profile, potential condensation amounts are very small, practically negligible. For the case of a water table at 200‐m depth, condensation occurs only when the relative humidity at the surface is above 95% and is less than 1.5% of the amount of water in the vapor phase in the profile.

Description: Spatial and temporal variability of water content in the upper soil layer, close to the surface, affects the intensity of hydrological processes. The impact of accounting for soil surface sealing on the spatial and temporal variability of the water content at the hillslope scale was studied in a semiarid environment. Relevant physical properties of the experimental site were derived by means of extensive field surveys, resulting in a detailed database used to characterize the 8240 cells used to represent the hillslope domain. The simulated spatial and temporal water content variability was achieved by aggregating a numerical 1-D simulation at each of these cells. Accounting for surface sealing improved water content predictions, more efficiently during the drying regime. Furthermore, extensive synthetic simulations show the sealed layer to be a highly efficient mechanism reducing temporal water content variability, compared to an unsealed system. It was also found that reduced evaporation in the sealed domain compensates for the loss in infiltrated water due to runoff enhancement. Depending on rainfall intensity and soil depth, a transition could occur from a positive feedback mechanism where the seal layer suppresses evaporation and conserves water stored in the profile to a negative one where the seal layer mainly reduces infiltration and increases water losses through runoff. Thus, the sealing process was found to substantially affect water budgets at all observed scales in the experimental site.

Description: Across many soil types and conditions, post-wetting soil internal drainage exhibits predictable dynamics that lead to a stable and repeatable hydration state termed “field capacity” ( FC ). Soil regulation of internal drainage towards FC has long been recognized as producing a useful hydrologic benchmark for modeling and for estimation of plant available soil water. To overcome ambiguities and inconsistencies in various ad-hoc definitions of FC , we propose using a soil intrinsic characteristic length (a matric potential value derived from drainable soil pore size distribution) to characterize the loss of hydraulic continuity associated with the attainment of FC . The resulting static criterion for FC was extended to formulate a self-consistent dynamic criterion based on soil internal drainage dynamics. A systematic evaluation of the proposed definitions of FC using numerical simulations and experimental data reveals remarkable consistency and predictability across a wide range of soil types. The new metrics add definitiveness and robustness of this widely used concept with potential expansion to additional agronomic, hydrologic, ecological and climatic applications.