Understanding soil moisture variability and its relationship with water content at various scales is a key issue in hydrological research. In this paper we predict this relationship by stochastic analysis of the unsaturated Brooks‐Corey flow in heterogeneous soils. Using sensitivity analysis, we show that parameters of the moisture retention characteristic and their spatial variability determine to a large extent the shape of the soil moisture variance‐mean water content function. We demonstrate that soil hydraulic properties and their variability can be inversely estimated from spatially distributed measurements of soil moisture content. Predicting this relationship for eleven textural classes we found that the standard deviation of soil moisture peaked between 0.17 and 0.23 for most textural classes. It was found that the β parameter, which describes the pore‐size distribution of soils, controls the maximum value of the soil moisture standard deviation.

The dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties. Measurements are made with a sensor that has two parallel cylindrical probes: one for introducing a pulse of heat into the soil (heater probe) and one for measuring change in temperature (temperature probe). We present a semianalytical solution that accounts for the finite radius and finite heat capacity of the heater and temperature probes. A closed‐form expression for the Laplace transform of the solution is obtained by considering the probes to be cylindrical perfect conductors. The Laplace‐domain solution is inverted numerically. For the case where both probes have the same radius and heat capacity, we show that their finite properties have equal influence on the heat‐pulse signal received by the temperature probe. The finite radius of the probes causes the heat‐pulse signal to arrive earlier in time. This time shift increases in magnitude as the probe radius increases. The effect of the finite heat capacity of the probes depends on the ratio of the heat capacity of the probes (C0) and the heat capacity of the soil (C). Compared with the case where C0/C = 1, the magnitude of the heat‐pulse signal decreases (i.e., smaller change in temperature) and the maximum temperature rise occurs later when C0/C > 1. When C0/C < 1, the magnitude of the signal increases and the maximum temperature rise occurs earlier. The semianalytical solution is appropriate for use in DPHP applications where the ratio of probe radius (a0) and probe spacing (L) satisfies the condition that a0/L ≤ 0.11.

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[1] The heat pulse probe (HPP) technique has been successfully applied for estimating water flux density (WFD)

Evaporation is a significant part of the water cycle in hyper‐arid environments. The subsurface of these deserts is characterized by deep groundwater with negligible recharge, whereby water flows from the water table to the surface and evaporates. We propose an analytical model to predict the evaporation rate and the position of the evaporative front. The model accounts for water table depth, atmospheric conditions, and soil hydraulic properties. We consider steady state flow, with two distinct regions separated by an evaporative front, liquid‐phase flow from the water table to the front and vapor‐phase flow from the front toward the surface. The driving forces are pressure head gradients for Darcian liquid flow, and thermal and relative humidity gradients for Fickian diffusive vapor flow. Evaporation rates are predicted for different soil types. The impact of constitutive models applied for characterizing these soils, groundwater depth, and atmospheric conditions are evaluated. Evaporation increases as groundwater levels are shallower, and as atmospheric temperatures increase and/or relative humidity values decrease. Evaporation decreases exponentially with groundwater depth, approaching a constant value of about 0.02 mm per year under typical atmospheric conditions and water table depths below 500 m. The impact of soil type and other related uncertainties are important when groundwater is shallower than 300 m. The relative portion of the liquid phase region increases compared to that of the vapor one as evaporation rates increase. The actual size of the liquid phase flow region, however, reaches its maximum when the water flux approaches zero at hydrostatic conditions.

[1] Surface-exposed fractures (SEFs) form a unique link between the atmosphere and the deep vadose zone. Quantifying evaporation and salt-accumulation rates within these SEFs is essential for understanding processes leading to groundwater salinization and contamination via these fractures. In this study, evaporation from SEFs (ESEFs) was quantified, mainly as a function of ambient atmospheric temperature, by using large-scale laboratory experiments and measuring evaporation under controlled conditions. In addition, ESEF was theoretically quantified based on the physical processes that govern it. The theoretical model was used to analyze ESEF rates as a function of ambient temperature, temperature gradient, fracture-aperture, and matrix pore size. ESEF was experimentally found to increase as the ambient temperature decreased. Measured evaporation rates were between about 110 and 260 g d À1 per square meter of fracture surface, for temperature differences between rock-bottom and the atmosphere of 0°and 13°C, respectively. Comparing these values with model results suggests that convection is the driving process for enhanced evaporation at low ambient temperatures. Finally, we show that ESEF rates decrease as a result of salt precipitation. During a $9-month period, with an imposed temperature difference of 13°C, ESEF decreased from $260 to $95 g d À1 m À2 due to salt accumulation near and on the fracture surfaces. Evaporation rates began decreasing after about 100 g m À2 of salt had precipitated and decreased to less than 50% of the initial rate after 160 g m À2 of salt had precipitated. We thus show that not only temperature, but also salt precipitation, largely affect ESEF rates.

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The dual-probe heat-pulse (DPHP) method is attractive for measuring soil thermal properties and volumetric water content. The purpose of this study was to develop and test a DPHP sensor having rigid probes made from thickwalled stainless steel tubing (2.38-mm outside diameter). The probes of this sensor are much more resistant to deflection than those of conventional DPHP sensors, decreasing measurement error caused by probe deflection during insertion into the soil. Laboratory experiments were conducted across a wide range of saturation levels with glass beads and three soils of different textures. For inferring soil properties from the proposed sensor, we applied the recently developed identical cylindrical perfect conductors (ICPC) model instead of the infinite line source (ILS) model that is typically used. The ICPC model improves solution for heat transport through the probe-soil system by accounting for the heat capacity and radius of the probes. Our results show a root mean square error of 1.4% volumetric water content and elimination of the measurement bias typically encountered with DPHP measurements. We conclude that the improved sensor, in combination with the ICPC model, provides a general, soil-independent water content estimate that is especially suitable for field soil water content monitoring because of its robust design with rigid probes. Because of its simplicity and measurements independent of soil type, we propose the presented DPHP method as an excellent alternative to other available measurement techniques for soil water content.Abbreviations: DPHP, dual-probe heat-pulse; ICPC, identical cylindrical perfect conductors; ILS, infinite line source.