The significance of the soil surface moisture (0s) with respect to daily bare soil evaporation (Ea) is analyzed using data from numerical simulations. This was performed by running a mechanistic model of heat and water flows in the soil. The mechanistic model was calibrated and validated on three different soils with contrasted hydraulic properties. Results show that Os does not allow an accurate estimate of the ratio of Ea to the daily potential evaporation (EPa). Added variables such as Epa and the wind velocity are necessary to improve the prediction of the Ea/Epa ratio. In a second attempt, a simple model of E a is proposed. It is based on three empirical parameters and required the measurement of Os obtained at midday, Epa, and the daily wind velocity. For a given soil, a single set of the three parameters allows an accurate estimate of Ea under soil moisture and climatic conditions encountered through the year in temperate areas.

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Plant water uptake has classically been supposed to depend on the leaf water potential (LWP) and LWP was assumed to control the stomatal conductance. A new approach to stomatal control has recently been proposed in which root water potential (RWP) determines stomatal aperture, and hence transpiration, by means of chemical messages. In such a case, models that provide precise calculations of RWP become essential for understanding soil‐root water transport. The objective of this paper is to propose a theoretical approach to calculate RWP and water uptake by roots in the X‐Y plane for any spatial root distribution. For the calculations of RWP, we assumed that the plant continuously adjusted its RWP so as to minimize the difference between the maximum evapotranspiration (MET) and the amount of water extracted by the root system. Water release from the roots in the soil was considered to be negligible, so roots in dry soil were temporarily removed from the pool of active roots. A Galerkin finite element method was used to solve Richards' equation. Examples of calculation are presented for a nonuniform root distribution under both wet and dry initial situations. Root water potential and the number of active roots varied greatly with time. Results suggested that no simple relationship existed between the plant water uptake and the root water potential. Furthermore, soil water‐potential maps showed a close relationship between water uptake and root positions, and great differences between RWP and the mean soil water potential could be calculated. A greater or lesser water redistribution toward the roots occurred during the night. Finally, the proposed approach may be validated with precise soil and plant measurements.

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots ('soil resistance') is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated 'soil resistances' were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that 'soil resistance' is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication. 2040

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A physical model is proposed for predicting the transport of the stable isotopes of water (H2 18O and HD16O) in soils under natural climatic conditions. The transport equation for stable isotopes in the unsaturated zone was coupled with a mechanistic model for water and heat exchange across the soil‐atmosphere boundary. Diffusive and convective isotope fluxes were each considered in liquid and vapor phases of the soil. Turbulent exchanges of isotopes between the soil surface and the atmosphere were modeled by use of similarity theory in the same way as water vapor. Boundary conditions at the soil surface are based on an energy balance and turbulent fluxes of heat, water, and isotopes. The resulting model provides a framework for understanding the behavior of stable isotopes in soils of variable hydraulic properties and under various climatic conditions.

To investigate the distribution of parathion [O,O-diethyl O-(4-nitrophenyl) phosphorothioate] and its highly toxic metabolite paraoxon [O,O-diethyl O-(4-nitrophenyl)phosphate] between the soluble and sorbed pools in the soil, batch experiments were conducted to evaluate the rate of adsorption and desorption of 14C-labeled parathion and paraoxon in soil. The mineralization and degradation of these products were also investigated during a 56-d experiment under controlled laboratory conditions. Adsorption patterns indicated initial fast adsorption reactions occurring within 4 h for both parathion and paraoxon. We also observed the formation of nonextractable residues. The paraoxon was more intensively degraded than the parathion, and production of p-nitrophenol and other metabolites was observed. A kinetic model was developed to describe the sorption and biodegradation rates of parathion, taking into account the production, retention, and biodegradation of paraoxon, the main metabolite of parathion. After fitting the parameters of the model we made a simulation of the kinetics of the appearance and disappearance of paraoxon. From the simulation we predicted a quantity of metabolite in the liquid phase amounting to 1% of the quantity of parathion initially applied. This is in agreement with the experimental data.

Summary The capacitance probe is an attractive device for monitoring soil moisture automatically. However, its sphere of influence is rather small (a few cubic centimetres only). We have analysed the possibility of monitoring moisture at the field scale using only a few probes (≤3). We calibrated each probe by establishing a direct relation between the field average soil moisture θf and the signal given by the probe. As in earlier studies, we found that a linear relation is generally suitable. A classical statistical analysis was performed to assess the error of a single probe. When replicate probes were installed, we obtained replicate estimates of θf. We proposed an estimator θf that combines all replications optimally. Three experiments each lasting several months were carried out on bare tilled fields to evaluate the probe against gravimetric measurements. Our results show that the calibrations differ significantly from one probe to another. Once calibrated, the capacitance probe provided accurate soil moisture measurements (70% of the calibration relations had residual standard deviations < 0.02m3m−3), but it is advisable to have at least two replicate probes. Soil water storage was well estimated by combining four to seven probes to establish the moisture profile, despite the error induced by each probe. Moreover, the temporal variations in water storage were accurately measured by the probes. We found an error of 0.6 mm day−1 (standard deviation) in daily variation of the water storage, which partly involved the error made on the reference measurements (gravimetric method).