The Alpilles-ReSeDA program was initiated to develop and test methods for interpreting remote sensing data that could lead to a better evaluation of soil and vegetation processes. This article presents the experiment that was setup in order to acquire the necessary data to achieve this goal. Intensive measurements were performed for almost one year over a small agricultural region in the South of France (20 kilometers square). To capture the main processes controlling land-atmosphere exchanges, the local climate was fully characterized, and surface energy fluxes, vegetation biomass, vegetation structure, soil moisture profiles, surface soil moisture, surface temperature and soil temperature were monitored. Additional plant physiological measurements and a full characterization of physical soil parameters were also carried out. After presenting the different types of measurements, examples are given in order to illustrate the variability of soils and plant processes in the area in response to the experienced climate. surface energy fluxes / evapotranspiration / soil moisture / soil physical properties / experiment / vegetation characterization Résumé-Suivi des échanges d'énergie et de masse au cours de l'expérimentation Alpilles-ReSeDA. Le programme Alpilles-ReSeDA a été mis en place pour développer et tester des méthodes permettant une meilleure utilisation des données de télédétection pour le suivi du fonctionnement des sols et des cultures. Cet article présente l'expérimentation qui a été réalisée pour acquérir un jeu de données permettant cette analyse. Des mesures intensives ont été réalisées pendant presque une année sur une petite région agricole du Sud de la France (20 kilomètres carrés). De façon à suivre l'ensemble des processus contrôlant les échanges surface-atmosphère, l'ensemble des paramètres climatiques locaux ont été mesurés, ainsi que les flux d'énergie de surface, les caractéristiques de structure de la végétation et du sol, l'humidité et les températures du sol, la température de surface. Des mesures des paramètres physiologiques des plantes et des caractéristiques physiques des sols ont également été entreprises. Après avoir présenté les différents types de mesures réalisées, des exemples présentant la variabilité des couverts végétaux et des sols dans la zone d'étude sont présentés.
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).
The influence of temperature on the dielectric permittivity of soil is the result of counteracting effect that depends on the soil's composition and mineralogy. In this paper, laboratory experiments showed that for a given water content, the soil dielectric permittivity was linearly related to the temperature, with a slope (α) that varied between samples taken in the same soil. These variations are difficult to predict and therefore, a simple and straightforward algorithm was designed to estimate α based on the diurnal patterns of both the measured dielectric permittivity and the soil temperature. The underlying idea is to assume that soil water content variations can be known with a reasonable accuracy over an appropriate time window within a day. This allows determining the contribution of the soil water content to the dielectric permittivity variations and then, the difference with the observed measurements is attributed to the soil temperature. Implementation of the correction methods in a large number of experiments significantly improved the physical meaning of the temporal evolution of the soil water content as the daily cycles for probes located near the surface or the long-term variations for more deeply installed probes.
Solving soil unsaturated flow problems requires knowledge of the water retention θ(h) and unsaturated hydraulic conductivity K(θ) relationships. The purpose of this study was to investigate more thoroughly the properties and accuracy of an evaporation method described by Wind (1969) for determining θ(h) and K(θ) from laboratory cores. Evaporation from a vertical column of soil was first simulated using numerical solution of Richards equation for a given set of soil hydraulic properties. The simulated data were then used to evaluate the ability of Wind's method to provide estimations of the retention and unsaturated conductivity curves when measurement errors were taken into account. The main sources of error were (i) errors due to the position of the tensiometers in the sample, (ii) errors due to the calibration of the transducers used for the pressure head measurements, and (iii) errors due to layering in the soil column. The estimated water retention curves were sensitive only to soil layering. On the other hand, small uncertainties in the position of the tensiometers (1 or 2 mm) and in the calibration curve of the transducers for the pressure head measurements (1–5%) had a great influence on the estimated hydraulic conductivity curves. A correction procedure was proposed and was satisfactory when errors of position in the tensiometers were taken into account. Results also showed that temperature corrections related to viscosity of liquid water were large. Finally, this method gives poor estimates of hydraulic conductivities when raw tensiometric data are corrupted with small errors in their position or calibration.
The influence of air convection on soil aeration has not been investigated in great detail. A study was conducted to measure air pressure changes by depth in the first 2 m of a prairie soil that was known for its denitrification properties. An absolute air pressure probe was used to measure air pressure fluctuations at the soil surface and newly designed differential air pressure probes were placed at depths of 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1, 1.5, and 2 m. A procedure of in situ recalibration was used every 6 or 7 wk for these probes. At the same depths, capacitive probes and thermocouples measured the volumetric water content and temperature, respectively. The bulk density profile, water table level, and rainfall amounts were recorded. The atmospheric pressure ranged between 97 000 and 101 000 Pa during the experiment. The signal drift of the air pressure probes was negligible for four of the probes and <50 Pa for the others. The accuracy of the measurements with signal drift compensation was superior to 10 Pa. Differences of air pressure between the soil at the surface and at depth may be >2000 Pa. These differences, which are not necessarily linked to the depth, are probably due to soil heterogeneities and generally to water infiltration. Air pressure seemed to level out quickly when the air‐filled pore space was continuous. This generally prevented “dephasing‐amortization” effects. When the air‐filled pore space was discontinuous, differential air pressures depended simultaneously on the atmospheric pressure and temperature fluctuations.
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