Jean-Louis Thony

Field measurement of soil surface hydraulic properties by disc and ring infiltrometers A review and recent developments

Abstract Soil management influences physical properties and mainly the soil hydraulic functions. Their measurement becomes one of the research preferences in this branch of applied soil science. Tension disc and pressure ring infiltrometers have become very popular devices for the in situ estimates of soil surface hydraulic properties. Their use for measuring solute–water transfer parameters of soils is now well established too. A number of publications testify that both devices have been extensively used all around the world for different purposes. In this review, a short introduction is devoted to the background theory and some examples are given to show how the theory can be used to determine hydraulic conductivity and sorptivity from measured cumulative infiltration. The methods of analysis of cumulative infiltration are based either on quasi-analytical solutions of the flow equation for homogeneous soil profile or on inverse parameter estimation techniques from the numerical solution of flow equation whether the soil profile is homogeneous or not. The disc infiltrometer has also been shown as a suitable device for inferring parameters describing the water-borne transport of chemicals through near saturated soils. Associated with conservative tracers, it has been recognized as a promising tool for the determination of both hydraulic and solute transport properties as well as for other parameters such as mobile/immobile water content fraction or exchange coefficient. An emphasis is put here on some published studies performed in different soils and environmental conditions focusing on heterogeneous soil profiles (crusted soils) or structured cultivated soils (aggregated soils), either when local water transport process is studied or when field spatial variability is investigated. Some new research studies such as water–solute transfer in structured or swelling–shrinking soils and multi-interactive solute transport are emerging. A number of challenges still remain unresolved for both theory and practice for tension and pressure infiltrometers. They include questions on how to consider and characterize saturated–unsaturated preferential flow or preferential transport process (including hydrodynamic instabilities) induced by biological activity (e.g. capillary macropores, earthworm holes or root channels) by specific pedagogical conditions (e.g. cracking, crusting) and by soil management practices (i.e. conservation tillage).

A simple soil-plant-atmosphere transfer model (SiSPAT) development and field verification

Abstract When examining the various soil-plant-atmosphere models proposed in the literature, it becomes obvious that, according to the speciality of their authors, one or several compartments of the model are generally very detailed, whereas the other compartments remain crude. The aim of this work was first, to build a model, including the main physical processes, but with equivalent degrees of simplification for all the compartments and, second, to provide a validation as complete as possible for the various compartments. The resulting model, driven by meteorological forcing at a reference level (incoming solar and long-wave radiation, air temperature, humidity and wind speed, and rainfall), can be divided into four main compartments. In the soil, coupled heat and mass transfer equations, including liquid and vapour phase transfer, are solved. In the atmosphere, stability is taken into account in the calculation of the aerodynamic resistances. At the soil-plant-atmosphere interface, one vegetation layer is considered, with two energy budgets: one for the bare soil fraction of the plot and one for the vegetated fraction. In the soil, root uptake is modelled using an electrical analogue scheme with various resistances (soil, root, xylem). Finally, in the case of rainfall (or irrigation), interception, infiltration and runoff is calculated. The model is first described and then compared with field data collected on a soybean plot of 0.72 ha. The soil is composed of three horizons, the hydraulic and thermal properties of which were determined experimentally. The atmospheric forcing and the net radiation were measured. The sensible heat flux above the canopy was deduced from wind speed and temperature profiles. In the soil, water pressure, water content and temperature were measured at several depths. Temperature profiles also allowed for the derivation of the soil heat flux at the ground surface and the latent heat flux was obtained from the energy budget. Plant height, leaf area index and leaf water potential were also recorded on several days. Seven days of complete measurements were available: 2 days were under dry conditions (19–20 August 1991) and 5 days under wet conditions (24–28 August 1991) following a rainfall of 46 mm on 22 August 1991. Missing parameters were calibrated using the first 3 days of the wet period (24–26 August 1991) and the model was validated on the remaining days. A fair agreement between the model and the data was observed for both atmospheric fluxes, for soil variables (water content and temperature) and for leaf water potential, provided only an accurate determination of the parameters was made.

Field measurement of the hydraulic properties of soil

Abstract Disc permeameters are used here on two contrasting soils to determine absorption and transmission characteristics in the potential range from saturation down to ψ 0 = −100 mm. One soil was a recently-ploughed loam, the other a cracking-prone heavy clay. Despite the dominance of gravity in the former case, and of capillarity in the latter, a rapid onset of geometrically-induced steady flows q ∞ , was observed from disc permeameters in both cases. For the loam a twin-disc analysis of q ∞ from discs of different radii, had to be used to determine the sorptivity S 0 . Whereas for the clay the ample square-root-of-time behaviour gave S 0 as well. The conductivity K 0 was then derived from q ∞ via Woodings equation. Another value of K 0 derived for the ploughed loam from a standard determination with a large, buffered ring, did not agree with that from the disc permeameter. This failure arose because of the vertical drop-off in K that rendered impossible the attainment of one-dimensional flow from the inner ring. The loam possessed the hydraulic properties expected of such a medium-textured soil that had recently been disturbed by ploughing. The clay however had a 40-fold jump in K right at saturation reflecting the presence of cracks.

Field measurements of water and nitrogen losses under irrigated maize

Abstract An intensive multidisciplinary experiment has been conducted over several years at La Cote Saint-Andre, near Grenoble, France. The major objective is to determine an optimal fertilizer application scheme for an irrigated agricultural system. Such a scheme would not degrade the quality of the environment, and yet would maintain a profitable level of crop production. This study is explicitly related to the cultivation of irrigated maize, a major crop in the area. The various terms of the water balance (consumption, drainage, soil storage) and of the nitrogen cycle (mineralization, plant uptake, leaching) were obtained from intensive monitoring in the upper layer of the 0.8 m of soil which corresponds to the root zone of the crop. This entailed the combined use of a neutron moisture meter, tensiometers and soil suction cups. To determine the specific effects of fertilization and crop growth, there were different treatments. These corresponded to a traditional fertilizer application of 260 kg N ha −1 , no fertilization, and bare soil, carried out within an area of approximately 2 ha. Several sites were instrumented on each treatment, one of them being specifically for the application and the monitoring of 15 N-tagged fertilizer. The results have shown that, in terms of the water balance, irrigation water management is extremely efficient, as drainage losses under the maize culture are negligible during the crop cycle. The situation is totally different, however, during the intercrop period (October–April), owing to rainfall. Then the soil is left bare and evaporation is very small, and now the drainage corresponds to about 90% of total inputs from precipitation. In terms of the nitrogen cycle, the results showed clearly that up to 150 kg N ha −1 was produced by mineralization in the soil. Nitrogen leaching beyond the root zone during the crop cycle is negligible, regardless of the rate of fertilizer application, as a result of the very small amount of drainage, despite irrigation. A very important contrast was found, however, between the fertilized and unfertilized treatments at harvest. There was a residue of 182 ± 64 kg N ha −1 in the fertilized sites, but none for the others. The whole quantity remaining in the root zone at harvest was then totally leached by winter rains. To decrease the risk of groundwater pollution, a reduction of about 100 kg N ha −1 from the traditional application rate has been recommended. Finally, the method of estimation of N balance has been successfully validated by a comparison between N uptake determined by direct analysis of the whole plant and the value estimated from the temporal variations of the N content in the soil.

Numerical modeling of coupled heat and water flows during drying in a stratified bare soil — Comparison with field observations

A general physically based formulation of water — both in liquid and vapor phases — and heat transport in a partially saturated soil coupled with a lower atmosphere boundary layer modeling is presented. It is driven by surface heat and moisture fluxes estimated from meteorological data. Soil water pressure head and temperature are used as the dependent descriptive variables and the resulting one-dimensional nonlinear equations are solved by a finite element Galerkin method. The numerical results are compared with field data obtained during an experiment conducted on a 3600 m2 bare soil presenting three different horizons in the first 80 cm including a crust of 5 mm thick at the soil surface. The plot was intensively equipped with appropriate sensors in order to measure the time evolution of soil temperature, water content and water potential at different depths and locations, the profiles of wind speed, air temperature and vapor pressure above the surface as well as the different radiation components. Due to experimental uncertainties in the estimation of some parameters (mainly soil hydraulic and thermal properties of the crust) and the lack of determination of others (such as vapor flow coefficients), the model was first calibrated on the first two days of the experimentation and then evaluated on the following five days. For the latter period, very fair agreement between computed and observed values of soil temperature and water content patterns as well as evaporation fluxes demonstrates the reliability of such a model, at least after a calibration phase and, for this specific experimental site, as long as the surface crust is not cracking.

Hydraulic conductivity measurements of the spatial variability of a loamy soil

Abstract The spatial variability of the parameters K fs and α of the exponential form of the hydraulic conductivity given by K ( ψ )= K fs exp (αψ) where ψ is the water pressure head in the soil has been examined on a bare agricultural soil, by performing surface infiltration measurements with the Guelph Pressure Infiltromer. Thirty two measurements were made at each node of a 4 m × 8 m grid. Both the early-time transient and steady-state flow rates, for three hydraulic heads (6, 16 and 25.5 cm) imposed at the soil surface have been analyzed. It is shown that the best estimates of the field-saturated hydraulic conductivity, K fs and of the alpha parameter (α) are given by calculations based on the steady-state regime simultaneously applied to the three heads. The results indicate that both, K fs and α are better described by a lognormal than a normal distribution. They also show that the parameters are autocorrelated up to about 25 m and to 20 m respectively and negatively spatially correlated together within a distance of 24 m. These findings tend to invalidate some assumptions classically used in the stochastic models of unsaturated water flow in porous materials.

Prediction of crust-induced surface runoff with disc infiltrometer data

The process of rainfall infiltration into crusted soil has been the focus of many analytical and numerical studies. However, most of these studies have been supported by laboratory data because no field data were available. Following recent field experiments on crusted soils, which provided crust co

Heat and water exchanges of fallow land covered with a plant-residue mulch layer: a modelling study using the three year MUREX data set

Abstract The MUREX (Monitoring the Usable Soil Reservoir Experimentally) experiment was conducted on fallow land in the Southwest of France. A three year continuous data set, including climatic variables, energy fluxes, surface water content, soil moisture profiles, surface and soil temperature, and evolution of vegetation characteristics was collected. The field possessed a plant-residue mulch layer, formed naturally by the accumulation of decaying and dead biomass. The three-year data set was used to analyse and model the long-term water and heat exchanges of the field using the SiSPAT (Simple Soil Plant Atmosphere Transfer) model. The original version was modified to take into account heat and water transfer within the plant-residue mulch layer. The 1995 data set was used for calibration of unmeasured parameters. Years 1996 and 1997 were used for validation of the approach, using the same parameter set obtained in 1995. Model results and observations were in good agreement for the three years when the plant-residue (mulch) layer effect was considered. The model properly reproduced contrasting responses to different rainfall conditions. Model simulations were used to understand some physical processes modified by the mulch layer. A decrease of 5–10% of annual total evaporation was obtained, as compared to the residue free case, associated with a decrease in soil evaporation and increase of transpiration. The decrease in soil evaporation was responsible for higher surface soil moisture. Daily soil and air temperature profiles were shown to be considerably modified by the mulch layer, an inversion occurring within the mulch, leading to colder averages and a smaller amplitude for soil temperature.

Field characterization of the relationship between electrical potential gradients and soil water flux

Electrical potential differences between electrodes installed vertically at four depths (0.3, 0.5, 0.7 and 0.8 m) were monitored continuously during a 2-month period in a measurement site under natural fallow. Simultaneously, changes in soil water content and in hydraulic head were measured on a daily basis at different depths of the soil profile at the same site. They were analysed to obtain daily values of the soil water flux at the depth z = 0.4 m. This was in particular carried out over a 10-day period following a rainfall event. At that depth the water flux was first oriented downwards (infiltration), then shifted progressively upwards (evaporation). It is clearly shown that there exists a very significant linear correlation between the electric potential gradient at that level and the value of the flux. Owing probably to electrode potential problems, there is a residual value when the flux is null. If the relationship is legitimately forced through the origin, it becomes clear that electrode measurements could be used to infer water circulation in the soil in terms of direction and amount of flow.

Automatic measurement of soil-water pressure using a capacitance manometer

Abstract A new, cheap and reliable pressure transducer has been developed. It is based on the use of a mercury manometer with the outer face of the glass tubing covered with a transparent metallic oxide. This acts as the fixed outer electrode of a capacitor, of which the capacitance linearly changes with the position of the mercury in the tube. This pressure transducer can be linked with an automatic recording device. Examples are given of the field use of a series of these transducers in which they are coupled to tensiometers in a natural water-balance study.