Jean-Pierre Vandervaere

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).

Estimating hydraulic conductivity of crusted soils using disc infiltrometers and minitensiometers

Although soil crusting has long been recognized as a crucial runoff factor in the Sahel, very few field methods have been developed for the measurement of the crust hydraulic conductivity, which is difficult to achieve because of the small thickness of most surface crusts. A field method, based on the simultaneous use of disc infiltrometers and minitensiometers is proposed for determining the crust hydraulic conductivity and sorptivity near saturation. On crusted soils, the classical analysis of the steady state water flow was found to be inadequate. The proposed method is based on sorptivity measurements performed at different water supply potentials and uses recent developments of transient flow analysis. A minitensiometer, placed horizontally at the crust-subsoil interface, facilitated the analysis of the infiltration regime for the crust solely. Results are shown for representative soil units of the East Central Super Site of the HAPEX-Sahel experiment: fallow grasslands, millet fields and tiger bush. Non-crusted soils were also considered and validated the transient method as demonstrated by comparison with Woodings steady state solution. This validation was obtained in the case of fallow grasslands soil but not for the millet fields. In this latter case, the persistent effects of localized working of the soil to remove weeds caused large variations in infiltration fluxes between the sampling points, which tended to dominate over effects of differences in applied potential. For the tiger bush crusted soils, the ratio of the saturated hydraulic conductivity of the crust to that of the underlying soil ranges from 13 to 16, depending on whether the crust is of a structural (ST) or sedimentation (SED) type. The method also allows the estimation of a functional mean pore size, consistent with laboratory measurements, and 40% less for the crusts in comparison with the underlying soil. The results obtained here will be used in hydrological models to predict the partition of rainfall between infiltration and runoff.

Soil measurements during HAPEX-Sahel intensive observation period

This article describes measurements made at each site and for each vegetation cover as part of the soils program for the HAPEX-Sahel regional scale experiment. The measurements were based on an initial sampling scheme and included profile soil water content, surface soil water content, soil water potential, infiltration rates, additional measurements on core samples, and grain size analysis. The measurements were used to categorize the state of the surface and profile soil water regimes during the experiment and to derive functional relationships for the soil water characteristic curve, unsaturated hydraulic conductivity function, and infiltration function. Sample results for different supersites and different vegetation covers are presented showing soil water profiles and total soil water storage on days corresponding to the experimental ‘Golden Days’. Sample results are also presented for spatial and temporal distribution of surface moisture content and infiltration tests. The results demonstrate that the major experimental objective of monitoring the supersites during the most rapid vegetative growth stage with the largest change of the surface energy balance following the rainy season was very nearly achieved. Separation of the effects of probable root activity and drainage of the soil profile is possible. The potential for localized advection between the bare soil and vegetation strips of the tiger bush sites is demonstrated

Unidimensional modelling of a fallow savannah during the HAPEX-Sahel experiment using the SiSPAT model

Abstract In the framework of the HAPEX-Sahel experiment, a data set was gathered on a fallow savannah site of the Central East Supersite. This includes 54 days of atmospheric forcing (air temperature and humidity, wind speed, solar and long-wave radiation and rainfall), net radiation, sensible, latent and soil heat fluxes and soil temperature series at a time step of 20 min. Furthermore, 17 soil moisture profiles, the evolution of the leaf area indices and some soil characteristics were available. The data set was used, at the field scale, to calibrate and validate the SiSPAT (simple soil plant atmosphere transfer) model, a 1D model of coupled heat and mass transfer in the soil-plant-atmosphere continuum. The objectives of the study were (i) to assess the performances of the model in the prediction of the diurnal cycle of net radiation, turbulent fluxes, soil temperatures and the evolution of soil water content over a period of 54 days (day of the year 239–292, 1992), characterized by early stage intense rainfall events and fast drying afterwards, (ii) to analyse the influence of soil surface crust on the water balance and (iii) to identify the 1D modelling limits when the surface area consists of two strates: a ground sparse herb layer, characterized by a large spatial variability of surface properties and water content with scattered bushes. The model was calibrated over a 2-week period and then run over the whole 54-day period. We were able to reproduce the main characteristics of the observed net radiation, turbulent fluxes, soil temperature and soil moisture for the intense rainfall events and for an elongated dry period. Nevertheless, when the crust was not taken into account, the rainfall-runoff-infiltration process and the evapotranspiration after rain were poorly predicted (overestimation of evapotranspiration of infiltration). When a crust was considered to model the water balance at the field scale, its influence was found to be substantial on the runoff generation and the infiltration, and consequently on the bare soil evaporation. However, runoff predictions were much larger than the observations. Indeed, at the field scale, no runoff was generally observed. Lateral redistribution of water between crusted and noncrusted zones was observed in the plot. However, this cannot be taken into account with the presented 1D deterministic modelling. Hence further model development is needed to yield a better representation of soil water fluxes at the field scale.

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

Error analysis in estimating soil water balance of irrigated fields during the EFEDA experiment: 2. Spatial standpoint

Abstract In Part 1 of this paper, an error analysis for estimating the variance of volumetric water content, soil water storage and actual evapotranspiration from a local standpoint was presented. This second part extends the analysis to the case of spatial average values corresponding to a field scale. For a low number of sampling points, the location component related to the spatial variability is the major component of the total variance of the foregoing variables. Results are presented for the two maize fields described in Part 1 and comparison is made with values of evapotranspiration provided by atmospheric latent heat flux measurements for one of them. Finally, the determination of the calibration, instrument, integration and location components of the variance of a given variable leads to an estimation of the number of sampling points required to estimate field average values with a prescribed degree of precision. This number varies significantly depending on the variable considered — water content, water storage or evapotranspiration — and proved to be different for the two fields.

An experimental analysis of hydrodynamic behaviour on soils and hillslopes in a subtropical mountainous environment (Western Sierra Madre, Mexico)

Many soils parameters and features play a role in explaining the hydrodynamic behaviour of a watershed. Textural data are relatively easy to obtain and to spatialise, due to their low spatial variability. Inversely, structural features usually exhibit great spatial variability and thus, are difficult to interpolate and to integrate in the framework of a hydrologic model. However, structural characteristics of the soils seem to have a greater influence on catchment hydrological balance than the textural ones. The objectives of this study were to identify which parameters control the soil surface hydrological behaviour and quantify the magnitude of their spatial variability. Measurements of soils characteristics, including bulk density and hydraulic conductivity, were carried out in five watersheds in the Western Sierra Madre (Northern Mexico). It is shown in this study that on a natural grassland under subtropical mountainous climate, spatial variability of soil hydraulic conductivity is almost as high at a 1-m2 scale as at a 1-km2 one. The main discriminating variables which account for the spatial variability appeared to be the lithology and overall the soil surface features, both being related. The latter seems to be a synthetic indicator of basin hydrodynamic behaviour, and to be easier than others to spatialise.

Runoff evolution due to land-use change in a small Sahelian catchment

Abstract Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall–runoff relationships for two periods (1991–1994 and 2004–2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km2 each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagener

Mapping topsoil field-saturated hydraulic conductivity from point measurements using different methods

Abstract Topsoil field-saturated hydraulic conductivity, Kfs, is a parameter that controls the partition of rainfall between infiltration and runoff and is a key parameter in most distributed hydrological models. There is a mismatch between the scale of local in situ Kfs measurements and the scale at which the parameter is required in models for regional mapping. Therefore methods for extrapolating local Kfs values to larger mapping units are required. The paper explores the feasibility of mapping Kfs in the Cévennes-Vivarais region, in south-east France, using more easily available GIS data concerning geology and land cover. Our analysis makes uses of a data set from infiltration measurements performed in the area and its vicinity for more than ten years. The data set is composed of Kfs derived from infiltration measurements performed using various methods: Guelph permeameters, double ring and single ring infiltrotrometers and tension infiltrometers. The different methods resulted in a large variation in Kfs up to several orders of magnitude. A method is proposed to pool the data from the different infiltration methods to create an equivalent set of Kfs. Statistical tests showed significant differences in Kfs distributions in function of different geological formations and land cover. Thus the mapping of Kfs at regional scale was based on geological formations and land cover. This map was compared to a map based on the Rawls and Brakensiek (RB) pedotransfer function (mainly based on texture) and the two maps showed very different patterns. The RB values did not fit observed equivalent Kfs at the local scale, highlighting that soil texture alone is not a good predictor of Kfs.

Impact of Land Use Changes in the Surface Hydrodynamics of a Water-harvesting Basin

Watershed management requires systematic measurements in order to reach good resources control. Some processes conditioning water transfers in the environment such as rain, run-off, infiltration, deep percolation, water uptake by plants, and evapotranspiration should be quantified to improve our knowledge of hydrological characterization and of the best management practices (Descroix/Nouvelot, 1997; Descroix et al., 2004). This work presents infiltration measurements and results produced with the Suction Disc Infiltrometer method realized in the Upper Nazas River Basin. This basin is the main water supply for Irrigation District 017 in northern Mexico, called the Lagunera region. Results show that soil surface controls the hydrodynamic behaviour of the watershed. Land use changes can be linked with productive practices that are causing strong hydrological consequences.