Georges Vachaud

Seasonal water balance of a sandy soil in Niger cropped with pearl millet, based on profile moisture measurements

Abstract In the Sahel, calculation of the field water balance from neutron-probe measurements is often difficult for pearl millet (Pennisetum glaucum (L.) R. Br.), which is due to the rapid drainage (D) of the sandy soils, on which it is typically grown. We present a simple method of calculating D in these soils from weekly neutron-probe data. The method divides the water balance into two phases. In the first, applicable early in the season, water flux across the maximum depth of probe measurement (Zm) is assumed negligible, and evapotranspiration (E) and D are calculated from the change in soil water content (θ) between the bottom of the rootzone (Zr) and Zm, thus allowing calculation of unsaturated hydraulic conductivity, K(θ), from the flux across Zr. In the second phase, when soil water starts to percolate across Zm, D is calculated from K(θ), assuming a hydraulic head gradient of −1. The method is used to calculate a one-dimensional water balance of a pearl-millet crop grown in a deep sandy soil at two fertility levels during a season of normal rainfall. Results show that the calculated K(θ) functions compare well with those based on laboratory measurements. An acceptable estimate of drainage, and therefore E could be made. Mean cumulative E and D were, respectively, 211 and 207 mm for the unfertilized crop, and 268 and 148 mm for the fertilized crop with 440 mm of rainfall received during the crop cycle. The fertilized millet crop water balance was simulated, which compared to the calculation method resulted in an about 10% higher seasonal E and a 10% lower seasonal D. Our study shows that E can be corrected for D using a simple but accurate method, and consistent with other studies in the region indicates that rainfall is usually not the primary limiting factor to pearl-millet production.

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.

A distributed physical approach for surface-subsurface water transport modeling in agricultural watersheds

Abstract Surface cover and soil type have a major influence upon groundwater recharge and groundwater quality in agricultural watersheds. However, several hydrological models focus on simulating groundwater recharge without including the influence of agricultural practices and soil characteristics. In this study, ANSWERS, a distributed parameters surface nonpoint source model has been modified to include the simulation of water transport in the vadose and saturated zones. This model takes into account the spatial and temporal variability of crop cover and management practices, and the spatial variability of soil type and rainfall distribution. It is physically based and uses parameters that can be easily determined from readily available soil and plant information. It has been validated at multiple scales: local scale, field scale and watershed scale. At the local and field scale, it predicts accurately drainage below the root zone and evapotranspiration on different type of soil cover. At the watershed scale, it reproduces well the piezometric levels and trends of variation.

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.

Modeling water quality in an urban river using hydrological factors--data driven approaches.

Contrasting seasonal variations occur in river flow and water quality as a result of short duration, severe intensity storms and typhoons in Taiwan. Sudden changes in river flow caused by impending extreme events may impose serious degradation on river water quality and fateful impacts on ecosystems. Water quality is measured in a monthly/quarterly scale, and therefore an estimation of water quality in a daily scale would be of good help for timely river pollution management. This study proposes a systematic analysis scheme (SAS) to assess the spatio-temporal interrelation of water quality in an urban river and construct water quality estimation models using two static and one dynamic artificial neural networks (ANNs) coupled with the Gamma test (GT) based on water quality, hydrological and economic data. The Dahan River basin in Taiwan is the study area. Ammonia nitrogen (NH3-N) is considered as the representative parameter, a correlative indicator in judging the contamination level over the study. Key factors the most closely related to the representative parameter (NH3-N) are extracted by the Gamma test for modeling NH3-N concentration, and as a result, four hydrological factors (discharge, days w/o discharge, water temperature and rainfall) are identified as model inputs. The modeling results demonstrate that the nonlinear autoregressive with exogenous input (NARX) network furnished with recurrent connections can accurately estimate NH3-N concentration with a very high coefficient of efficiency value (0.926) and a low RMSE value (0.386 mg/l). Besides, the NARX network can suitably catch peak values that mainly occur in dry periods (September-April in the study area), which is particularly important to water pollution treatment. The proposed SAS suggests a promising approach to reliably modeling the spatio-temporal NH3-N concentration based solely on hydrological data, without using water quality sampling data. It is worth noticing that such estimation can be made in a much shorter time interval of interest (span from a monthly scale to a daily scale) because hydrological data are long-term collected in a daily scale. The proposed SAS favorably makes NH3-N concentration estimation much easier (with only hydrological field sampling) and more efficient (in shorter time intervals), which can substantially help river managers interpret and estimate water quality responses to natural and/or manmade pollution in a more effective and timely way for river pollution management.

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.

Stochastic approach of soil water flow through the use of scaling factors: Measurement and simulation

Abstract The method of scaling, which is extensively used in hydraulics and fluid mechanics, has not been widely developed in soil physics, most probably due to the fact that vigorous assumptions are rarely met in nature. It will, however, be shown that heterogeneity from location to location within a field or at the scale of a watershed may be approximated with the use of a scaling coefficient for each site.

Assessment of arsenic concentration in stream water using neuro fuzzy networks with factor analysis.

We propose a systematical approach to assessing arsenic concentration in a river through: important factor extraction by a nonlinear factor analysis; arsenic concentration estimation by the neuro-fuzzy network; and impact assessment of important factors on arsenic concentration by the membership degrees of the constructed neuro-fuzzy network. The arsenic-contaminated Huang Gang Creek in northern Taiwan is used as a study case. Results indicate that rainfall, nitrite nitrogen and temperature are important factors and the proposed estimation model (ANFIS(GT)) is superior to the two comparative models, in which 50% and 52% improvements in RMSE are made over ANFIS(CC) and ANFIS(all), respectively. Results reveal that arsenic concentration reaches the highest in an environment of lower temperature, higher nitrite nitrogen concentration and larger one-month antecedent rainfall; while it reaches the lowest in an environment of higher temperature, lower nitrite nitrogen concentration and smaller one-month antecedent rainfall. It is noted that these three selected factors are easy-to-collect. We demonstrate that the proposed methodology is a useful and effective methodology, which can be adapted to other similar settings to reliably model water quality based on parameters of interest and/or study areas of interest for universal usage. The proposed methodology gives a quick and reliable way to estimate arsenic concentration, which makes good contribution to water environment management.

Carhyd a computer-aided system for characterisation of the hydraulic conductivity of field soil

Summary A computer-aided system has been developed to determine easily the hydraulic conductivity and soil water pressure potential relationships with the soil water content from an internal drainage experiment. This software has three different features: a) data acquisition (neutron count rate or soil water content; tensiometers (if any) measurement) b) determination (tables and graphs) of soil water content profiles, total head profiles at various times; computation of changes of water storage with time from the surface to various depths, analytical estimation of soil water flux at a given depth from a fit of the water storage-time dependence by a semi-long or log-log correlation; determination of h ( θ ) c) direct estimation of the K ( θ ) relationship using a simplified technique, first described by LIBARDI et al. (1980), and avoiding the use of tensiometers. It also contains a subroutine related to calibration of the neutron probe.