Alessandro Comegna

Stochastic analysis of unsaturated steady flows above the water table

Steady flowtakes place into a three-dimensional partially saturated porous medium where, due to their spatial variability, the saturated conductivity Ks and the relative conductivity Kr are modeled as random space functions (RSF)s. As a consequence, the flow variables (FVS), i.e. pressure-head and specific flux, are also RSFs. The focus of the present paper consists into quantifying the uncertainty of the FVS above the water-table. The simple expressions (most of which in closed form) of the second-order moments pertaining to the FVS allow one to follow the transitional behavior from the zone close to the water-table (where the FVS are non-stationary), till to their far-field limit (where the FVS become stationary RSFs). In particular, it is shown how the stationary limits (and the distance from the water-table at which stationarity is attained) depend upon the statistical structure of the RSFs Ks, Kr and the infiltrating rate. The mean pressure head kψl has been also computed, and it is expressed as kψl = ψ0(1+ψ), being ψ a characteristic heterogeneity function which modifies the zero-order approximation ψ0 of the pressure head (valid for a vadose zone of uniform soil properties) to account for the spatial variability of Ks and Kr. Two asymptotic limits, i.e. close (near field) and away (far field) from the water-table, are derived into a very general manner, whereas the transitional behavior of ψ between the near/far field can be determined after specifying the shape of the various input soil properties. Besides the theoretical interest, results of the present paper are useful for practical purposes, as well. Indeed, the model is tested against to real data, and in particular it is shown how it is possible for the specific case study to grasp the behavior of the FVS within an environment (i.e. the vadose zone close to the water-table) which is generally very difficult to access by direct inspection.

Mapping solute deep percolation fluxes at regional scale by integrating a process-based vadose zone model in a Monte Carlo approach

Abstract Interpreting and predicting the evolution of non-point source (NPS) pollution of soil and surface and subsurface water from agricultural chemicals and pathogens, as well as overexploitation of groundwater resources at regional scale, are continuing challenges for natural scientists. Accordingly, in this study we present a regional-scale modeling approach for vadose zone solute leaching that is based on stochastic application of a deterministic vadose zone model describing the water flow and solute transport processes in the unsaturated zone using the Richards equation (RE) and the advective-dispersive equation (ADE), respectively. The stochastic framework (Monte Carlo technique) allows accounting for uncertainty in the vulnerability outputs. As the approach is built on physically-based equations, it may be extended to the predictions of water fluxes (i.e., groundwater recharge) in the vadose zone. The approach relies on available datasets coming from different sources (detailed pedological information, hydrological properties in different soil horizons, water table depth, spatially distributed climatic temporal series and land use) and offers quantitative answers to soil and groundwater vulnerability to NPS of chemicals at regional scale within a defined confidence interval. Interpolation of these local distributions by geostatistical tools provides areal distributions of the statistical moments of solute and water fluxes. A preliminary evaluation of methodology was carried out for quantifying contaminant transport and groundwater recharge profile in the Metaponto plain in Southern Basilicata, Italy. Results showed large differences in the magnitude of the different travel times and related uncertainties among different profiles. The lower or higher vulnerability was found to be mainly related to the average silt content of the soil profiles.

A Review of Approaches for Measuring Soil Hydraulic Properties and Assessing the Impacts of Spatial Dependence on the Results

The movement of water in the soil and associated solute transport perform a role of primary importance in many applications in the field of hydrology and agriculture. In the sound management of irrigation water, in relation to specific environmental conditions and cropping systems, knowledge of local water flow conditions in zones affected by the root systems is indispensable. Once the irrigation method has been established, only knowledge of the laws governing water flow allows the necessary irrigation frequencies and rates to be established to optimise the distribution of soil moisture, reducing within established limits the effects of water stress and containing water wastage.

TRANSPORT OF SOLUTES IN THE FIELD AS AFFECTED BY IRRIGATION

This study documents and compares the transport of a conservative solute in near saturated soil profiles under flood and sprinkler irrigation. The experiments were carried out on a clay Vertic-Usthortens soil located near Potenza (Italy). Two 2x2 m2 plots were clipped of their native grass vegetation. After spraying on the surface a Cl- pulse as KCl salt; water was applied in five increments over two months as flood irrigation on the first plot and as sprinkler irrigation on the second one. Chloride resident concentration Cr, was sampled by soil coring at four different days after chemical application. Cr(z,t) profiles were analyzed by spatial moment method. The recovered mass of Cl- and location of center of mass were comparable for the two types of irrigation. The spread around the center of mass, however, was higher for the flood-irrigated plot. In the flood-irrigated plot, more mass leached below the depth of 90 cm. The velocity of the center of mass was consistently 10-20% larger than the piston displacement velocity. To evaluate the nature of transport, the Cr(z,t) distributions were modelled using quasi-steady solution of convection-dispersion equation(CDE). At the scale of our experiments the profiles of Cl- resident concentration are well-simulated.

Evaluating the Role of Soil Variability on Potential Groundwater Pollution and Recharge in a Mediterranean Agricultural Watershed

Regional-scale studies on groundwater vulnerability assessment of non-point source agrochemical contamination suffer either from no evaluation of uncertainty in data output, in that of qualitative modelling, or from prohibitively costly computational efforts, in that of deterministic modelling. By contrast, a methodology is presented here which integrates a solute transport model based on transfer function (TF) and a geographic information system (GIS). The methodology (1) is capable of solute concentration estimation at a depth of interest within a known error confidence class, (2) uses available soil survey and climatic and irrigation information and requires minimal computational cost for application and (3) can dynamically support decision-making through thematic mapping. Raw data (coming from different sources) include: i) water table depth, ii) soil texture properties, iii) land use, and iv) climatic information with reference to a study area located in southern Italy. Such information has been then manipulated in order to generate data required for the subsequent hydrological modelling. Simulated breakthrough curves were generated for each soil textural class. They are texture-based travel time probability density functions (TFtb), describing the leaching behaviour of soil profiles with similar soil hydrological properties. The latter, in turn, were estimated by indirect estimation techniques such as pedotransfer functions (PTFs) to overcome the trouble of intensive in situ and/or laboratory determinations of soil hydraulic and hydrodispersive properties, which are generally lacking for regional-scale studies. Results showed large differences in the magnitude of the different travel times and related uncertainties among different profiles. The lower or higher vulnerability was found to be mainly related to the average silt content of the soil profiles.

USE OF THE JONASSON’S MODEL TO ESTIMATE THE VAN GENUCHTEN PARAMETERS FROM TEXTURAL DATA OF SOME SOILS IN SOUTHERN ITALY

In the light of potential application of deducing soil water retention from some simply determined physical properties, due consideration was given to physical and empirical models in the literature, based on the transformation of a granulometric curve PSD into a retention curve θ(h). In particular, forecasting po- 5 tentials as proposed by Jonasson’s model were evaluated in 15 soils from Southern Italy. The estimated θ(h) curves provided a good reliability only when the weighing factor Wf was optimised. Even if the results obtained should be further verified in soils having different pedological characteristics, the model, used, may be particularly suitable to determine θ(h) curves, when experimental observations are poor or lack.