Abstract
Abstract. Unsaturated hydraulic properties and their spatial variability today are analyzed in order to use properly mathematical models developed to simulate flow of the water and solute movement at the field-scale soils. Many studies have shown that observations of soil hydraulic properties should not be considered purely random, given that they possess a structure which may be described by means of stochastic processes. The techniques used for analyzing such a structure have essentially been based either on the theory of regionalized variables or to a lesser extent, on the analysis of time series. This work attempts to use the time-series approach mentioned above by means of a study of pressure head h and water content θ which characterize soil water status, in the space-time domain. The data of the analyses were recorded in the open field during a controlled drainage process, evaporation being prevented, along a 50 m transect in a volcanic Vesuvian soil. The isotropic hypothesis is empirical proved and then the autocorrelation ACF and the partial autocorrelation functions PACF were used to identify and estimate the ARMA(1,1) statistical model for the analyzed series and the AR(1) for the extracted signal. Relations with a state-space model are investigated, and a bivariate AR(1) model fitted. The simultaneous relations between θ and h are considered and estimated. The results are of value for sampling strategies and they should incite to a larger use of time and space series analysis.
Highlights
The increasing need for water for domestic and industrial purposes under ever more stringent environmental protection measures, combined with advances in irrigation, makes it necessary to gain in-depth knowledge of water and solute flow in the vadose zone, understood as the zone roughly extending from the soil surface to the water table
Based on laws of water flow in unsaturated porous media, in order to be applied such models are known to require mathematical relations linking the local value of water content in volume θ to the water tension h and soil hydraulic conductivity k
We will analyze individually the two parameters which characterize the soil water status in terms of θ and h measured at 0.3 m depth, along the N–S line of the plot so as to highlight their intrinsic structure linked to regional variability and, for 3 of the 12 measuring sampling times, the variations occurring in time
Summary
The increasing need for water for domestic and industrial purposes under ever more stringent environmental protection measures, combined with advances in irrigation, makes it necessary to gain in-depth knowledge of water and solute flow in the vadose zone, understood as the zone roughly extending from the soil surface to the water table. Deterministic evaluation of spatial heterogeneity of soil physical and hydraulic properties requires a large number of measurements and can only be performed for limited areas This has led to the increasing use of statistical models in which hydraulic variables are considered stochastic (Freeze, 1975). In the case of isotropy, could be considered as the realization of a stochastic process which is a function of coordinates on a horizontal plane and, in the case of anisotropy, a function of direction Applications of such techniques have proved promising for describing variability in space of soil hydraulic properties and have led to defining the number and distance at which to make determinations, thereby reducing sampling costs (Vieira et al, 1981).
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