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Temporal Stability of Spatially Measured Soil Water Probability Density Function¹

ABSTRACT

Soil water data collected from three different fields are analyzed by two techniques (temporal analysis of the differences between individual and spatial average values; and Spearman's rank correlation) to search if time-invariant characteristic statistical properties of the probability density functions can be assigned to individual locations. A grass field was equipped with 17 neutron access tubes and surveyed 24 times during a 2 1/2 yr-period. In another field planted with olive trees, nine neutron access tubes were installed and quarterly measurements were performed during two consecutive years. The latter field cropped in wheat was gravimetrically sampled on a regular spatial pattern five different times and was routinely surveyed during a 1-yr period at four selected locations by using a neutron moisture meter. All data show the existence of a very significant time-stability of particular individual locations characterized by the same parameter in the statistical distribution of the observations taken over the field. It is shown that some locations conserve the property to represent the mean and extreme values of the field water content at any time along the year. This stability seems to be explained to a large extent by relationships between soil texture and water content.

¹ Contribution from the Institute de Mécanique de Grenoble. Laboratoire associé au CNRS n° 6 B.P 68-38402 Saint Martin d'Heres Cédex, France.

² Senior Researcher, Post Graduate Students, and Senior Researcher, respectively.

Received for publication April 6, 1984. Accepted for publication December 6, 1984.