Fjölrit RALA - 15.06.2004, Page 71

Effects of drying on volcanic soil degradation:
Practical implication on hydrological behaviour
A. Basile1 and A. Coppola2
1 Institute for Mediterranean Aericultural and Forest Systems (ISAFOM-CNR), Ercolano
(NA), Italy
2Department DITEC, TJniversity of Basilicata, Potenza, Italy
Many volcanic soils (Andisols) show an irreversible volume changing after drying. The
irreversible changes occur when elementary unit particles, weakly bonded together, come
sufficiently close to enable strong bonding; larger size aggregates are formed and they are not
broken up on re-wetting. These modifications affect very much the type, volume and pore-
sizes distribution and therefore modifies several physical properties (i.e. water retention,
hydraulic conductivity and apparent hydrodynamic dispersivity) producing remarkable effects
on many hydrological processes taking place in the soil.
Several basic mechanism are not yet know on this distinctive effect: among others the
energy level at which the irreversible change starts and develop, its development rate, and the
effect of the time on the energy accumulation process. This deficit of knowledge limits our
capability in distinguishing the status, in respect of the time scale, of the field degradation and
therefore the prediction capability of the process.
Such scenario is especially relevant considering that in much research and practical activities
many physical properties are obtained on dried samples. One of the aim of this study is
therefore the prediction of the hydrological behaviour of soil profiles studying the influence
and the effects of samples drying.
Two different case studies were investigated: the first one in an agricultural area with
intensive tillage and inputs (SARNO plain); the second one on a natural slope (Monte
FAITO).
The drying effects on the Ap horizons of these different soil profiles have been compared in
terms of the single physical property (i.e. water retention, saturated and unsaturated hydraulic
conductivity, pore sizes distribution, PDF, etc..). Moreover, this drying effect on the solute
and water balance has been tested.
Undisturbed soil samples were collected in cylinders from the main horizons of volcanic
soils classified as Humic Haplustand and Typic Hapludand. The Humic Haplustand was
located in the SARNO plain, an alluvial plane surrounded by limestones covered by volcanic
material; they were formed by pyroclastics fall and volcanic colluvial material. The Typic
Hapludand soil was located at 1120 m a.s.l. on Umestones relief covered by volcanic ash and
pumices.
Soil water retention and hydraulic conductivity functions were measured by means of
tension table and Wind’s method, saturated hydraulic conductivity by means of constant head
permeameter and solute transport characteristics by means of a miscible flow experiment.
These measurement were performed twice, the second one after the samples were oven dried.
Main results of the comparison in the SARNO soil were that the drying induces: (i)
reduction in the total porosity of the 20%; (ii) the translation of the soil water retention curve
(at least for values of potential ranging between -5 and -350 cm), along with the
corresponding pore-size distribution. Taking into account the invariance of the bulk density,
this confirms that the porosity reduction occurs in the small pores region; (iii) an augmented
fraction of large pores, as indicated by the pore-size distribution analysis; (iv) the saturated
hydraulic conductivity values agree with the new distribution; (v) the breakthrough curve
become shghtly asymmetric and a preferential flow can be hypothesised.
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