Restricting layers, flow paths and correlation between duration of soil saturation and soil morphological features along a hillslope with an altered soil water regime in western Victoria

Abstract

This paper is the second of two describing how soil macromorphological and chemical data can be combined with soil hydrological data to distinguish between, and to quantify, past and present hydrological processes relevant to waterlogging and dryland salinity. The first paper provides a methodological framework for the study, and describes the initial interpretation of the macromorphological features of the toposequence studied, using the soil feature-system-domain grouping method. This second paper deals with the added value of extensive piezometric and other hydrological observations relative to soil macromorphological studies, and with quantitative relationships between soil colour and duration of waterlogging.

As with the first paper, this paper focuses on a soil toposequence at Gatum on the eastern Dundas Tableland in western Victoria. For the the broad crest with yellow gradational soils or Dermosols (Plinthoxeralfs), the hydrological data confirmed the conclusion from macromorphological observations that:

(1) There are three levels at which downward flow of water is restricted: at the top of the largely unaltered mottled zone, that is at 0.8-1.0 m depth, well below the top of the yellow Bt1-horizon (fresh water); at the top of the pallid zone, at about 3-3.5 m depth (fresh water); and on top of the unweathered ignimbrite (saline water).

(2) Below about 30 mm depth, down at least as far as 2.0 m and sometimes even more, the major pathways for downward movement of water are indeed root channels, with or without live roots, and not interpedal cracks. The 3-dimensional spacing of the rootholes above and through the less-permeable mottled zone, and the (horizontal) conductivity, storage capacity, and thickness of overlying horizons determine the extent of ponding, runoff, and deep infiltration taking place.

In addition, the hydrological observations showed that:

(3) If rainfall is regular, well-spaced, and not excessive, ponding is unlikely to take place even where there is a rainfall surplus and there are layers restricting downward flow of water.

Furthermore, along the toposequence with yellow duplex soils or Dermosols, Chromosols, and Hydrosols,

(4) There were generally good correlations between duration of saturation at the bottom of the E-horizon and colour aspects of the E-horizon (value and chroma of the matrix) and of the B2-horizon (hue, value, and chroma of the matrix).

Based on these findings additional suggestions are made for improving identification and management of water logging and salinisation processes.