Hydrological investigations of soil and vegetation types in the lower Cotter catchment

Abstract

The quality of Canberra’s domestic water has deteriorated in recent years, and various conditions within the catchment area have been blamed. This project was designed to investigate the stability of the land surfaces in the lower Cotter catchment, and to determine ground cover requirements necessary to keep soil and water losses to a minimum. Surface run-off and soil loss were measured for each natural rainfall for a year on 41 permanent plots covering a range of soil-vegetation types in the lower Cotter catchment. A portable rainfall simulator was constructed for use in measuring infiltration rates, and the artificial rainfall (at an intensity of 320 points per hour) had drop size and velocity characteristics similar to those of natural rain of the same intensity. Infiltration measurements were made on the 41 permanent plots and on 33 additional roving plots during the summer of 1963-1964. Each plot was characterised for type and amount of ground cover, and the soil physical properties of bulk density, capillary and non-capillary porosity, and particle size distribution. On soil types with low non-capillary porosity in the subsoil (restricted drainage) the non-capillary porosity of the surface soil was found to be the most important factor in controlling infiltration and surface run-off. Non-capillary porosity of the surface soil was related to weight of ground cover, and reductions in the ground cover below 4,000 g per 60 square feet resulted in a marked increase in surface run-off. On soil types with high non-capillary porosity in the subsoil (even drainage) surface run-off was at a low level at all times, and the amount of ground cover present had little or no effect. Soil loss on soil types with restricted drainage in the subsoil was dependent on the amount of ground cover present. Ground cover in excess of 4,000 g per 60 square feet resulted in negligible soil loss; but reductions in ground cover below thi s value resulted in a large increase in soil loss. For a given amount of ground cover (below 4,000 g per 60 square feet) soil loss increased exponentially with rain fall intensity . Most of the soil loss which occurred during the year resulted from a small number of high in tensity rains. Chemical analyses of the so i l loss indicated high fertility erosion. Soil loss was negligible on soil types with even drainage throughout the profile irrespective of the amount of ground cover. Infiltration rate s were slightly higher under eucalypts than under pines and a hydrophobic e f f e c t was present under both types when the ground cover was dry. Low intensity prescribed burning in a pine plantation did not result in any significant reduction in infiltration rates. However, the concentration of nutrients in the run-off water from two subsequent intense artificial rain falls was significantly increased by the burning. Native eucalypt forests and well established pine plantations provided excel lent catchment protect ion on all soil types. Young pine plantations on soil types with restricted drainage in the subsoil provided insufficient protection. Most of the soil loss measured during the year originated on very bare areas such as firebreaks. Improvement in catchment protection could be attained by vegetating bare areas such as roads and firebreaks . Show more