A COMPARISON OF NEAR-SATURATED HYDRAULIC-PROPERTIES MEASURED IN SMALLCORES AND LARGE MONOLITHS IN A CLAY SOIL

The effect of measurement scale on hydraulic properties close to satur ation was investigated in a day soil. Results from measurements on und isturbed ''standard'' small soil core samples were compared with resul ts from three large intact soil monolith samples which were assumed to reflect natural soil hydraulic behaviour. An intermediate sample size , which was used to characterize soil water retention relations (theta (psi)) in each layer in the intact monoliths, was obtained by cutting the intact monoliths in three layers. The small cores were subsequentl y sampled from the cut monoliths so that all measurements were made on the same soil material. Measurements of theta(psi) and saturated hydr aulic conductivity (K-s) were compared, as well as the exponential rel ationships between K-s and macroporosity (phi(ma)), the latter derived from the theta(psi) data for small cores and from specific yields for intact monoliths. Between soil water pressure heads of -60 cm acid -1 5 cm the small core data and the cut monolith data showed similar thet a(psi) relations, whereas they diverged closer to saturation. When com paring K-s and phi(ma), small core aata were correlated with the intac t monolith data although the small cores had smaller values. Thus, the ''standard'' small cores can satisfactorily predict the investigated soil hydraulic properties for a natural soil at pressure heads equal o r less than -15 cm, but they may be in error, especially in soils with macropores, in the pressure head range -15 cm to saturation, i.e. in the largest macropores.