To study the effect of soil texture, aggregate size and initial moisture content on soil water diffusivity, D(θ), and unsaturated hydraulic conductivity, K(θ), horizontal absorption experiments were conducted on samples of loamy sand, sandy loam and clay–loam soils as well as on artificially prepared water-stable aggregates of a clay–loam sample (sizes 0.·25–0–25, 0·25–0·5, 0·5–1, 1–2, and 2–4 mm). For comparable moisture contents, D(θ) followed the order loamy sand > sandy loam > clay–loam. The effect of initial moisture content on D(θ) varied with soil texture. K(θ), which was evaluated using D(θ) for air-dry initial moisture content and the slopes of the water retention curves, also varied with soil texture.
The D(θ) function for air-dry initial moisture content increased with the decrease in aggregate size, the increase being more pronounced below a size of 1 mm. Values of D(θ) obtained from initially airdry soil and at 10% of saturation moisture content did not differ greatly from one another. The K(θ) function was almost the same for aggregate sizes 1–2 and 2–4 mm. In the size ranges of < 1 mm, K(θ) increased with the decrease in aggregate size. The particle/aggregate size range of 0·1–0·5 mm was the most conducive to unsaturated water flow.