Abstract
This paper describes the lateral flow of water quantitatively in a field with mole drains and a lot of drying cracks, and discusses the physical properties of the flow. The lateral flow of water was measured using an undisturbed soil block of 150 cm in width, 500 cm in length and 70 cm in depth (where a vinyl sheet was placed) in a clayey soil field of 2, 100 m2.
The hydraulic conductivities of the field, estimated from the data of the soil block, was higher 4 to 6 by orders in magnitude than the hydraulic conductivities of the small core samples of each100 cm3. The Reynolds number of the flow through a mole drain, which lies 35 cm below the soil surface, was estimated to be 40 to 50. These values were too large to guarantee application of the Darcy law. On the other hand, the Reynolds number of the flow through soil cracks was estimated to be 1 to 3, which were within the tolerance for application of the Darcy law.
When the water table was low and water flowed mainly in the soil cracks, there were no differences among the permeabilities of water through 500 cm, 250 cm and 125 cm long soil blocks. On the other hand, when the water table was high and water flowed mainly in the mole drain, the fluxes corresponding to the same hydraulic gradients changed with the lengths of the soil blocks. This is probably due to the existence of narrow sectional areas formed at various places in the mole drain
The phreatic surface of the soil block of 200 cm long was estimated through the investigation of the crack walls colored with water-soluble white paint. The predicted phreatic surface calculated by the Darcy law was very close to the investigated water table at 80 cm far from the mole drain, while it was extremely different from the investigated water table around the mole drain. This suggests that, in the field with a lot of cracks and mole drains, the lateral water flow occurs in the condition that Darcy-type flow and non-Darcy-type flow are mixed. The apparent hydraulic conductivity of this field, including the flow in soil matrix, in cracks and in mole drains, was estimated to be 0.01 to 0.1 cm/s.
