Modelling of water movement in a clayey agricultural field with shrinkage cracks

Abstract

I n this paper, following aspects are discussed so as to describe quantitatively the role of cracks on water movement in a clayey agricultural field with shrinkage cracks. (1) Hydrological properties of drainage according with rainfall events. (2) Quantitative estimation of the cracks which developed in the field. (3) P hysical properties and routes of water flow through an undisturbed soil block (1.5m X5.0m). (4) P hysical properties of water flow in the entire field estimated by recession curve of drain flow. A test field was 30mX70m in area and has a impervious plastic sheet at 0.7m deep. A drain pipe, 60mm in diameter and 70m long was installed on the sheet and mole drains, 100mm in dia-meter, were constructed at about 0.35m deep and at 1.2m spacing. Soil texture was heavy clay (48%). Soil profile was able to be divided into two layers, plowed layer (about 0.1m thick) and subsoil. The subsoil had large prismatic structure due to shrinkage cracks penetrating to the bottom of the field. Matrix of the subsoil showed a massive structure and saturated hydraulic conductivity was no more than the order of 10-6 to 10-7 cm/s. Results of the experiments are as follows. (1) Most of rain water was drained from the field by pipe drainage. The pipe drainage started soon after beginning of rainfall, and the drain flow responded quickly on the rainfall patterns. ⑵ In the subsoil of the field, cracks 1—2mm wide developed and the length of the cracks on horizontal sections was 20m/m2 at the depth shallower than the mole drains and 10m/m2 at the depth deeper than the mole drains. From these value, volume of the cracks could be estimated 1.5—3 % of the volume of entire field, and 1—2 % of the volume of the field deeper than 35cm. ⑶ Experiment by using the undisturbed soil block showed that water flowed only along the mole drains and cracks. When the water table was high and water flowed mainly along mole drains, rapid and non- Darcian flow occurred. On the other hand, when the water table was low and water flowed along only cracks, the flow showed D arcian property and the hydraulic conductivity was in the order of 10-2cm/s. (4) Recession curve of the flow of the pipe drainage could be divided in to 2 parts. When water table was high, flow rate of the pipe drainage decreased linnearly with time. This was similar to flow property from an orifice of a water tank, and was interpreted by rapid flow along the mole drains. On the other hand, when wather table was low and water flows along the cracks, the flow rate decreased exponentially with time. This could be described very well by the equation of unsteady Darcy flow, and volume of the cracks and hydraulic conductivity was calculated 2% and in the order of 10—2cm/s respectively. From these results, it can be said that most of water flowed only along the mole drains and the crack in this clayey field, and that when the water flows along the cracks, the flow showed D ar ci an property and the hydraulic conductivity was in the order of 10-2cm/s.