Study on the Influencing Factors of Specific Yield of Unconfined Aquifer Using Laboratory Column Drainage Experiment

Specific yield is one of the most important hydrogeological parameters of unconfined aquifer. In order to study the influencing factors of specific yield, the laboratory column drainage experiment was developed to estimate specific yield of homogeneous and stratified heterogeneous sand columns. The groundwater level was decreased according to segmentation of 2 cm or 5 cm and each section kept different drainage times to obtain the specific yield value of different depth to water table. When the depth to water table was within the capillary rise height, the relationship between specific yield and water table depth can be expressed by a nonlinear function. The specific yield value increased with the drainage time and it became constant when the drainage time of each drawdown section was more than one hour. Specific yield value of coarse sand with particle size 0.6-0.9 mm was always larger than that of fine sand with particle size 0.2-0.4 mm. When groundwater level was 20 cm under the layer interface, two different columnar structures of upper-coarse-lower-fine and vice versa exerted distinct impacts on the magnitude of specific yield. For shallow unconfined aquifer, specific yield varied with the initial depth to water table, duration of drainage, soil texture and heterogeneity near the water table.