Variations of Water Table Profile and Outflow of Bi-level Drains in a Layered Soil

Two issues of the drainage systems which are less studied in the literature are drainage in layered soils and bi-level derange systems. Hence، it is necessary to do more research in these topics. Therefore the present study was carried out under above conditions in the Imam Khomeini Farming and Industrial Lands، located in the north of the Khuzestan province، Iran. In this area based on primary studies، lateral spacing was estimated about 70 m and they were installed at the depth of 2. 1m. But after two years of establish these drainage systems، it was observed that due to existence a soil layer with a very low infiltration rate (less than 10 time in compare with its above and below layers) at the depth of 110 to 160 cm، drainage systems in 500ha of these fields had low performance. Then، in order to overcome with this problem، the implementation of bi-level drainage system was proposed. Hence، a new drain line was installed at the depth of 1. 2m between the old drain lines. After establishment of these new drains one field was selected to evaluation the performance of this new system. In the selected field three rows of observation wells were installed (in total 20 observation wells). After equipment of these observation wells، water table fluctuation، inflow and outflow water values from the field were measured in 3 irrigation periods. Then the results of field observations were compared with the obtained results from analytical solution in the literature. Results show with establish new drain lines the problem of these fields are solved and the water table profile will drop-off to the below of root zone in less than 36hr. Also it was concluded that the rate of water table decline by moving from the end of field to the open collector of field will be increased and the discharge of deep drains are more than low-deep rains. Results show that analytical solution is not able to predict the water table profile with well accuracy and predicted values by this equation only in a small area of water table profile were consistent with the observed values in the field. The main reason of this difference was the specific situation of layers of the soil.