Abstract: Abstract: Hanging curtain drains are widely installed at a shallow depth during the processes of lowering groundwater tables in a subsurface drainage system. The presence of hanging curtains has posed a great challenge to the theoretical formula of subsurface drainage discharge. The heads above the pipe and at the middle location between the pipes were also considered as parameters. In this study, a modified formula was proposed for the subsurface drainage discharge considering hanging heads. Three formulas were also selected to explore the equivalent impervious depth, including the Kirkham equation, classic drainage formula in the Soviet Union and Hooghoudt formula. A reasonable assumption was then made for the six kinds of series formulas. Furthermore, an HYDRUS model was used to simulate the drainage discharge and heads above the pipe and at the middle location between the pipes using the theoretical formula under different drain spacing, drain depth, depth of impervious layer, and soil texture. Specifically, three types of drain spacing (6, 20, and 40 m), three depths of impervious layer (2, 5, 10 m), three drain depths (0.8,1, and 1.2 m), and four soil textures (sand, silt, loam, clay) were set in the comprehensive tests. 409 groups of relevant data were then obtained during simulation. The better theoretical formulas of subsurface drainage discharge were determined to compare the calculated and simulated values considering the hanging curtain section. The applicability of formulas was also verified in various soil textures at the different heights of hanging curtains. A Hooghoudt formula was selected to evaluate the simulation. Additionally, a correlation analysis was made on the hanging head, as well as the ratio of discharge per unit length and hydraulic conductivity. The results showed that the formula of equivalent impervious depth given by van der Ploeg was smaller than that by van der Molen and Wesseling. There was a larger difference between the two aforementioned formulas, as the increase in the ratio of impervious depth and drain spacing. The calculated value of the Hooghoudt formula was also significantly smaller than the simulated one without considering the hanging curtain. In the case of the hanging curtain, the calculated discharges using six kinds of series formulas were all matched well with the simulated values with the correlation coefficients larger than 0.99 and mean absolute errors smaller than 11%. Meanwhile, a series of formulas were established using the Hooghoudt formula with the equivalent impervious depth by van der Molen and Wesseling, Kirkham or В.И.Аравин, and С.Н.Нумеров equation. It was found that better performance of modified formulas was achieved to well match with the larger correlation coefficients and the smaller mean absolute errors than other cases. In soil texture, the theoretical formula considering hanging curtains in silt and loam performed the highest applicable levels, followed by that in the sand. There was a smaller change of the head at the middle location between pipes, while a larger drain spacing during the decreasing process of hanging head. Once the hanging head was not available, the formulas can be estimated by the discharge per unit length of the pipe and hydraulic conductivity. There was also a better linear correlation between the hanging head and the ratio of discharge per unit length of the pipe and hydraulic conductivity with the correlation coefficient of 0.96. The finding has a great significance to enriching and developing the theory and technology of agricultural drainage.