Comparison of methods for calculating farmland drainage modulus

Abstract: The correct calculation of drainage modulus is of important practical significance for the determination of the suitable scale of water logging control projects. Taking the Luoshan drainage area in Four-lake Watershed in Hubei Province as an example, based on the runoff generation characteristics of underlying surfaces in the drainage area, the drainage area was classified into 4 kinds of underlying surfaces, i.e. the paddy field, the arid land (including wasteland), the water surface, and the building plot. Then the runoff of different underlying surfaces was calculated respectively. Drainage moduli under different water logging control standards were calculated with the empirical formula and average draining method respectively. Then the relationship between days of drainage for water logging control, drainage moduli and return periods derived from both calculation methods was analyzed. Results showed that the corresponding days of drainage for water logging control calculated by the empirical formula were 3.5 days. When drainage moduli calculated by the empirical formula were less than that by the average draining method, days of drainage for water logging control were less than 3.5 days in the drainage area. Then drainage moduli calculated by the average draining method were suggested to ensure the project safety. However, when drainage moduli calculated by the empirical formula were more than that by the average draining method, days of drainage for water logging control were more than 3.5 days in the drainage area. So drainage moduli calculated by the empirical formula were relatively safe. In addition, if drainage moduli were determined, the drainage standard checked with different methods was different. The design standards with return periods of 10 years and 20 years and the water logging discharge duration of three days derived from the average draining method were equivalent to design standards with return periods of 15 years and 33 years derived from the empirical formula under one-day rainstorm respectively. However, design standards with return periods of 10 years and 20 years and the water logging discharge duration of five days derived from the average draining method were equivalent to design standards with return periods of 4 years and 7 years derived from the empirical formula under three-day rainstorm, respectively. This research can be used as a reference for reasonable selection methods for calculating drainage modulus.