Comparison of different methods for calculating farmland drainage modulus

Abstract

【Objective】 Drainage modulus is an important parameter for designing drainage systems in farmland. Taking Suibin county in Sanjiang Plain as an example, this paper analyzes its runoff and drainage modulus. 【Method】 The study site was an area prone to waterlogging. We first calculated the runoff (RO) using historical data on precipitation, pumping stations and land usage, and then estimated it using the runoff generation method (RGM) and the SCS-CN method (RSCS). The drainage modulus associated with each runoff was calculated using both empirical formulae (QE) and the average draining method (QA). The runoff and drainage modulus calculated by different methods were tested against measured data in the field. 【Result】 In the same year and under the same rainfall return period, the measured runoff increased with rainfall duration, while under the same rainfall duration and the same rainfall return period, it decreased with passage of years. The runoff and drainage modulus calculated from different methods differed from the experimentally measured data. For heavy rainfalls, the runoff estimated from different methods was ranked in the order of RGM>RSCS>RO, while for other rainfalls, it was ranked in the order of RSCSQE, whereas for rainfalls lasting 3 days, QA<QE. 【Conclusion】 The SCS-CN method works better for calculating runoff in the studied area, although its accuracy is sensitive to the CN value. The drainage formulae that include channel storage coefficient effectively describe the regulating function of watersheds. Regions with less development and good water storage capacity allow a prolonged drainage duration.

Keywords

• drainage modulus
• runoff
• runoff generation method
• scs-cn method
• empirical formula
• average draining formula