Effects of underlying surface changes on drainage modulus in Four-lake Watershed

This study takes the Luoshan drainage area in Four-lake Watershed in Hubei Province as an example. The drainage modulus is calculated with an empirical formula. The effects of water surface ratio, proportion of paddy field to dry land area, urbanization, irrigation mode and underlying surface changes on the drainage modulus are analyzed. Results show that the drainage modulus decreases with an increase in the proportion of paddy field to dry land area, and decreases with an increase in the water surface ratio, as well as increases with an increase in ground hardening rate. In addition, the effects of water surface ratio on the drainage modulus are greater than those of the proportion of paddy field to dry land area, and the ground hardening rate. The drainage modulus is smaller with intermittent irrigation than that with traditional irrigation. Based on the water logging control standard of a three day rainstorm with a 10 year return period, the drainage modulus had increased from 0.38 m³/(s·km²) in 1994 to 0.46 m³/(s·km²) in 2011 because of underlying surface changes. In 2011, the drainage modulus met approximately the water logging control standard for a three day rainstorm with a 19 year return period, based on the underlying surfaces in 1994.