Effects Of Underlying Surface Changes On Farmland Drainage Discharge

Water logging is one of the natural disasters which exists in everywhere and affects various industries; especially it has brought about tremendous impacts on the agricultural production. China is one of the countries where water logging frequently occurred in the world. Water logging has a huge negative impact on economic and social development. According to statistics, there is about0.245million hectares cultivated land which is easy to occur water logging. From1978to2008, the area suffered from flood and water logging reaches0.128hectares and the inundated area of water logging and flood disaster reaches0.074hectares every year on average. The yield reduction of grain, cotton and oil caused by water logging accounts for about5%of the total production every year. So it has threatened badly China's food security. With the changes of the global climate, the rapid development of social economy, the continuous growth of population and the accelerated process of urbanization, water logging occurs more and more frequent, and water logging disaster losses are more and more serious. Then, higher requirements will be put forward to water logging control. At present water logging control projects in our country were mainly built in the1970s, with the rapid development of economic society and the change of the underlying surfaces, the social needs for water logging control is increasing, and the hydrological regime of water logging area also have changed greatly. Therefore, studying the drainage discharge under condition of change of underlying surfaces is of practical significance for the determination of the suitable scale of projects, controlling water logging and ensuring food security.The study is financially supported by the "12th Five-Year Plan of National Science and Technology Support". The subject of this study is water logging control. This paper summarizes the research status of the principle to determine water logging control standard, expression and drainage index for water logging control, influence factor and the method for calculating drainage modulus, the runoff yield and concentration simulation in farmland drainage area and effects of underlying surface changes on runoff yield and concentration at home and abroad. On this basis, the effects of the underlying surface changes on drainage modulus is studied.Taking the Luoshan drainage area in Four-lake Watershed in Hubei Province as an example, the study is carried out from the aspects of the comparison of methods for calculating farmland drainage modulus and influence factor analysis, the development of runoff yield and concentration model and the effects of underlying surface changes on drainage modulus. The main results are as follows:On the basis of introducing the runoff yield and concentration and the process of calculation the drainage modulus, the principle, characteristic and application scopes of common methods for calculating drainage modulus at home are summarized. And various methods are compared qualitatively. Then this study takes the Luoshan drainage area in Four-lake Watershed in Hubei Province as an example, and the quantitative comparison of three methods for calculating farmland drainage modulus (the empirical formula, average draining method and water balance method) is studied. Then the corresponding relationship between the empirical formula and average draining method is established. In addition, the drainage capacity of water logging control projects is checked, and the selection of calculation methods is discussed.On the basis of the comparison of methods for calculating farmland drainage modulus, the factors influencing drainage modulus are analyzed. The effects of return period and duration of rainstorm and underlying surface changes (water surface ratio, proportion of paddy field to dry land area, ground hardening rate, irrigation mode) on the drainage modulus are analyzed by the empirical formula and average draining method respectively. Then the quantitative relationship between drainage modulus and proportion of paddy field to dry land area, ground hardening rate, water surface ratio is established.On the basis of widely collected data such as rainfall, land use, soil type, river section, water level and flow and so on, the coupled hydrologic and hydrodynamic model at a drainage area scale is constructed by combining SCS with MIKE11. While the runoff of paddy field and water surface and the simulation of sluice and pump are modified. The process of runoff yield and concentration in flood season in1980,1983,1991and1996is simulated by the coupled hydrologic and hydrodynamic model. Then the CN and roughness coefficient of the model are calibrated and validated. On this basis, the drainage capacity of the drainage area is checked. In addition, the influence of flood level of the outer river to drainage is analyzed quantitatively by simulating drainage modulus under the combination of different design storm and flood level of the outer river.Based on the underlying surface in1994and2011, the effects of underlying surface changes on drainage modulus is analyzed quantitatively by the coupled model. On the basis of analysis the main trends of the underlying surface changes from1994 to2011, the runoff and flood peak flow with different return period rainstorm under the underlying surface in1994and2011are simulated respectively by the constructed runoff yield and concentration model. And the drainage modulus in1994and2011underlying surface are calculated respectively by four methods (the empirical formula, average draining method, water balance method and hydrologic and hydrodynamic model method). Then the effects of underlying surface changes on drainage modulus is analyzed by comparing the drainage modulus under different underlying surfaces. In addition, the effects of the proportion of paddy field to dry land area, water surface ratio and ground hardening rate on drainage modulus is analyzed by the constructed model And the linear relationship between drainage modulus and the proportion of paddy field to dry land area, water surface ratio and ground hardening rate is established. Then the combination schemes of the proportion of paddy field to dry land area, water surface ratio and ground hardening rate are established, and different schemes are simulated and analyzed by the constructed model.