Risk analysis on agricultural drainage ditch filling and flood disasters in lower plain area of North China

Abstract: The construction of irrigation and water conservancy was initiated on the North China Plain in the 1950s, which played a significant role in the saline-alkali soil improvement and flood discharge. However, the phenomenon of filling agricultural drainage ditches has become common in the North China Plain since the 1990s. It is necessary to know the condition of filling agricultural drainage ditches and flood disasters related with this phenomenon. Nowadays, there are few researches to analyze the condition of filling agricultural drainage ditches. In order to enrich existing studies, this research took Cangxian County which was battered by flooding and soil salinization in lower plain area as a case study, and explored the condition of filling agricultural drainage ditch and the flood disaster. The changes of drainage ditch area and spatial variation were analyzed based on land use databases of Cangxian County in 1992 and 2010 using the method of GIS (geographic information system). Then, the condition of filling agricultural drainage ditch was investigated by means of the field research in Nan Gutun Village. This village was one of the most densely populated agriculture villages in Cangxian County. Interviewing with the village committee members and the villagers over 70 years old, we learned about the local agricultural production mode, the way of life, the changes of agricultural drainage ditch and its mechanisms from 1960s to now. Finally, we analyzed the risk of flood disasters from the aspects of precipitation trends, percentage of precipitation anomalies, underground water level, drought/flood frequency, relationship between soil water capacity and rainfall, and upland water condition. The results showed that from 1992 to 2010, the area of drainage ditches in Cangxian County reduced by 37.73%. Meanwhile, the proportion of drainage ditches in Cangxian County decreased by 2.03% and the farm ditches was the most serious in being filled. The results of flood disaster analysis showed that the annual precipitation had an obvious downtrend and the seasonal precipitation was evenly distributed. With the underground water level lowering at the average speed of 1-2 m/a, soil water supplement through soil capillarity was decreasing. This made the storage capacity of soil water enlarged before rainy season. Based on soil physics, 1 m water-free soil could contain approximately 140 mm precipitation, which played a significant role in slowing down the surface runoff. At the same time, the actual water flow in the upstream was decreasing. Moreover, China's rapid urbanization and industrialization is accompanied by a continuous growth of water use, and upland water is mainly used for cities, which results in the decrease of upland water. In consideration of all condition, the risk of flood disasters was reducing according to the precipitation, soil water capacity and upland water. In other words, under the constant climate circumstances, the condition of filling agricultural drainage ditches did not increase the risk of flood in the majority of years. But we found in the field research that the risk of flooding increased after the heavy rain. Based on the results above, we suggest that the government should take terrain, underground water level and soil into consideration in the redesigning of drainage ditch systems, and attain the goal of maintaining high and stable yield of grain, farmland infrastructure protection and requisition-compensation balance of cultivated land through the scientific planning of water conservancy construction and land consolidation.