Research On Applicability Of Calculation Method Of Farmland Drainage In Huaibei Plain

In order to improve the applicability of farmland drainage calculation formula in the Huaibei Plain,farmland drainage flow was taken as the research object in this paper.The variation trend of precipitation characteristics at typical hydrological station over the years and the proportion of impervious surface in typical region in the Huaibei Plain were analyzed.The changes of drainage capacity in rural areas were concluded through comprehensive analysis.The applicability of the typical farmland drainage calculate method empirical formula method was discussed and the modification range of key parameters was given.（1）Based on the precipitation data from 1972 to 2016 from seven representative stations among northern,middle and southern areas of the Huaibei Plain,the annual average precipitation and the maximum 1,3,5 days precipitation under different return periods were calculated.It was found that the precipitation decreased from southwest to northeast.Aiting hydrological station was chosen as typical example,the variation trend of precipitation characteristic values from 1985 to 2016 was calculated.It was found that the continuity of precipitation process has changed,and the proportion of three-day rainfall event has increased since 2000.Compared with the period before 2000,the precipitation increased significantly in July and August from2000 to 2010,and also increased significantly in September from 2011 to 2016.The annual precipitation data were tested by Mann-Kendall method,and it was found that there were 6 changes in the trend of increase and decrease,but no significant change occurred in the overall trend of change.（2）An rural area of about 300 km² on the southwest side of Guoyang county was chosen as typical area.The area was divided into three pieces of small area depending on geographic location and development degree.Satellite image,which was pre-processed by Photoshop software,was used to analyze the difference of impervious surface area proportion among areas.The results showed that the rural area which was influenced by the urbanization showed a big increase in impervious area proportion,up to 6 times to the underdevelopment rural area.SWMM model was used to simulate precipitation-drainage process with preset impervious surface proportion of different areas data and hydro-meteorological data from Wudaogou hydrological experiment station in 2010.By comparing the difference of drainage flow from different areas,it was found that area with higher proportion of impervious surface show greater drainage flow than the area with low proportion.The maximum difference was 2.91m³·s^-1/km²,which means much less flow would seepage into natural land.Considering that the area which was influenced by the urbanization mostly rely on mechanical pumping and drainage equipment to discharge waterlogging,while the pumping and drainage equipment were often influenced by manual operation and equipment integrity,and the drainage channels can’t be smooth due to the building layout.The possibility of occurrence of severe waterlogging is much greater than underdevelopment areas.（3）The applicability of the empirical formula method for calculating the drainage flow in the Huaibei Plain was analyzed.The peak value relation index（m）was adjusted or not in contrast calculations with empirical formula and average draining formula.The adjustment rationality and adjustment range of the m were researched.Results of calculations were analysed and taken to improve the applicability of empirical formula.The spatial distribution of precipitation showed a trend of decrease from southwest to northeast.Before the adjustment,the result of the empirical formula is equivalent to the flow of 1 d draining water.After adjusting the m to0.84～0.87,the result is equivalent to the flow of 1.5 d draining water.With the decrease of the rainfall frequency,the drainage flow of average draining formula was greater than that of empirical formula and the ratio was 1.13 times when the runoff depth was 50 mm.The m can be selected from 0.84～0.87 for calculating drainage flow in Huabei Plain by using empirical formula,and the value increases from the northern area to the southern area.