Research On Transportation Of Nitrogen And Phosphorus In The Subsurface Drainage Watershed And Spatiotenporal Modelling

Agricultural non-point source pollution is the main form of non-point source pollution, and in the agricultural watershed with subsurface drainage systemthe non-point source pollution problem is more complex, therefore to analysis of nitrogen and phosphorus pollutants transportation patterns andapply the specific method to control non-point source pollution in subsurface drainage watershed is very important.By using numerical simulation method, this thesis analyzed nitrogen and phosphorus in both micro- and macro- perspectives, and then combine those two process in the whole process.After that,the image analysis technology and the numerical analysis method were usedto figure out the regular pattern of nitrogen and phosphorus transportation in those two process, and nitrogen and phosphorus transportation process influence factors were analyzed when they transport in soil layers. A holistic approach for recognizing transportation mechanism of pollutants was built, and this made up the micro pollutants transportation process numerical simulation research.This study combined with a variety of mathematical and statistical methods, and10-year monitored multi-parameter data set of watershed were analyzed to identify the most important factor.Found that subsurface drainage system couldtransfer more than 15% of rainfall into drainage water, and for surface runoff flow only accounted for 1.5%-3% of rainfall. And correlation between many factors were significant. By dimension reducing, three main influence principal components were detected, the three components according to its actual meaning can be named as: biological principal component which including steaming temperature, cover factor, vegetation type, volume, dry weight;hydrology principal components which including flow and rainfall; anthropogenic components including fertilizer application load and management. Regression analysis showed thatcorrelation coefficient in As A, B, Bs, C, Cs site was 0.594, 0.171, 0.838, 0.566, 0.814, 0.566,respectively,which significantly higher than the phosphorus regression process of 0.353, 0.356, 0.371, 0.276, 0.206, 0.276,respectively. From those results, we can find that nitrate was more likely to transport. Inthose influence factors, through the nearest neighbor vector space analysis method,flow was found as the most influential factors of nitrogen and phosphorus transportation, but the importancy index of nitrogen was 17%, compared with only 9% for phosphorus.Based on long period field monitoring data in this watershed, DRAINMOD and Arc SWAT mode were used and calibrated according to the presence of subsurface drainage system in this watershed, those two model were applicable to describe the process of nitrogen and phosphorus transportation both in micro- and macroperspectives, and the quantitative of nitrogen and phosphorus during transportation were more efficiency. Correlation coefficient R2 and model efficiency coefficient values NS are: DRAINMOD calibration rate were 0.81, 0.77, respectively; validation period 0.75, 0.72;SWAT calibration rate for the model efficiency is 0.83 validation period is 0.84. In this study, using Latin hypercube sampling method(LHM) algorithm to do the sensitivity and uncertainty analysis, the result showed that hydraulic parameters such as hydraulic conductivity K, surlag were sensitivity param caneters, which were directly affect the model stabilization tendency.In addition, the transportation and transformation of nitrogen and phosphorus results were given, and the profit and loss value nitrogen and phosphorus were calculated, nitrogen loss of 7.3 kg/km2, and phosphorus is even, nitrogen loss seriously.Using the simulation results, we can identify and applied control structure to reduce 78.94% loss of phosphorus by setting filter belt in there.Based on the simulated results by DRAINMOD and SWAT model, Fourier transform and wavelet analysis method were used to analyze the regular pattern of nitrogen and phosphorus transportation on the time line.Results show that the flow in the frequency 1/year(0.1-1, 1-10, 10-100) showed a downward trendenergy spectrum value, means that there are a regularity, the wavelet analysis results showed that the flow could show certain periodicity in the scale of 90 days in three years time.And nitrogen and phosphorus concentration in the drainage pipe had strong energy distribution in 120 days and continued throughout the entire monitoring period. But for correlation of flow and concentration, the energy spectrum peakcan be found a inscale 365 days. By comparing the wavelet coherence of nitrogen and phosphorus concentration in water, the results showed that nitrogen and phosphorus transportation process correlated wellin surface runoff and drainage process, but there are significant difference in the river process.For nitrogen and phosphorus spatial transportation process analysis, this study did it by two separately steps. The point transportation process focused on microscopic process, and whole watershed process targeted macro process. Rainfall event which return period was 1 year, duration were 5, 10, 48 hours,intensity was 4 mm/hr were choenas the initial condition of soil nitrogen and phosphorus transportation, the result showed that there was no flowafter 5 hours rain in drainage pipe,the highest nitrate concentration difference was 2-3 mg/L in same soil profile, for phosphate, the highest concentrate in difference is 0.01 mg/L, there is no obvious change after 48 hours. For 10 hours rainfall events, nitrate concentration in pipe was 5 to 6 mg/L, maximum concentration reached to 8 to 9 mg/L after 48 hours. 48 hours of continuous rainfall nitrogen migration increasing over time, and nitrogen phosphorus is far less than the increase of amplitude, characterized by a relatively stable state.