| The occurrence of farmland runoff is generally random,but it also shows the character of regularity under the influence of specific conditions such as rainfall,soil water content,and field vegetation coverage.In this case,determining the characteristics of farmland surface runoff and nutrient loss is of great significance to formulating nutrient reduction strategies according to specific temporal and spatial characteristics.Therefore,this study first clarified the high-risk period of rice-wheat rotation farmland runoff in the Taihu Lake area through historical rainfall data analysis and literature review.Then,the field rainfall simulation device was used to study the characteristics of farmland runoff and nutrient loss under different rainfall intensities at different growth stages（vegetation coverage）of rice and wheat farmlands.Finally,a comparative study of different sowing and planting methods was carried out while considering the impact of wheat sowing and planting methods on runoff.The research findings listed as follows can provide technical and scientific support for runoff reduction in rice and wheat farmlands in southern China.（1）The average annual precipitation in the Taihu Lake area was 1,164.8 mm（657.7-1,643.7 mm）,and the anomaly percentage of annual precipitation basically varied around±25%（normal）.After excluding the precipitation probability and daily average precipitation in the wheat-growing season after the flood year and the drought year,it is found that the monthly time series changes showed a trend of declining first and increasing then.Among them,the probability of precipitation and average daily precipitation in December was the lowest.Meanwhile,according to the statistical results of the 12-year runoff occurrence time of the wheat-growing season,February（19.05%）and March（18.10%）had a relatively high number of runoff occurrences,which were the high-risk periods for wheat farmland runoff.The actual minimum precipitation that drove runoff in different years was 8.1-19.4 mm,indicating that the probability of runoff was not completely consistent with the change in the probability of precipitation.In addition,in the rice-growing season,the highest rainfall probability（63.33%）and rainfall（20.33 mm）happened in June,indicating that June is more likely to produce runoff,a high-risk period for rice farmland runoff.（2）The time to runoff of wheat farmlands under different vegetation coverage was only significantly related to rainfall intensity.Runoff coefficient under low vegetation coverage was significantly affected by rainfall intensity and sowing methods,but it was not affected by sowing methods or rainfall intensity under high vegetation coverage.It also found that the peak runoff rate under different vegetation coverage all appeared in the middle period and late period of runoff.In particular,the peak runoff rate of drilling treatment(193.27-521.83 m3·ha-1·h-1)of the low vegetation coverage under the same rainfall intensity was higher than that of the broadcasting treatment(144.62-414.76 m3·ha-1·h-1).Also,the peak runoff rates of different planting methods under high vegetation coverage were relatively close.The initial period of runoff was a high concentration period of nutrient loss（the change with runoff time can be described by logarithmic function and power function）.Generally,it increased with the increase of rainfall intensity,and was affected by vegetation coverage and planting methods.The peak nitrogen（N）concentration（2.25-4.57 mg·L-1）of the drilling treatment under low vegetation coverage is higher than that of the broadcasting treatment（2.19-5.00 mg·L-1）,but the case was opposite under high vegetation coverage（drilling: 1.24-1.95 mg·L-1;broadcasting: 1.58-2.74mg·L-1）.The phosphorus（P）performance of the drilling treatment under low vegetation coverage is consistent with N,but the peak P concentration of the broadcasting treatment under high vegetation coverage is higher under high-intensity rainfall（drilling: 0.93 mg·L-1;broadcasting: 1.94 mg·L-1）.（3）Affected by the loss rate,the loss rate of N and P in wheat farmlands in the middle and late stages of runoff is relatively high,especially under medium-and high-intensity rainfall.Under the same rainfall intensity,the N and P loss rate of the drilling treatment under low vegetation coverage was higher than that of the broadcasting treatment.Under high vegetation coverage,it was equivalent to the broadcasting treatment.The N loss of the runoff under different vegetation coverage and planting methods was mainly inorganic N,and nitrate nitrogen（NOX--N）accounted for a relatively high proportion.In comparison,the P loss of the runoff was mainly particulate phosphorus（PP）.The N loss under the two vegetation coverages had different responses to rain intensity.The total P loss under different vegetation coverage and planting methods increased with the increase of rain intensity(low vegetation coverage:0.08-0.44 kg·ha-1;High vegetation coverage: 0.02-0.1 kg·ha-1).The N loss under low vegetation coverage increased with the increase of rain intensity.The N loss under the same rain intensity(0.21-0.80 kg·ha-1)was higher than that of sowing(0.16-0.61 kg·ha-1).However,there was no significant difference in the amount of N loss in different treatments under high vegetation coverage.The amount of N loss was closely related to the runoff rate and nutrient concentration before and during the runoff,while the amount of P loss is closely related to the runoff rate and nutrient concentration in the middle and late stages of runoff.（4）Under different vegetation coverages,the outflow rate of rice farmland runoff showed a trend of rising first and decreasing then,and the peak outflow rate increased obviously with the increase of rain intensity.The change characteristics of N concentration with runoff time under different rainfall intensities can be fitted with a logarithmic function,presenting an overall trend of decreasing.The variation characteristics of P concentration with runoff time under different vegetation coverages and rain intensities can be fitted by a power function,presenting an overall trend of downward.The initial runoff period was the risky period of high N and P concentration.The peak value of N concentration under the low vegetation coverage and high vegetation coverage was 1.32-1.72 mg·L-1 and 0.96-1.03 mg·L-1,respectively.The peak value of P concentration under the low vegetation coverage and high vegetation coverage was 0.19-0.31mg·L-1 and 0.24-0.55 mg·L-1,respectively.The initial period（0-20 min）and the middle period（20-40 min）of runoff were the high-risk periods of N and P loss.The N and P loss rate in runoff was less affected by rainfall intensity because of the effect of rain intensity reduction under high vegetation coverage.Therefore,the N and P loss of different rainfall intensities under low vegetation coverage was significantly different.Especially,the N and P loss in high rainfall intensity increased significantly.The N loss in rice farmland runoff was mainly NH4+-N,whose proportion under different rainfall intensities was relatively close（41%-52%）.The P loss was mainly TDP（69%-85%）,which proportion had a negative correlation with rainfall intensity.The N loss load was closely related to the runoff rate and N concentration in the early period and middle period of runoff.The P loss load was closely related to the P concentration in the field surface water before the experiment,the P loss concentration in the middle period of the runoff,and the runoff rate in the middle period and the late period. |
