| In Northeast China,seasonal freezing and thawing are common.Snow melting erosion is dominant during thawing period.Snow melting runoff will carry a large amount of nitrogen,phosphorus and ions to the downstream water body,which is the main reason for the loss of nutrients on the slope and the occurrence of agricultural non-point sources during this period.Ridge farmland is widely distributed in Northeast China.However,up to now,little research has been done on the migration of runoff,nitrogen,phosphorus and ions in ridge farmland during snowmelt period,which can not provide theoretical basis for the layout of agricultural non-point source pollution control measures in this period.Taking farmland with different ridge directions in Northeast China as the research object,through two years of field fixed-point observation experiments,the physical characteristics of snow-melting runoff,thawing depth and snow-melting rate of different ridge directions（named T1,T2,T3,T4,T5,T6,T7）and the anion and cation,p H value of snow-melting runoff in farmland with different ridge directions were analyzed.The migration process of chemical indicators such as conductivity,total nitrogen and total phosphorus was studied,and the loss rule of chemical indicators in snow melting runoff and snow melting process in different ridge directions was explored.The main conclusions are as follows:（1）During the snowmelt period,the maximum snow depth is on T1 day and the smallest is on T7 day.From T2 to T7,the snow depth decreased gradually.During the 7 days from March 18 to March 24,the difference of snow depth between T1 and T7 was 0.24,2.00,2.31,1.73,3.05,4.89 and 1.53 cm,respectively.On the last day,the snow depth of T1 decreased significantly,and a large amount of runoff broke through the ridge.（2）During the snowmelt period,the radiation received from the ridges to the farmland,the snowmelt rate,the runoff and the thawing depth changed similarly.The correlation between received radiation and snow melting rate and thawing depth reached a significant level,and the influence on snow melting rate gradually decreased with the increase of temperature.The correlation between snow melting rate and runoff is significant.（3）Runoff varies greatly in three days when the ridge flows to the farmland.The runoff from T2 to T7 on March 23 was larger and significantly higher than that on March 22 and 24.The runoff was 2.06,1.09,0.89,0.88,0.7 and 0.17 times higher than that on March 22,and2.02,1.47,1.39,1.9,0.75 and 0.44 times higher than that on March 24.The runoff of T1 is smaller in the first two days because of its strong snowwater storage capacity.On the third day,the runoff reaches a peak value of 350.39 ml/s when it breaks through ridges and platforms.Among the ridge-oriented farmland,the loss of T2 was the lowest at 272 L,which was 0.51times more than that of T1,but there was no significant difference between them.（4）During runoff generation,the variation of ions in ridge runoff was NO3->SO42->Ga2+>Mg2+>Cl->K+>Na+>NH4+>NO2->PO43-,and NO3-was the main loss ion.The correlation between the electrical conductivity of ridge and ion loss was significant.The changes of ion content and loss amount in ridge farmland were T1>T7>T6>T5>T4>T3>T2,T7>T1>T6>T4>T5>T3>T2,respectively.The proportion of ion loss was22.25%,18.45%,16.3%,12.1%,11.63%,11.43%and 7.85%respectively.（5）During runoff generation,the nitrogen and phosphorus content and total amount of some ridge farmland changed significantly.Compared with the traditional transverse ridge T1plot,the total nitrogen content in T2-T7 plot decreased by 58.88%-34.4%,57.3%-39.7%,64.85%-29.2%in three days,and the total nitrogen content in T2 plot was the lowest,while the total nitrogen loss in T2,T 3 and T4 plot was lower than that in T1 plot plot,with a reduction of 30.21%,17.61%and 3.96%respectively.The total phosphorus loss in each plot was small,and that in T1 plot was the lowest,with a total loss of 18G. |
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