Research On The Characteristic Of Seasonal Snow At Low Hill Area In Northeast China

Snowmelt erosion is one of the main types of erosion in the seasonal snow cover area.Northeast is one of the four major snow areas in our country,which has heavy snowmelt erosion.The loss of nitrogen and phosphorus is different from that of hydraulic erosion under the action of snowmelt erosion,for snow is an important chemical ion wet deposition process,and the chemical source of snowmelt runoff is not only from the soil,but in the precipitation process as well.At present,researches on snowmelt erosion at the northeast region are concentrated on loss in runoff and sediment,while study on the chemical ion analysis of snowmelt runoff is relatively few,moreover,even fewer on both the physical properties and the chemical ion contents of snow cover during snow period.Therefore,this study had selected the Jixing small watershed,which lies in Meihekou,Jilin province,to observe the snow depth,snow density and liquid water content during the snow processes,and to analysis their variations on vertical profile by combining with meteorological data.Besides,the fresh snow and snow cover were collected hierarchically in order to determine its chemical ion contents,and during the snowmelt process,the relevant runoff were collected for determining chemical properties of it,so as to analysis the physical and chemical properties variation from snow processes to snowmelt ones.The main conclusions are as follows:?1?The main period of snow cover was from November to next February,snow depth has negative correlation with snow and air temperatures,and it was positively correlated with snowfall.Snow melting processes were mainly concentrated between 10:00 and 18:00.The snowmelt could exerted hysteretic effect on air temperature;At the beginning of snow accumulation,from the bottom layer?0-5cm?to the surface one,the snow cover density,under its own gravity,decreased gradually,with the formation of depth hoar in snow cover's bottom layer,the variation rule of snow density,on vertical profile of snow layer,in the 5-10cm of middle layer was greater than the surface and bottom ones;Affected by upper snow pressure and late depth hoar,the snow cover density,in the bottom layer,showed a tendency of increasing first and then decreasing.At the beginning of snow accumulation,the liquid water content of snow layer decreased gradually from the bottom layer to the surface one,with extension of snow cover time,this content of each snow layer showed a single peak type with the variation of depth,apart from these,this content,in the 5-10cm of middle layer,was higher than the surface layer and bottom one,under this circumstance,the "retaining water"was formed;the snow layer temperature was the main factor that affecting the magnitude of liquid water content,and there was a significant positive correlation between them.?2?In the aspect of fresh snow chemical properties,the mean concentration of anion and cation followed the order as NO₃^->SO₄^2->Ca²⁺>cl^->NH₄⁺>K⁺>Na⁺>PO₄^3->Mg²⁺>NO₂^-,with the NO3-as the main anion,the Ca²⁺ as the main cation,which accounted for 39.4%and 13.51%of the total ion contents.In the aspect of snow cover chemical properties,the NH₄⁺,NO₃^-and SO₄^2-,in different snow depth,are changing similarly,and their contents of the bottom layer,which were increasing first and then decreasing with time varying,were less than the surface one,where the NH₄⁺ and SO₄^2-contents increased gradually with the extension of snow cover time.Likewise,the change of K+ and Na+ were similar,and their contents,which were highest in January,in the 0-5cm of bottom layer,were higher than the surface one.Similarly,the changes of Mg²⁺ and Ca²⁺ were similar,and their contents,which were higher in the surface layer than the bottom one from January,in the 0-5 cm of bottom layer,from December to next January,were higher than the surface one.The content of NO2-was less,and it was not detected in the upper layer.The chemical ion contents within snow cover were much higher than that of fresh snow,and snow cover played a role in preserving the chemical ions.?3?Organic nitrogen and nitrate nitrogen were the main components of the total nitrogen in fresh snow,the change range of them within the total nitrogen were 19.9%-53.2%and 23.8%-69.9%respectively.In the surface layer,the total nitrogen content of snow cover was highest,in the middle layer,however,this content was lower than the surface and bottom ones.The average content of the total nitrogen within snow cover was between 2.27 and 4.66mg/L,and the ratio of inorganic nitrogen to total one was between 53.9%and 69.8%,with the NO₃^--N as the main element,the organic nitrogen as rest.The total phosphorus content in fresh snow was between 0.69 and 2.16mg/L,the content of PO₄^3--P was relatively low,and the ratio of which to total phosphorus was between 6%and 37%.The total phosphorus content in snow cover was concentrated mainly in the bottom layer,the highest content of it was 4.78mg/L,and the ratio of PO₄^3-P to total one was between 3.0%and 15.4%,besides,with the extension of snow cover time,the ratio of PO₄^3--P was falling??4?The pH value of fresh snow was between 4.4 and 6,which showed acidic,and the main reason for the decrease of it was acidic material input;According to the calculation of neutralizing acidity,65.7%acid materials were neutralized,and the neutralization ability of Ca²⁺ and NH₄⁺ were more stronger;In the aspect of conductivity range,its variation range was from 14.9 to 52.3us/cm,and there was a significantly positive correlation between pH value and conductivity;In the aspect of pH value in snow cover,it was less than 5.6 overall,during that period,the acidity of snow cover in the surface layer was higher than the bottom one,and it decreased gradually from the bottom layer to the surface one.In the aspect of the snow cover conductivity,except the mid-December,during which the snow cover conductivity,in the surface layer,was lower than the bottom one,the content during rest time was opposite;According to the Hysplit Air Mass Backward Trajectory Analysis,it showed that,in the study region,the snow cover period was mainly influenced by the northwestern wind,the air mass was mainly came from inland area,and the Ca²⁺,Mg²⁺,K+ and Na+were mainly came from terrigenous dust,while the NO₃^-,Cl-and SO₄^2-were mainly originated from burning straw and coal.?5?Considering the snowmelt runoff,the ion content within it was related to the volume of it as well,which was higher(except PO₄^3-)when the runoff was comparatively large.The ion content within runoff followed the order as NO₃^->SO₄^2->Cl>Ca²⁺>K⁺>Mg²⁺>PO₄³>NH₄⁺>Na⁺>NO₂^-.The ion content within runoff was much larger than that in snow cover;The total nitrogen content in runoff was from 6.62 to 20.52mg/L,which was higher than that within fresh snow and snow cover.The content ratio of inorganic nitrogen to total nitrogen was comparatively high,which was between 77.0%and 92.8%,and the proportion was slightly increased by comparing with the average ratio of snow cover,moreover,the main element within the total nitrogen was inorganic nitrogen.Considering the total phosphorus content,it was between 1.24 and 3.70mg/L,the variation of which was silmilar to the flow rate,the ratio of P043-P to total phosphorus in runoff was from 12.4%to 23.8%.The pH value of runoff,however,was between 5.01 and 5.16,which showed acidic.By calculating the neutralization acidity,about 86.8%acids could be neutralized by the alkaline ions.The variation of runoff conductivity was similar to that of ion concentration,which was a reflection of the total contents of ions.