| The agricultural plastic film in China have been used in a large scale in the past decades,since the agricultural cultivation with plastic film mulching has a lot of advantages including saving water,heating preservation,increasing production and salt controlling,etc.However,the recycling pattern for agricultural plastic residual film is not perfect,and about 30%of plastic film would be left in the soil every year.The worst of it is,the plastic film is very difficult to degrade in the soil,and it should take more than one century for it to disappear under natural conditions.However,the study on the influence of soil water transport of residual film is still less,and the mechanism is still unclear.There are few reports on soil water infiltration,evaporation,hydraulic characteristics and soil water characteristic curve variation of the residual film containing.To solve this problem,the combination of plastic film in North China distribution characteristics was studied through the method of questionnaire investigation and field sampling.Five residual film treatments(0,50,100.200,400 kg/hm~2)and different soil factors were conducted to study the effects of different amounts of residual film on soil water the characteristics and process of water movement,soil evaporation,soil structure,soil water characteristic curve and its model construction etc.This study provides theoretical basis of the soil water movement under the condition of agricultural plastic film residues,and can raise people awareness of the dangers of residual plastic film.What is more,it could ensure sustainable development of grain production and agriculture in china.The main results are as follows:(1)The amount and mulching area of plastic film in Hetao irrigation district of Inner Mongolia increased by 125.81%and 147.62%respectively in recent 10 years,and the typical research area even increased by 240%and 265.81%,while the mulching area per unit area decreased by 8.82%and 7.08%.Different mulching years and irrigation methods significantly affected the film residual rate(R)and film damage rate(D)(P<0.05),and the R of mulching 5 years,10 years and 20 years increased by 80.14%?163.70%?273.64%than 2 years respectively on average,and for D it increased by 20.97%?38.140/%?60.20%respectively on average than 2years.Compared with surface irrigation,the R and D of drip irrigation increased 42.92%and 20.01%on average.The residual plastic film mainly concentrated in the soil depth of 0?10 cm,which accounting for 64.89%;and the residual film area between 0?4 cm2 and 4?20 cm2 was 58.86%of the total amount in the farmland;and with the mulching age increase,the total amount of residual film residues in the lower soil depth gradually increased,which the 0-10cm soil residual film was reduced from 71.47%to 41.40%for the mulching years from 20 to 2.but the 10?20cm soil residual film was increased from 23.37%to 32.24%;and the different irrigation methods had few effect on the distribution of residual plastic film in soil.With the increase of mulching years,the proportion of the different period area of residual film was more significance,which because of the small plastic film proportion increased;and the residual film area below 20 cm2 were 1.43,1.44,1.51,1.59 times than the area number more than 20 cm2(P<0.05)for the mulching years of 20 years,10 years,5 years and 2 years.(2)The threshold CT images showed that black spot area clearly increased with the increase of residual film,and there was proportional relation between black spot area and residual film amount.The black spot areas were 19 times than non-residual film treatment,so it showed residual film significantly increased in soil with quantification and visualization.With the increase of film residues,two kinds of soil saturated water content showed a decreasing trend,but the sandy loam of SL1?SL4 treatment had no significant difference(P>0.05),and it showed significant differences(P<0.05)until 400 kg/hm~2 residual film treatment(SL5).The soil capillary water content showed a decreasing trend for both sandy loam and sandy soil,and the reduction degree of sandy soil was less than sandy loam.The water-holding capacity decreased with the increase of the amount of residual film more obviously,and the treatment of SL5 decreased by 52.75%compared with the SL1 treatment,SS5 decreased by 56.83%compared with SS1 treatment.For the sandy loam,with the gradual increase of the amount of residual film in soil,soil saturated hydraulic conductivity has stepped a clear downward trend.When the film reached 100 kg/hm~2,it decreased by 41.37%.For sandy soil,the overall trend was slowly reducing.The Boltzmann parameter values of different treatment decreased with increasing soil moisture content,in addition,with the increase of the amount of residual film in the soil,the soil water holding capacity was gradually reduced.(3)The ability of soil water retention decreased gradually with the increase of residual film.The equivalent aperture ratio was increased in the low suction stage(mainly draining soil water in large pore),and on the opposite,the equivalent aperture ratio was decreased in the high suction stage(mainly draining soil water in medium and small pore).Especially when the residual film reached to 400 kg/hm~2,the equivalent aperture ratio in the low suction stage increased by nearly 20%than the non-residual film treatment.The parameters of RPF-SWCC model showed that the saturated soil water content decreased with the increase of residual plastic film,and the accuracy of RPF-SWCC model generally higher than van Genuchten(VG),Brooks Corey and Log normal distribution model.The RMSE(root mean square error),GMER(geometric mean error ration)and R2 of the RPF-SWCC model were all better than these models.So,it can be concluded that RPF-SWCC model has better performance for fitting soil water characteristic curve with residual plastic film.(4)With the increase of residual film in the soil,the infiltration rate of sandy loam and sandy soil decreased slowly,and the time required for wetting front migration increased significantly,and the sandy loam with 400 kg/hm~2 residual film treatment(SL5)was increased by 27.56%compared with that of non-residual film treatment(SL1)at 30 cm soil layer.In addition,the cumulative infiltration amount was significantly decreased(P<0.05)in the same infiltration time,and the cumulative infiltration amount of SL5 treatment reduced by 52.01 mL(23.12%)compared with SL1 at the end of infiltration.The evaporation rate and cumulative evaporation were significantly reduced(P<0.05)with the residual film increasing,and the cumulative infiltration rate of SL5 treatment was 30.63%less than that of SL1 after evaporation,and the influence of different residual film amounts on sandy loam was greater than that of sand soil.The four major soil water infiltration and evaporation model were evaluated,it was showed that both Kostiakov and Philip infiltration models can well simulate the soil water infiltration for the soil with residual plastic film,but the Philip model had better performance than Kostiakov model,and had better adaptability for sand with plastic film residue.The fitting accuracy of Black evaporation model decreased with the increase of residual film,but the Rose evaporation model were less influenced by the amount of residual plastic film.So it can be concluded that the Rose evaporation model was more suitable for the film residual soil cumulative evaporation estimation.(5)The deeper buried depth of plastic film residue caused the smaller the wet zone after drip irrigation.The wet zone around the plastic film was irregular.There was remarkable different between the drip infiltration in residue layer and the same layer 's infiltration of corresponding other treatments(P<0.01).In addition,soil water around the residue zone was apparently higher,and maximum soil water in the study zone increased with deeper buried depth of plastic film residue,and there were the similar laws for different drip discharges.So it can be concluded that the buried depth of plastic film residue in soil has a large influence on the soil water infiltration.It is meaningful for designing the irrigation schedule to make clear the effect of burial depth of plastic film residue on drip infiltration. |
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