| Irrigation water with high content of sand particles in irrigation area of Ningxia along the Yellow River,should be properly treated in order to meet the requirement of drip irrigation.Proper treatment of sandy water used for drip irrigation is a key technical and engineering theoretical problem.For treatment of sandy water,suitable filters are pressure filters including centrifugal,screen,media(sand or gravel packs),and disc filters.Although being widely used in irrigation projects,these various kinds of filters have their own limitations due to relatively high sand content in irrigation water,since more frequent backwashing and filter screen burst happen.To overcome the limitations of the aforementioned filters applied in Ningxia irrigation area,the non-all-flow sandy water filtration method and device for drip irrigation was proposed by some researchers.The core idea of non-all-flow sandy water filtration method is that the process of sand particles releasing and sandy water infiltration-filtration happen simultaneously,providing a good reference for sandy water treatment for drip irrigation.Based on the non-all-flow sandy water filtration method,the non-arable land soils or the farmland natural soils are chosen as porous media for sand releasing and infiltration-filtration of water in this study.Aiming at the scope of sand releasing-infiltration-filtration-clogging of the natural soils used as filter media,the mechanism for sand releasing and infiltration-filtration of water using natural soils are system-atically studied,by indoor experiment,field experiment and numerical simulation.Main results obtained in this study are as follows:(1)Considering that the clogging process of soil media used for sandy water filtration is a process in which the clay and silt content increase and that of sand content decreases,i.e.,the texture of soil becomes viscous.The ponded infiltration experiment using soil column and numerical simulation using HYDRUS-1D had been carried out to study the effect of soil texture i.e.,the clay content on soil water infiltration characteristics and steady-state infiltration rate,for filter media such as sandy loam soil,sandy soil and loamy sand soil,respectively.Results show that infiltration characteristics for these three soil media are influenced by clay content apparently.The steady-state infiltration rate for sandy loam soil decreases exponentially with clay content.Philip model was the proper infiltration model for sandy loam soil.The derived predicting model for cumulative infiltration of sandy loam soil using clay content performs well,with relative error less than 8.3%.At the same time,the steady-state infiltration rate for sandy soil and loamy sand soil both show significantly negative correlation with clay content(p<0.01).The modified Kostiakov model was the proper infiltration model for both sandy soil and loamy sand soil.The derived predicting model for cumulative infiltration of sandy soil as well as loamy sand soil using clay content perform well,with relative error less than 10.7%and 11.4%,respectively.(2)To study the effect of clay content on parameters of soil water characteristic curve and the clogging of soil pore space,two soil texture such as sandy soil and sandy loam soil were chosen as filter media.Results show that the soil water characteristic curve and pore size distribution for both sandy soil and sandy loam soil are influenced by clay content apparently.Parameters ?s and a show significantly positive correlation with clay content(p<0.01),and the parameter ?r shows significantly negative correlation with clay content(p<0.01),the parameter n decreases exponentially with clay content(R2>0.99).The volume of transmission pores in the soil(the effective diameter 0.03?1 mm)decreases,while that of storage pores(the effective diameter 200 nm?0.03 mm)increases with increasing clay content.What's more,the volume of the rest of pore space remains unchanged.Main reason for the clogging of pore space is due to contrast volume change of transmission pores and storage pores.The derived predicting model for soil water characteristic curve using clay content performs well,with relative error between predicted and observed values for volume of transmission pores less than 7.82%,while that of storage pores is less than 9.89%.(3)For the interface problem existing in numerical scheme for simulating one-dimensional unsat-urated flow in layered soil with water head-based Richards equation,the Immersed Interface Method(IIM)was introduced to modify the finite difference schemes near the interface to treat the irregularities.Such modified scheme allows cutting the spatial grid in an arbitrary manner,what's more,a second order accuracy of solution can be globally obtained.(4)Aiming at practical issue of non-all-flow sandy water filtration method and device for drip irriga-tion in Ningxia irrigation area,a sandy water filtration method by using non-arable land or natural sandy loam soil and sand soil+subsurface drainage pipes(depth of 1 m)+non-woven fabrics was proposed.The ponded sandy water infiltration experiment was performed using sand tank with ponded depth of sandy water above the soil surface 5 cm,and sand content 2 kg/m3 for sandy water.The filtration performance by using these two soils was investigated and compared.In addition,the impact of sandy water filtration on physical properties of sandy loam soil and sandy soil as well as the clogging mechanism for soil pore space during sandy water filtration was analyzed.Results show that both the sand content and filtration velocity of the filtered water decrease exponentially with time.The largest diameter of sand particles in the filtered water is less than 0.1 mm,which could be used for micro-irrigation.The filtration fineness for sandy loam soil as filter media is better that of sandy soil as filter media.Theoretical clogging time for sandy loam soil and sandy soil as filter media are 107 days and 123 days,respectively.For sandy loam soil as filter media,the average theoretical available water amount is 3.20m3/(m2·d),for per unit area of subsurface PVC drainage pipe,including the filtered water available for screen filter.Theoretical available water amount for drip irrigation is 1.64 m3/(m2 · d),for per unit area of subsurface PVC drainage pipe.In addition,the average theoretical available water amount is 1.35m3/(m2 · d),for per unit area of soil filter media,while theoretical available water amount for drip irrigation is 0.68m3/(m2 · d),for per unit area of soil filter media.For sandy soil,the average theoretical available water amount is 3.51 m3/(m2·d),for per unit area of subsurface PVC drainage pipe,including the filtered water available for screen filter.Theoretical available water amount for drip irrigation is 1.88 m3/(m2·d),for per unit area of subsurface PVC drainage pipe.In addition,the average theoretical available water amount is 1.44 m3/(m2 · d),for per unit area of soil filter media,while theoretical available water amount for drip irrigation is 0.80m3/(m2·d),for per unit area of soil filter media.During sand releasing and infiltration-filtration process of sandy water,the texture as well as structure change for both sandy loam soil and sandy soil.During the experiment,the clay and silt content increase while that of sand content decreases on the whole soil profile for both sandy loam soil and sandy soil.After the experiment,the sandy loam soil changes to a silty-clay soil,and that of sandy soil changes to a sandy-clay loam soil.The volume of transmission pores decreases significantly while that of storage pores increases significantly for both sandy loam soil and sandy soil.The clogging of soil pore space is mainly caused by clay and silt particles carried by sandy water.Clogging depth of the soil media is 0?100 cm for sandy loam soil and 0?80 cm for sandy soil,respectively,with the percentage of clogging of pore space 66%and 63%,respectively.The predicting model for cumulative infiltration of sandy soil using clay content performs well,with relative error less than 11.7%during sandy water infiltration process.(5)Based on the results of non-all-flow sandy water filtration experiment using sand tank indoors,the results of 3-year field experiment of non-all-flow sandy water filtration method and device were com-pared and analyzed.The filtration performance as well as the clogging characteristics were taken into consideration.Results show that when the sand content in the sandy water is 5.66 kg/m3,considering the sand content in the filtered water,the filtration time span,the particle size distribution of sand particles in the filtered water and the filtration velocity,as well as clogging depth of the filter media,the treatment of natural farmland sandy soil+subsurface non-fine concrete filtering pipes(external diameter 400 mm,internal diameter 300 mm,depth 1.5 m)+seepage control+350 g/m2 non-woven fabrics covering the surface of the soil media,is the optimal treatment and is recommended in the field actual use for treatment of sandy water for micro-irrigation.The predicting model for soil pore space of sandy soil and sandy loam soil performs well,with relative error less than 11.34%and 19%for transmission pore and storage pore volume,respectively.(6)The non-all-flow sandy water filter meets the demand of drip irrigation for an entire irrigation season.What's more,removal efficiency for clay particles of original sandy water is good.For field non-all-flow sandy water filter,the corresponding removal percentage for clay particles is 87.50%for sandy soil as filter media,with sand content in the original sandy water 5.66 kg/m3. |
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