Estimating Methods For Soil Hydraulic Properties

An important step for understanding transport of water and solute in soils is to obtain soil water properties which can represent field conditions. The parameters include soil water characteristic curve, hydraulic conductibility (K) and water diffusivity(D). Among the three parameters, it is difficult to estimate K because of its difficulty to be measured directly. To understand the spatial variability of the parameters is beneficial to predict water and solute transport in soils and to use related estimating methods. A lot of methods have been reported in obtaining directly or inquiring into indirectly those parameters. Three representative methods are selected and each merits/shortages and applicable water content range are discussed in this thesis. Experimentally measured soil water characteristic curve is selected as a standard. These methods are: l.van Genuchten water retention model, based on experimental measurement of soil water characteristic curve, which is the most popular model in use and can gain K expression by combining with hydraulic conductibility model such as Mualem model or Burdine model. 2. Shao' s integral method to obtain parameter a and n in van Genuchten model. Same as van Genuchten water retention model, it can get K by combining with hydraulic conductibility model and measuring Ks. 3. Through observing the horizontal and vertical water redistribution processes directly obtain K and D. Some conclusions are presented.l.The K of four undisturbed soils is greatly different, and its spatial variability is significant. By analyzing K's semi-variance, we can see that the value of semi-variance increases while the distance between two measured spots increase, but it reaches a stable value quickly. This value is called its spatial size. Within this size, the K value can be considered as the same. The experimental results show that the size of sandy loam and Heilu soil is within 2m, Huangmian soil' is about 2. 24m, and Lou soil' is less than 0.5m.2. The experiments on water redistribution method show that the calculating C value agrees with experimental C value mostly when index function is used to simulate the relationship between wetting front humidity and average humidity, especially for sandy loam, Huangmian soil and Lou soil.3. By using integral method to estimate parameter a and n, the author finds when S(suction) increases, a and n increase. Thus, the accuracy of a and n is mainly determined by the accuracy of S.4. Among the three methods, water redistribution method gains worse accuracy, but it is a direct method to obtain K and D, further obtains water characteristic curve. The experiment is very simple, and needn' t special equipment. Thus, it is a suitable method for the researchers lack of equipment to measure soil hydraulic properties.Integral method has the best accuracy, and finer texture can gain better estimate results. It solves the un-sole problem of a and n in van Genuchten model. The experiment is very simple, and only 2 days are needed in one experiment.5.Integral method is fit for the whole water content range. Water redistribution method has superiority in lower water content range.