推求土壤非饱和运动参数的方法

推求土壤非饱和运动参数的方法

硕士生：景为

学科专业名称：土壤学

研究方向：土壤水分动力学

指导教师：邵明安研究员

准确获取能代表田间土壤条件的土壤水分运动参数（土壤水分特征曲线（或比水容重C）、土壤导水率K和土壤水分扩散率D）是模拟土壤中水和溶质运动的基础。三个参数中，以预测非饱和导水率最为困难，原因之一在于直接测定困难。对土壤水分运动参数空间变异性认识的加深将有助于预报田间水分和溶质迁移过程，也有助于完善参数确定的方法，使之更具普遍性。在以往的研究中，已有许多直接测定或间接推求这些参数的方法。本文选取了其中的三种代表方法，以实测水分特征曲线作为标准进行比较，评价各自的优缺点及适应范围。三种方法是：(1)实测土壤水分特征曲线；(2)用简单入渗法推求van Genuchten水分特征曲线模型中的参数α和n,通过实测饱和导水率Ks，结合导水率模型而获得非饱和导水率K；(3) 根据土壤水分水平和垂直再分布过程直接推求非饱和导水率K和扩散率D。研究结果表明：

1．四种非扰动土壤饱和导水率具很大的差别，其半方差随间距加大而增加，但很快达到一个稳定值,此值即为其变异性的空间尺寸，沙土、黑垆土的空间尺寸为2m，黄绵土的为2.24m，娄土的则更小。

2．土壤水分再分布实验表明，用三种函数拟合湿润锋湿度与平均湿度的关系时，以指数函数拟合计算的比水容重值与实测值最为吻合，尤其是沙土、黄绵土、娄土。

3．利用简单入渗法估计van Genuchten水分特征曲线模型模型中的参数时，α和n值推求的准确度就主要取决于S值测定的准确度，而S的准确测定较易实现，由此可断定简单入渗法的准确性较高。

4．在三种推求导水参数的方法中，水分再分布方法准确性较差，但它无需测定水分特征曲线即可直接得到土壤导水参数K和D，是一种非常简便的方法，尤其适宜于黄土高原沙土导水参数的测定；由简单入渗法获得的水分特征曲线与实测值吻合最好，随着质地变细，拟合效果更好，适合于黄土高原黄绵土、黑垆土和娄土导水参数的测定，而且还解决了Van Genuchten模型中参数不唯一的问题，实验简便，省时（约需2天），计算简单，结果准确，具有很大的优越性。

5．三种方法的适用范围，简单入渗法在整个测定范围内均适宜，水分再分布方法在低含水量段较有优势。

关键词：土壤水分运动参数推求方法比较

Estimating Methods for Soil Hydraulic Properties

Graduate: jing Wei

Supervisor: Prof. Shao Mingan

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: 1.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 αand 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.

1. 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, Huangm ian soil’ is about

2.24m,and L ou 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 αand n, the author finds when S(suction) increases, αand n increase. Thus, the accuracy of αand 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 αand 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.

Keywords: Soil water movement parameter, Estimating method, Compare