分散流控制壁对无压渗漏计收集效率影响的定量评价

无压渗漏计(Zero-tension lysimeter,ZTL)多用于非饱和带土壤溶质通量的监测,但由于ZTL安装时与原状土壤相接触会存在毛管障碍界面,易形成分散流使其土壤溶液收集效率降低。为准确描述田间水分渗漏量或土壤溶质的运移过程与规律,基于HYDRUS模型模拟结果,对ZTL不同设计(加装不同高度分散流控制壁)和不同适用环境条件(土壤质地、灌水量、土壤蒸发量和初始土壤含水率)的土壤渗漏水收集效率及影响因素进行数值模拟和定量评价。结果表明,无分散流控制壁的ZTL(ZTL0),在0.35 cm3/cm3土壤初始含水率、0.2 cm/d蒸发量和1 000 mm灌水量条件下的砂壤土、壤土和粉土处理,收集效率分别仅为11%、13%和26%,而在相同环境条件下安装分散流控制壁的ZTL(ZTLd),当控制壁高度为20 cm时可使收集效率提升到50%以上。安装的分散流控制壁高度随灌水量的降低、土壤持水能力的提高和土壤蒸发量的增大而升高,初始土壤含水率降低会使偏砂性土壤中安装的ZTLd收集效率降低,但在壤土和粉土中安装时可使其收集效率增大。增加ZTLd安装深度可能会导致其收集效率降低,在某一特定安装深度对ZTL收集效率计算的结果并不适用于其他深度。

Quantitative Evaluation of Effects of Divergence Barrier on Collection Efficiency of Zero-tension Lysimeter Collection Efficiency of Zero-tension Lysimeter

Zero-tension lysimeters (ZTL) have been widely applied to monitor soil solution flux and solute transport through soil layers in the vadose zone to the ground water under various conditions in research areas, such as the comprehensive utilization of agricultural water and soil resources as well as soil and water environment conservations. A capillary barrier that can cause flow divergence around the system will be created between undisturbed soil and ZTL as the ZTLs are installed in the field. Meanwhile, the water can overcome the capillary barrier and enter into ZTL only when the soil water is saturated above the contact plane of soil and ZTL. Thus, the efficiency of adopting ZTL for water collection decreases correspondingly. To improve the collection efficiency of ZTL, divergence barrier was added to ZTL (ZTLd), and the factors that can affect the collection efficiency of ZTL were examined, such as soil hydraulic property, soil texture, irrigation rate, initial soil water content and height of divergence barrier, and HYDRUS model was adopted. By measuring soil water flux from seepage face in a two-dimensional model and comparing it with applied flux in a one-dimensional model at the same depth, the collection efficiency was obtained. Three soil textures including sandy loam, loam and silt were considered, and the values of van Genuchten model parameters for test soils were not changed in HYDRUS model. Moreover, irrigation rates ranged from 100 mm to 1 000 mm, and height of divergence barrier ranged from 0 cm to 70 cm. Two initial soil water contents (i.e. 0.15 and 0.35 cm3/cm3) and two soil evaporation rates (i.e. 0.2 and 0.5 cm/d) were adopted. According to the results, the collection efficiency of ZTL without divergence barrier (ZTL0) was low due to lateral diversion of water above the seepage face. As for the ZTL0 under 1 000 mm irrigation rates, the collection efficiencies were 0~11%, 5%~13% and 6%~12% in the sandy loam, loam and silt soil, respectively. Due to the fact that more water was accumulated above the seepage face, the measured soil water flux increased as the divergence barrier was installed. However, the height of divergence barrier was reduced with the increase of irrigation rates as well as the decrease of water holding ability of soil and soil evaporation rates. When the height of divergence barrier was less than 20 cm and irrigation rate reached 1 000 mm under the condition of 0.35 cm3/cm3 initial soil water content and 0.2 cm/d soil evaporation rate, the collection efficiency of ZTLd was increased to 50%. Thus, coarse-textured soils, divergence barrier, low soil evaporation rate and high irrigation rates were preferred so as to measure water flux rate accurately by ZTLd. High initial soil water content (0.35 cm3/cm3) would lead to low collection efficiency of ZTLd in sandy loam, and collection efficiency of ZTL at a certain burial depth might not be suitable for other depths. According to the assumption of the study, all simulations were based on homogeneous soil without consideration of preferential flow, thus there was no effect of size on the collection efficiency. In conclusion, the results of this research can exert significance to the improvement of ZTL.