中国根层与表层土壤水分关系分析

根层与表层土壤水分的关系, 是由较易获取的表层土壤水分信息去探讨较难获取的深层土壤水分信息的重要桥梁。已有的根层与表层土壤水分关系(简称根表关系)大都基于一种作物或一种生态系统。该文根据我国生态系统研究网络, 包括森林、草地、农田、荒漠和沼泽生态系统的31个站点109个观测场2006年全年3437对根层和表层土壤水分数据, 研究了根表关系以及生态系统、土壤质地、湿润度、植被、土壤厚度和土壤水分量级对根表关系的影响。研究发现, 表层和根层土壤水分存在着线性关系。森林和沼泽的根层与表层土壤水分相关程度较高, 无论是率定段还是校核段, 其决定系数(R²)均大于0.79。农田和草地生态系统的率定段相关性较好, R ²均大于0.80, 校核段相关性稍弱, R ²分别为0.70和0.50。荒漠生态系统的相关关系最弱, 率定段的R²为0.62, 校核段的R²为0.49。土壤质地和生态系统因素对根表关系的影响较为一致。半湿润带、半干旱带和干旱带的根表关系空间分异性最强; 十分湿润带的根表关系与壤土和森林生态系统的根表关系相对应。湿润带内部的根表关系较为一致。将植被对根表关系的影响分为4类, 前两类为根表关系微弱的植被, 由植被本身或者植被以外的地域因素导致, 不适合用根表关系去由表层推算根层土壤水分; 后两类为根表关系良好植被, 区别为服从和不服从关系总线, 可分别用各自的根表关系或者关系总线从表层土壤水分获取根层土壤水分。表层土壤水分与0-20、0-30、……、0-100 cm土层的土壤水分均分别具有较好的相关关系, 但二者的相关性随土层厚度的增加而降低。不过, 即使是土层厚度抵及100 cm, R ²仍能维持在0.79。通过将土壤水分分别除以所有观测数据的最大值(“标甲”法)和各个生态系统数据的最大值(“标乙”法), 发现根表关系不受土壤水分量级本身的影响。

Relationship analysis between soil moisture in root zone and top-most layer in China

Aims It is more difficult to determine soil moisture in root zones (RSM) than soil moisture in the top-most layer (TSM). The relationship between TSM and RSM based on point A is useful for acquiring RSM at point B from its TSM if the relationship between TSM and RSM is general. The general relationships established so far have been based on only one crop or one ecosystem. Our aims are to determine the general relationship between TSM and RSM in China over various crops and ecosystems and to explore the effects of ecosystem, soil property, precipitation, vegetation, soil thickness and the order of magnitude of soil moisture on the relationship.
Methods We used 3 437 pairs of TSM and DSM data from 31 stations at 109 observation sites. The data represent all the terrestrial ecosystems, including forest, grass, agriculture, desert and mire, in 2006 from the China Ecosystem Research Network. We used linear regression in mathematics and effect analysis in physics to study the general relationship between TSM and RSM and to determine the influence of ecosystem, soil property, precipitation, vegetation, soil thickness, the order of magnitude of soil moisture on the relationship. The data are divided into calibration and verification groups for producing the relationship.
Important findings There is generally a linear relationship between TSM and RSM. The coefficient of determination (R²) between RSM and TSM is high for forest and mire ecosystems (R² > 0.79 for both calibration and verification), intermediate for agricultural (R² > 0.80 for calibration and 0.70 for verification) and grass ecosystems (R² > 0.80 for calibration and 0.50 for verification) and low for desert ecosystem (R² = 0.62 for calibration and 0.49 for verification). The effect of soil properties on the relationship is similar to that of ecosystem, with mire ecosystem corresponding to meadow soil with white pulp, desert ecosystem corresponding to aeolian soil, grass ecosystem corresponding to sandy-loam and sandy soil, forest ecosystem corresponding to loam and agricultural ecosystem corresponding to various soil types. The spatial variability of the parameters for the relationship is very high for soil moisture within semi-humid, semi-arid and arid zones and low for humid zone. The relationship is similar among very-humid zone, loam and forest ecosystem. The effect of vegetation on the relationship can be divided into four categories: poor relationship due to the influence of vegetation itself, poor regional relationship due to the difference between the location sites of vegetation, good relationship at single point and good relationship over a region. It is not recommended to derive the RSM from TSM based on the relationship for the first two categories. It is suitable to use the relationship only at single point for the third category. And it is practicable to use the regional relationship to derive RSM from TSM for the fourth category. Correlation is higher between TSM and RSM where the soil layer is thicker. The R² can remain as high as 0.79 when the soil
layer is as thick as 100 cm. By scaling the original soil moisture data with maximum value, it is found there is no effect of the order of the magnitude of soil moisture on the relationship.