半干旱黄土丘陵区撂荒坡地土壤水分循环特征

为深入了解半干旱黄土丘陵区土壤水分循环特征和为开展荒地造林工作提供背景数据,在宁南上黄生态试验站,选取典型多年撂荒坡地,进行土壤水分的长期定位观测,分析其土壤水分补给、消耗特征与时空变异性。结果表明:研究区降雨入渗量和入渗深度随降雨量增加而增加,入渗补给系数约为0.44,雨水资源化率有待提高。定义全年一半以上的次降水事件中能被有效补给的土层深度为降水普遍入渗深度,则研究区降水普遍入渗深度为0—40 cm,观测期内最大入渗深度不超过300 cm。同时,土壤水分的蒸散发量在丰水年平水年干旱年,主要蒸散发作用层位于0—200 cm土层范围内,最大蒸散发深度达到300 cm以下。该区土壤储水量的季节变化为"V"型,剖面土壤平均含水量的垂直变异则呈现反"S"型。土壤水分的变异系数随土层深度的增加表现出幂函数递减趋势,结合有序聚类法的分层结果,可采用0.20和0.05两个CV值将撂荒地土壤剖面划分为水分活跃层(0—40 cm)、次活跃层(40—200 cm)和相对稳定层(200 cm以下)3个层次。

Characteristics of the soil water cycle in an abandoned sloping field in the semi- arid Loess Hilly Region

To better understand the characteristics of soil water cycle in the semi-arid Loess Hilly Region and to provide background data for the change in land cover, a typical sloping filed which was abandoned for many years was selected at the Shanghuang Experimental Station in southern Ningxia. Dynamics of the soil water were continuously monitored by using a neutron probe in fixed positions. The results showed that the rainfall was not frequent and precipitation was limited. The amount of rainfall recharge (y) was positively correlated to the precipitation (x), as expressed in the equation y = 0.44x - 0.58 (R² = 0.980, P<0.01). The recharge coefficient was 0.44, and the rate of rainwater harvesting was to be improved. Similarly, the recharge depths were increased with the precipitation. The impact of rainfall on soil water content was mainly concentrated to the top 40 cm of the soil. There was a time lag between the rainfall recharge and the soil water in deeper soil layers. The soil depths could be effectively recharged by rainfall of more than 50% precipitation events in a whole year were defined as the general infiltration depth. This concept was helpful to redefine assessment standards of dried soil layer (DSL). In the study area, the general infiltration depth was 0-40 cm. Although continual rainfall increased the movement of soil water to deeper soil layers, the maximum infiltration depth did not exceed 300 cm during the observation period. Meanwhile, evapotranspiration in this region was always positively correlated to air temperature and soil water content. During the growing season, evapotranspiration was stronger than in the non-growing season and it was higher in the wet year and lower in the dry year than in the year with normal level of precipitation. Soil water content, which was influenced by evapotranspiration, decreased with soil profile. The major depth that was affected by evapotranspiration was 0-200 cm, with the maximum depth reaching to more than 300 cm. Furthermore, seasonal changes in the soil water storage showed a V-shaped trend, with the minimal value appearing at June or July. The average soil water content in the soil profile showed a reversed S-shaped trend, with the maximum and minimum values at the depth of 40 and 200 cm, respectively. Additionally, variance coefficient of the soil water from the soil profile showed a decreasing power function trend with the increase of soil depth. Referring to the results of vertical division of soil water profiles by the sequential clustering method, the variance coefficient of 0.20 and 0.05 could be used as criterions to divide the soil water profile into three levels: the active layer (0-40 cm), the second active layer (40-200 cm), and the relatively stable layer (deeper than 200 cm). In the wet year, due to continual rainfall infiltration and strong evapotranspiration, the variance coefficients at the soil depth of 0-200cm were increased and the range of active layer was broadened.