Impacts of Soil Conservation Measures on Vadose Zone Drainage in the Loess Plateau, China

The Chinese Loess Plateau has one of the highest rates of soil erosion in the world as a result of sediment, topography, climate and land use factors. Soil and nutrient loss from cultivated fields has been a major concern for agricultural sustainability and economy. Extensive remediation measures have been implemented since the 1970s and involve land use changes such as terracing and replacement of crops with shrubs and trees. These measures reduce runoff and therefore would be expected to increase infiltration/recharge and/or evapotranspiration. The aim of this study is to evaluate the effects of soil conservation-related land use changes on drainage/recharge through the vadose zone. Four boreholes (6 to 20 m deep) were drilled and sampled in the Zhifangou watershed (Shaanxi Province) in four different soil conservation settings which are common in the central Loess Plateau: cultivated hill slope terrace, cultivation above a check-dam, planted forest, and planted shrubs. Each of these conservation measures was implemented in the 1970s.

High matric potentials in the tensiometric range (~ 0-1 m) and downward head gradients indicate drainage beneath the cultivated terrace and check-dam sites. In contrast, matric potentials are about three orders of magnitude lower in profiles under shrubs and trees, suggesting near-zero moisture flux. Soil moisture contents mirror this pattern (0.12 and 0.16 g/g beneath cultivated sites compared to 0.08 and 0.09 g/g beneath trees and shrubs, respectively) and are consistent with the post-afforestation reduction in soil moisture documented in the area. Vadose zone chloride concentrations beneath cultivated sites indicate drainage rates of 50 – 100 mm/yr. Vadose zone chloride accumulation is not apparent in the afforested sites, which may be attributed to recent onset of the low moisture flux conditions. Hydrologic impacts of diminished drainage beneath the forest and shrubland could include reduced groundwater recharge and salt buildup in soils.