黄土塬区苹果种植对深剖面土壤水分和地下水补给的影响

通过对长武黄土塬区不同林龄苹果林地下0~20 m深剖面土壤湿度及土壤水氯离子浓度测定,定量分析了黄土塬区苹果种植对厚深黄土剖面土壤水分及地下水补给的影响。结果表明：随着林龄增加,苹果林地深剖面土壤水分由浅及深逐年降低,深层土壤储水量呈倒“S”曲线趋势下降,27龄之后进入稳定期。丰水年份形成的活塞流是黄土塬区深层渗漏以及地下水补给的主导方式,农田下地下水年均潜在补给量为30.2 mm,占年均降水量的5.2%。农田转换为苹果林地后形成的深厚土壤干层将阻断降水对地下水的补给,减弱地下水补给过程中活塞流的主导作用。需通过政策引导协调农果面积比例,保证地下水资源持续补给,达到可持续利用的目的。

Impacts of apple tree planting on deep soil water content and groundwater recharge for the Loess Tableland of China

As an important component of terrestrial water resources, soil water and groundwater are critical factors in forestry development and human life on the Loess Plateau of China. In the past 20-30 years, large areas of farmland were converted to apple orchards, which resulted in excessive consumption of deep soil water and reduction of groundwater recharge. To quantify the impacts of apple trees on soil moisture and groundwater recharge, the soil water content and chloride concentration of soil water in 0-20 m deep profile were measured in apple orchards with different ages on the Changwu Tableland. The results showed that: the soil water content in apple orchard along the deep profile decreased from shallow to deep layer with increasing tree age, and the mean soil water contents in 5-yr, 10-yr, 13-yr, 15-yr, 18-yr, 20-yr, and 27-yr apple orchards were 20.84%, 20.31%, 19.23%, 17.88%, 15.54%, 14.93%, and 13.38%, respectively. The soil water storage in deep profile decrease in invert “S” with tree age and stabilized after 27 years. The conversion from farmland to apple orchards increased soil water depletion and further deepened the accumulation of Cl ^- in the deep profile. The average Cl^- concentrations of soil water within 4-10 m layer in farmland, 10-yr, and 27-yr apple orchards were 30.8, 36.6, and 114.3 mg/L, respectively. Both the Cl^- profiles and the 1963-tritium peak in the homogeneous loess profile reflected that the piston flow formed in the wet years is the dominant mechanism for deep percolation and groundwater recharge in the Loess Tableland. The average infiltration rate of groundwater recharge in cropland was 105 mm/a, and the average recharge rate was 30.2 mm/a, accounting for 5.2% of the annual precipitation. The conversion from farmland to apple orchard led to severely dried soil layer in the deep profile, further prevented the recharge of rainfall to groundwater and weakened the dominance of piston flow in the process of groundwater recharge. Therefore, an appropriate areal proportion between farmland and apple orchard should be coordinated based on the policy guidance, so as to achieve sustainable recharge and utilization of groundwater resources for the Loess Tableland.