The growth characteristics of 20-year-old alfalfa (Medicago sativa L. ) and soil water consumption and recovery process at various eco-sites in semiarid Loess Plateau were systematically studied. Results show that alfalfa productivity varies with growth stages. Alfalfas have higher aboveground biomass from June to August, which is the vigorous growth period of a year under synchronous conditions of both temperature and precipitation. However, the soil water is excessively consumed to its bottom point. Alfalfa annual yield has significant difference. Alfalfa grows vigorously during 4 to 8 years in all tested ecosites. In the 10th year, soil water drops to the minimum level and reaches a severe deficit. Dry soil layer reaches 300～720 cm. After 10th year, soil water begins to recover slowly with alfalfa growth declining. Soil moisture above 500 cm soil layer recovers the same level as before planting in 15th to 20th years. However, the soil below 500 cm is seriously dry, and soil water recovery is extremely slow and hard. Overall, alfalfa production reaches peak during 4～8 years after planting, and alfalfa should be replanted at 8 to 10 years after planting in semiarid Loess Plateau.%本文系统研究了黄土高原半干旱地区不同立地条件下,20年生紫花苜蓿(Meducagi-sativa L.)生长变化特征与土壤水分的消耗与恢复过程.结果表明:紫花苜蓿不同生长阶段生产力差异较大,年生长的6-8月份,即在水热同步条件下,紫花苜蓿形成较高地上生物量,为年生长的盛期,但土壤水分消耗过度,降至年最低点；紫花苜蓿产草量不同生长年限均达极显著差异,在山地、塬地和川地3种立地条件下均呈规律性变化特征,即在生长的4～8年间,3种立地条件下紫花苜蓿均为旺盛生长阶段,第10年土壤含水量降到最低,土壤干层厚度高达300～720 cm,土壤水分严重亏缺；10年后随紫花苜蓿的基本衰败,土壤水分开始缓慢恢复；第15～20年,3种立地类型500 cm土层以上土壤水分可恢复到接近种植前的土壤含水量,而500 cm以下土壤通体干燥化严重,水分恢复极为缓慢,且恢复难度较大.因而,在黄土高原半干旱区紫花苜蓿适宜生长年限应为8～10年,第4～8年为苷蓿生长的高峰期.
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