渭北旱塬苹果园地产量和深层土壤水分效应模拟

为了研究实时气象条件下渭北旱塬不同生长年限苹果园地产量变化趋势和深层土壤水分变化规律，在模型适用性与模拟精度验证基础上，应用WinEPIC模型模拟研究了1962—2001年期间洛川旱塬苹果园地产量演变动态和深层土壤水分效应。结果表明：(1) 在模拟研究期间，洛川旱塬4—40年生苹果园产量整体上呈波动性下降趋势，初期产量逐渐增加，11—23年生达到最大值(平均为28.8 t/hm2)，之后随降水量年际波动呈现出明显的波动性降低趋势。(2) 40年间苹果园地遭受的干旱胁迫日数呈波动性上升趋势，与年降水量波动趋势相反。(3) 1—15年生期间苹果园地平均年耗水量高于同期年降水量，导致苹果园地0—10 m土层土壤强烈干燥化，逐月土壤有效含水量波动性降低，1—10年生、11—20年生和21—40年生期间发生土壤干燥化并且程度逐渐加剧，但干燥化速率逐渐减缓，土壤干燥化速率分别为95.4 mm/a、12 mm/a和1.5 mm/a。(4) 随生长年限的延长，苹果园地0—10 m土层土壤湿度逐渐降低、土壤干层分布深度逐渐加大，在14年生时超过了10 m，20年生以后2—10 m 土层形成稳定的土壤干层。因此，基于土壤水分利用的苹果生长与果园利用的合理年限为20 a，最长不宜超过23 a。

Modeling the changes of yield and deep soil water in apple orchards in Weibei rainfed highland

Based on model applicability and precision validation, the WinEPIC model was used to simulate the changes of yield and deep soil desiccation of rainfed apple orchards, using real-time weather data and soil data from 1962 to 2001. Variation of apple productivity and soil moisture in apple orchards were analyzed, based on the simulated results at Luochuan, one typical area of Weibei Rainfed Highland. The simulated results showed as follows: (1) Fruit yield showed a decline tendency with fluctuation as a whole from 4 year-old orchard to 40 year-old orchard during the simulation period. The fruit yield increased gradually from 4^th year to 11^th year, and reached the highest value ( averaged 28.8 t/hm² ) during 11^th year to 23^th year, then decreased obviously with the fluctuation of annual rainfall. (2) During the simulation period, annual rainfall at Luochuan fluctuated and decreased gradually, therefore, water stress days of 1-40 year-old apple orchards showed an upward trend which was opposed to the change trend of annual rainfall. And a peak value of 147.8 stress days occurred at 38 year-old apple orchard (in 1999). (3) Mean annual consumption of soil water was higher than mean annual rainfall in apple orchards at Luochuan. Therefore, soil had been drying strongly in apple orchards, and the total amount of soil water overuse was 1009 mm during the 40 years. The monthly available soil water contents in 0-10 m soil layers declined obviously during the simulation period. However, the fluctuation range was higher in the beginning period than in the later period. The soil desiccation of apple orchards occurred in the period of 1-10 years, 11-20 years and 21-40 years and aggravated gradually, but the desiccation rate slowed down, with the soil desiccation rates of 95.4, 12 and 1.5 mm per year, respectively. (4) As the soil moisture decreased in 0-10 m soil layers, the thickness of drying layers increased and became deeper gradually with the growth years of apple trees and the deeper of apple roots distribution, and the depth of drying layers distribution was more than 10 m in 14 year-old apple orchard. However, rainfall infiltrated no more than 2 m at Luochuan and there was no water supply below 2 m soil layers, so soil water content in 2-10 m soil layers was stable relatively in 15-40 year-old apple orchards. When apple trees grew to 23 years, soil water in 2-10 soil layers almost could not be used by apple trees, and soil water in 0-2 m soil became the main water resources for the growth of apple trees. Therefore, soil water content varied sharply with the variation of annual rainfalls in 0-2 m soil layers. Both fruit yield and soil desiccation were significantly different in different ages of apple orchards. Considering economic and ecological benefits, reasonable growth and utilization years of apple orchard is 20 years, the longest should not exceed 23 years.