基于树干液流及涡动相关技术的葡萄冠层蒸腾及蒸散发特征研究

针对西北干旱区绿洲经济作物葡萄树冠层蒸腾及蒸散发特征的相关问题,在甘肃省敦煌市南湖绿洲开展无核白葡萄树液流速率及蒸散发观测试验,采用基于热平衡原理的包裹式茎流计,详细分析了典型生长季7—9月份葡萄树蒸腾耗水规律,使用"单位叶面积上的平均液流速率SF×叶面积指数LAI"的方法,实现了从单株到林分冠层蒸腾的尺度扩展,并通过与涡动相关技术所测蒸散发数据对比,详细研究了葡萄地冠层蒸腾及蒸散发规律。结果表明:典型生长季中葡萄树液流速率日变化为单峰型曲线,日均耗水量从2.76 kg到10 kg不等,胸径越大的葡萄树日均耗水量越大;冠层蒸腾及蒸散发日变化曲线亦为单峰型,白天8:00—12:00与17:00—20:00期间,葡萄冠层蒸腾与蒸散发曲线均比较吻合,该时间段葡萄地蒸散发绝大部分来源于葡萄冠层蒸腾,而12:00—17:00之间由于午后太阳辐射强烈土壤蒸发量增加,葡萄蒸散发大于冠层蒸腾;典型生长季3个月中,葡萄冠层蒸腾量的变化范围在1.88—8.12 mm/d之间,日均冠层蒸腾量为6.12 mm/d,蒸散发在1.74 mm/d至10.78 mm/d之间,日均蒸散发量为7.13 mm/d;日均土壤蒸发量约为1.01 mm/d,只占总蒸散发量的14.2%,日均冠层蒸腾占日均蒸散发的比重达到85.8%,说明该生长阶段冠层蒸散发以作物蒸腾为主。

Research of transpiration and evapotranspiration from a grapevine canopy combining the sap flow and eddy covariance techniques

In agriculture ecosystems, more than 90% of all water input is lost by evapotranspiration (ET). Thus, the accurate measurement of ET and its associated components (i.e., canopy transpiration and soil evaporation) are essential for many agricultural applications, such as irrigation scheduling, drainage, and yield forecasts. In the arid region of northwestern China, water resources are rare and are often the restricting factor for plant production. By comprehensively using the sap flow method and eddy covariance (EC) technique, the single tree sap flow velocity and evapotranspiration flux from the grapevine in a desert oasis ecosystem located in Nanhu County, China, were measured during a typical growing season from July 13 to September 12, 2013. Canopy transpiration was then obtained by multiplying the average sap flow per leaf area unit by the leaf area index (LAI), and compared with the EC-measured evapotranspiration flux. The results showed that the diurnal dynamics of sap flow velocity of all sample trees exhibited single peak curves. With increasing diameter of the grapevine at breast height (DBH), the peak in the diurnal variation of sap flow velocity tended to increase. Thus, the sap flow velocities of different single trees were mainly controlled by DBH. The average daily water consumption of the grapevine was exponentially associated with DBH, while the regression relationships differed significantly in different months. Mean water consumption of each sample trees ranged from 2.76 kg to 10 kg during a typical growing season, which was consistent with previous studies on the water consumption of Tamarix ramosissima in the hinterland of Taklimakan Desert in China. Daytime dynamics of canopy transpiration upscaled by sap flow (E_sf) and evapotranspiration measured by EC (ET_ec) both exhibited the unimodal type, and their variation trends were also relatively identical. From 08:00 to 12:00 and 17:00 to 20:00, there was good agreement between the values of E_sf and ET_ec, which means that evapotranspiration primarily originated from canopy transpiration for this period. However, from 12:00 to 17:00, the values of E_sf were lower than those of ET_ec, due to the contributions of soil evaporation to the evapotranspiration process. During the study period, the daily evapotranspiration varied from 1.74 mm/d to 10.12 mm/d, with a mean value of 7.13 mm/d, while daily canopy transpiration ranged from 1.88 mm/d to 8.12 mm/d, with a mean value of 6.12 mm/d. The ratios of daily canopy transpiration to daily evapotranspiration varied from 60% to 98.3%, with a mean value of 85.8%. On cloudy and rainy days, the difference between daily evapotranspiration and daily canopy transpiration was very small, with both being less than 5 mm/d. These results indicate that evapotranspiration was dominated by canopy transpiration during the growth stage of the grapevine. The difference in daily evapotranspiration and daily canopy transpiration was daily soil evaporation with a mean value of 1.01 mm/d. However, this result still needs further experimental verification. Thus, the comprehensive use of the sap flow method and eddy covariance technique may help us to identify the characteristics evapotranspiration and its different components (canopy transpiration and soil evaporation) in ecosystem efficiently. This information will improve our understanding about how the evapotranspiration process interacts with environmental factors.