土壤蒸发和植被蒸腾对三江源退化高寒草甸蒸散的影响

Abstract

蒸散（ET）主要由土壤蒸发（E）和植被蒸腾（T）组成，然而难以把E与T从陆地生态系统ET中区分开。为阐明位于青海省境内的三江源区（89°24'—102°23'E，31°39'—36°16'N）高寒草甸E和T对生态系统ET的影响，利用小型蒸渗仪和微气象系统定量研究了三江源退化高寒草甸ET、E和T的变化，以及植被和环境因子对其的影响。结果表明：2017和2018年的ET分别为467.7 mm和479.2 mm，其中生长季（5—9月）约占72%，且E对生态系统ET的贡献（56%）大于T（44%），年降水量（P）的90%以上通过ET返回大气（ET/P > 90%）。根据生长季中不同植被覆盖度的蒸渗仪观测结果发现，ET随植被覆盖度的降低而增加。逐步回归分析表明，净辐射（R_n）是驱动生态系统ET、E、T最主要的因子；另外，E对饱和水汽压差（VPD）的响应更敏感，而T受空气温度（T_a）的影响更大；土壤含水量（SWC₅）对蒸散的影响相对较小，可能由于研究区降水相对较多的原因。结果说明，草甸退化将加剧土壤蒸发，进而导致生态系统散失更多的水分。;Evapotranspiration (ET) is mainly composed of evaporation (E) and transpiration (T), however it is often difficult to separate E and T from ET in a terrestrial ecosystem. To better understand the effects of E and T on ET of alpine meadow in the Three-River Source Region (TRSR) (89°24'—102°23'E, 31°39'—36°16'N) in Qinghai Province, lysimeter and micrometeorological measurements were used to quantitatively study the variations of ET, E and T, and effects of vegetation and environmental factors on evapotranspiration processes. The results showed that the ET in 2017 and 2018 was 467.7 mm and 479.2 mm, respectively, and ET during the growing season (May-September) accounted for about 72% of annual total ET, while the contribution of E to ET (56%) was greater than that of T (44%). Furthermore, more than 90% of annual precipitation was returned to the atmosphere through the ET processes (ET/P > 90%). Based on the results from lysimeters with different vegetation coverages during the growing season, it was found that ET increased with the decreasing of vegetation coverage. The results of stepwise regression analysis indicated that net radiation (R_n) was the most important factor driving ecosystem ET, E and T; in addition, the response of E to vapor pressure deficit (VPD) was more sensitive than T, while the air temperature (T_a) was more significantly associated with T in comparison to the E. The effect of soil water content (SWC₅) on ET was relatively small maybe due to the precipitation was relatively abundant in this alpine meadow. The results suggested that the degradation of alpine meadow would lead to the increase of E, as a result more water is lost from the ecosystem through evapotranspiration.