Actual evapotranspiration (AET) is the main component of water balance and comprises evaporation (E) and transpiration (T). Despite widespread recognition of the importance of water in terrestrial ecosystems, knowledge about key processes in the water balance is surprisingly limited, especially for how water is partitioned between E and T. We used a daily time step soil water model (SOILWAT2) and 10 years of data to describe AET partitioning and water balance of a degraded meadow and an artificial pasture in the Three-River Source Region on the Qinghai-Tibetan Plateau. Our results showed that there are two important environmental factors influencing water balance processes: evaporative demand of the atmosphere and precipitation. Due to the same evaporative demand and precipitation regime, the AET between our two sites was similar in seasonal trends and annual amounts. On an annual basis, AET and deep drainage accounted for almost 100% of the precipitation received by both the two sites. Despite the similarity of annual AET between the degraded meadow and the artificial pasture, the AET partitioning between the two sites was substantially different. Mean annual transpiration/actual evapotranspiration (T/AET) was 27% in the degraded meadow and 52% in the artificial pasture. The key controls on AET partitioning were vegetation and soil, and compared to soil, our study emphasized the influence of vegetation, and high amounts of biomass resulted in the highest values of T/AET in the artificial pasture. Additionally, we identified deep drainage as an important and previously overlooked component of water balance accounting for an average of approximately 21% of the annual precipitation in both the two study sites.

中文翻译：

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青藏高原三河源区退化草甸与人工牧场AET分配及水分平衡比较

实际蒸散量 (AET) 是水分平衡的主要组成部分，包括蒸发量 (E) 和蒸腾量 (T)。尽管人们普遍认识到水在陆地生态系统中的重要性，但关于水平衡关键过程的知识却出奇地有限，特别是关于水如何在 E 和 T 之间分配。我们使用了每日时间步长土壤水模型 (SOILWAT2) 和 10 年青藏高原三河源区退化草甸和人工牧场的AET划分和水分平衡数据。我们的研究结果表明，有两个重要的环境因素影响水平衡过程：大气的蒸发需求和降水。由于相同的蒸发需求和降水情况，我们两个站点之间的 AET 在季节性趋势和年度数量上相似。按年计算，AET 和深层排水占两个站点接收的降水量的近 100%。尽管退化草地和人工牧场之间的年度 AET 相似，但两个站点之间的 AET 划分却大不相同。退化草甸的年平均蒸腾量/实际蒸散量 (T/AET) 为 27%，人工牧场为 52%。AET 划分的关键控制是植被和土壤，与土壤相比，我们的研究强调了植被的影响，大量的生物量导致人工牧场的 T/AET 值最高。此外，