水量平衡稳态假设是流域水量平衡计算中通常使用的一个假设。该假设认为在较长时间尺度上（如年尺度或多年尺度），流域水储量的变化远小于该时间尺度上的水文通量（降水、蒸发或径流），进而可以忽略不计。尽管这一假设在流域水量平衡计算中被广泛采用，但却极少有研究对该假设的合理性进行验证，基于该假设进行的流域水量平衡计算中的误差亦是未知。本论文针对流域水量平衡的稳态假设进行研究，选取了全球1057个未受人类活动干扰的流域，基于各流域30年（1982-2011年）的长序列径流和降水观测数据，结合四个独立的基于遥感或机器学习算法生成的蒸散发数据产品，分析了各流域不同时间尺度上水储量的变化，采用阈值方法确定了流域达到水量平衡稳态所需的时间，并分析了稳态所需时间受流域气候及下垫面条件的影响，首次在全球尺度上评估了流域水量平衡稳态假设的合理性。主要结论如下：（1）当稳态阈值设为月平均降水量的5%时，约70%的流域能够在10年内达到水量平衡稳态，但同时有约6%的流域在30年内仍无法达到水量平衡稳态；（2）流域达到水量平衡稳态所需时间和流域气候、植被覆盖度呈现出显著的相关性。干旱区及植被稀疏覆盖的流域达到水量平衡稳态的时间显著地长于湿润区及植被覆盖度较高的流域；（3）降雪会影响流域达到水量平衡稳态所需的时间，降雪比例高的流域通常需要更长的时间达到水量平衡稳态；（4）水文变异性的分配与传递过程对水量平衡稳态有重要影响。在上述分析的基础上，本文进一步量化了在流域水量平衡计算中应用稳态假设而产生的误差。结果显示：（1）忽略流域水储量变化所导致的误差会随着水量平衡计算时间尺度的增加而下降；（2）对于典型的10年尺度，在干旱区流域，应用稳态假设所带来的水量平衡计算误差约为同期平均降水量的7%，而在湿润流域该比例会下降至约3%。上述结果表明，在应用稳态假设时（尤其在干旱/半干旱流域中），应当考虑充分流域的气候与下垫面条件，以减少由此带来的计算误差。
It has long been assumed that over a sufficiently long period of time, changes in catchment water storage (ΔS) are a relatively minor term compared to other fluxes and can be neglected in the catchment water balance equation. Despite its widely application in water balance calculations, the validity of this fundamental assumption, however, has rarely been tested, and the associated uncertainties in water balance calculations remain unknown. Here, we use long‐term (1982–2011) observations of monthly streamflow (Q) and precipitation (P) for 1,057 global unimpaired catchments, combined with four independent evapotranspiration (E) estimates to infer ΔS and to provide a global assessment of the steady‐state assumption in catchment water balance calculations. Results show that when the threshold for steady state is set to 5% of the mean monthly P, ~70% of the catchments attain steady state within 10 years while ~6% of the catchments fail to reach a steady state even after 30 years. The time needed for a catchment to reach steady state (τs) shows a close relationship with climatic aridity and vegetation coverage, with arid/semiarid and sparsely vegetated catchments generally having a longer τs. Increasing snowfall fraction also increases τs. Additionally, the partitioning and propagation of hydrologic variability has significant impact on water balance steady state. The imbalance (ewb) caused by ignoring ΔS decreases as averaging period for water balance calculations increases as expected. For a typical 10‐year averaging period, ewb accounts for ~7% of P in arid, but that decreases to ~3% of P in humid catchments. These results suggest that catchment properties should be considered when applying the steady‐state assumption and call for caution when ignoring ΔS in arid/semiarid regions.
