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. However, the validity of this fundamental assumption 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 (τ_s) shows a close relationship with climatic aridity and vegetation coverage, with arid/semiarid and sparsely vegetated catchments generally having a longer τ_s. Additionally, increasing snowfall fraction also increases τ_s. The imbalance (e_wb) caused by ignoring ΔS decreases as averaging period for water balance calculations increases as expected. For a typical 10‐year averaging period, e_wb 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.

中文翻译：

---

评估全球集水区水平衡计算中的稳态假设

长期以来，人们一直认为，在足够长的时间内，与其他流量相比，集水储量（ΔS）的变化相对较小，在集水平衡方程中可以忽略不计。但是，这种基本假设的有效性很少得到检验，水平衡计算中的相关不确定性仍然未知。在这里，我们使用长期（1982-2011年）观测的1,057个全球未受损流域的月流量（Q）和降水量（P），结合四个独立的蒸散量（E）估计值来推断ΔS并对集水区水量平衡计算中的稳态假设进行全局评估。结果表明，将稳定状态的阈值设置为月平均P的5％时，约有70％的流域在10年内达到稳定状态，而约6％的流域甚至在30年后仍无法达到稳定状态。所需的集水的时间达到稳定状态（τ_小号）示出了具有气候干旱和植被覆盖有密切的关系，与干旱/半干旱和植被稀疏集水区通常具有较长τ_小号。此外，增加降雪分数也增加τ_小号。忽略Δ引起的不平衡度（e _wb）随着水平衡计算的平均周期增加，S降低。在典型的10年平均期内，干旱地区的e _wb约占P的7％，而在湿润地区则下降至P的约3％。这些结果表明，在应用稳态假设时应考虑集水性质，而在干旱/半干旱地区忽略ΔS时应谨慎行事。