Analysis of nonstationarity in low flow in the Loess Plateau of China

An approach for nonstationary low-flow frequency analysis is developed and demonstrated on a dataset from the rivers on the Loess Plateau of China. Nonstationary low-flow frequency analysis has drawn significant attention in recent years by establishing relationships between low-flow series and explanatory variables series, but few studies have tested whether the time-varying moments of low flow can be fully described by the time-varying moments of the explanatory variables. In this research, the low-flow distributions are analytically derived from the 2 basic explanatory variablesthe recession duration and the recession coefficientwith the assumption that the recession duration and recession coefficient variables follow exponential and gamma distributions, respectively; the derived low-flow distributions are applied to test whether the time-varying moments of explanatory variables can explain the nonstationarities found in the low-flow variable. The effects of ecosystem construction measures, that is, check dam, terrace, forest, and grassland, on the recession duration and recession coefficient are further discussed. Daily flow series from 11 hydrological stations from the Loess Plateau are used and processed with a moving average technique. Low-flow data are extracted following the pit under threshold approach. Six of the 11 low-flow series show significant nonstationarities at the 5% significance level, and the trend curves of the moments of low flow are in close agreement with the curves estimated from the derived distribution with time-dependent moments of the recession duration and time-constant moments of the recession coefficient. It is indicated that the nonstationarity in the low-flow distribution results from the nonstationarity in the recession duration in all 6 cases, and the increase in the recession duration is resulted from large-scale ecosystem constructions rather than climate change. The large-scale ecosystem constructions are found to have more influence on the decrease in streamflow than on the increase in watershed storage, thus resulting in the reduction of low flow. A high return period for the initial fixed design value decreases dramatically with an increasing recession duration.