| Hydrologic partitioning is an interesting and essential topic in catchment hydrology, which is also the theoretical basis of integrated watershed management. In the context of climate change, not only does the amount of annual precipitation and potential evapotranspiration alter, but also do the seasonal characteristics of climate, including the precipitation state and intra-annual distribution of water and energy. Yet, the knowledge gaps of hydrologic responses to the change in climates seasonality still persist. In this thesis, by developing water-energy balance models which take the effects of climate seasonality into consideration, efforts are made to achieve a better understanding of mean annual runoff change and ecologic responses due to the climate change.Firstly, the past 50-year climatology seasonality was investigated by analyzing the daily meteorological records of 743 national weather stations across the China. Obvious spatial pattern of climatology seasonality emerged in China. The trend analysis indicated that there is significant increase in annual snowfall amount and decrease in precipitation seasonality, leaving other seasonal characteristics, such as peak time of climate forcing unchanged.Subsequently, catchment water-energy balance models considering the effects of snow and rainfall-potential evapotranspiration seasonality were developed, respectively. Within the Budyko framework, by analyzing the effective available water and energy for evapotranspiration, the snow ratio was introduced to establish an equation depicting the effect of snow on annual runoff. The result showed that catchments with higher snow ratio tend to have larger runoff index(ratio of runoff to precipitation) given the same dryness index(ratio of potential evapotranspiration to precipitation). Meanwhile, with the aid of stochastic soil moisture model, the evolution of average soil moisture under the seasonal climate forcing was derived, which is also testified using field observations of daily soil moisture and actual evapotranspiration from seven global micrometeorological tower sites in the FLUXNET database. The effects of seasonality of rainfall and potential evapotranspiration were investigated by employing the developed seasonal stochastic soil moisture model. The numerical simulations concluded that for the same dryness index, the evaporation index(ratio of evapotranspiration to precipitation) decreases with the larger amplitude or phase difference between rainfall and potential evapotranspiration. And the conclusions are consistent with the case study of 190 MOPEX catchments.Finally, the above two developed models were applied to investigating the catchment hydrologic and ecologic responses to climates seasonality change. The attribution analysis of the past 50-year runoff change in the Heihe upstream, a typical cold catchment, showed that the raise in snow ratio is responsible for 30% increase in the annual runoff. In addition, a significant increase of runoff in this region was also predicted in the future climate change scenarios. As a representative of ecologic responses, the relationship between optimal root depth and climate seasonality was explored. Simultaneously, the validity of this hypothesis was verified by using long-term average soil moisture storage delivered from catchment observations. As a representative of hydrologic responses, the contributions of variations in climate, especially in precipitation seasonality, and land use to runoff change of 282 catchments in China were evaluated. The results showed that the decline of precipitation seasonality has a significant influence on runoff change in the Yellow River, Haihe River and Liaohe River. Meanwhile, it also indicated that the contribution of land use change to runoff change is overestimated by the common runoff change attribution methods. |
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