Monthly discharge of four rivers with various permafrost coverage and little anthropogenic influence was used to identify effects of permafrost degradation during the last 50 years,which has occurred because of significant increases in air temperature in the river regions.The basins of the Shule,Heihe,Shiyang and upper Yellow Rivers in northwestern China have 73%,58%,33% and 43% permafrost coverage,respectively.There is snow cover in the basins and no rain to supply rivers during winter. The monthly recession coefficient(RC) in winter reflects groundwater conditions.The RC has increased obviously for the Shule and Heihe rivers with 73% and 58% permafrost coverage,respectively,but did not increase for the Shiyang River,and decreased insignificantly for the upper Yellow River,which had less permafrost coverage.There is a distinct positive relationship between RC and annual negative degree-day temperature(NDDT) at the meteorological stations in the basins with high permafrost coverage.These results imply that permafrost degradation due to climate warming affects hydrological processes in winter.The effect is obvious in the basins with high permafrost coverage but negligible in those with low permafrost coverage. Permafrost degradation increases infiltration,enlarges the groundwater reservoir,and leads to slow discharge recession.The result means that hydrological processes are affected strongly by permafrost degradation in river basins with high permafrost coverage,but less in river basins with less permafrost coverage.
