Research On The Land Surface-Hydrological Process Over The Upper Huaihe River Basin

The sub-grid land surface-hydrological process is the key component of the terrestrial water cycle system.The complex mechanism and the profound influence from the sub-grid land surfacehydrological process have always been the crucial topic of the hydrometeorological researches.On the one hand,the land surface-hydrological process is the most important part of the water cycle system in terms of regulating mass,energy,water and ecology balance between land and atmosphere.Varying synoptic phenomena and terrestrial flash floods are inseparably correlated to the land surface-hydrological processs.On the other hand,with the rapid development of computer technology and hydrometeorological measurments,numerical weather prediction models require much higher resolution for the weather forecasting.While increasing model resolution can give more accurate and explicit predictions of meso-scale and small-scale precipitation systems,it also amplifies the influences derived from the land surface thermal dynamic and water vapor transfer,which are highly correlated to the sub-grid land surface-hydrological process.Therefore,an appropriate description for the sub-grid hydrological process will help to explain the mechanism of the hydrological-atmospheric interactions.This paper adopts a high resolution,distributed and explicit physics land surface-hydrological model to study the effects of the sub-grid land surface hydrology on the water circulation system over the Upper Huaihe River Basin.Researches have been carried out from four aspects.First,based on the analysis of hydrological parameter uncertainty,influences of the land surface heterogeneity on the hydrological processes are discussed.Second,hydrological post-processing methods are studied to improve the accuracy and reliability of land surface streamflow forecasts.Third,the short-term climatic characteristics of the hydrological-atmospheric interactions are studied to discover the mechanism between the sub-grid hydrology and the atmospheric systems.Last,the typical synoptic events are investigated to understand the effect of the sub-grid hydrological process on the rainfall-induced weather systems.With the thorough analyses from the four aspects,the main conclusions are addressed as follow:1.The soil hydraulic parameters(SHPs)have large uncertainties on the river discharge over the Upper Huaihe River Basin,and the SHPs are appropriate for constructing the parameter ensemble for streamflow probabilistic forecast.The Upper Huaihe River Basin is featuring on the plain dominated landscape and moist soil condition,which enhance the effect of the soil water movement(conductivity and diffusion)on the surface runoff generation and groundwater refiltration.The spatially distributed structure of the soil components is complex and diverse.Hence,different methods for the estimations of the model SHPs can directly affect the sub-grid soil water movement over basin,which can cause great uncertainty on the surface runoff generation.Constructing SHP ensemble based on different transform methods can reflect the uncertainty of the hydrological process derived from the sub-grid land surface heterogeneity,and explain the mechanism of the surface runoff and streamflow changes from the aspects of soil water movement and soil water retention.Meanwhile,results of the SHP based ensemble streamflow showed that using SHPs with slower soil water movement could enhance the streamflow peak and mitigate the recession duration,and improve the sensitivity and quality for the flood events.2.The post-processing methods can effectively improve the quality of probabilistic estimation of the streamflow ensemble.Different post-processing methods have their own advantages and disadvantages.Two post-processing methods,including the logistic regression(LR)and Bayesian model average(BMA),are applied on the SHP based streamflow ensemble,which has a drawback of small ensemble spread.The results show that the LR method has better improvements of the peak streamflow probabilistic forecasts,which is more dispersive and better alarm for the flood events.The LR results are under-dispersive in the small and medium streamflow,while the BMA method is more dispersive and better reliable in the small and medium streamflow.Moreover,the BMA results are more sensitive to the variations of small and medium streamflow than the LR results.3.The sub-grid land surface hydrological processes can have significant impacts on the land surface condition and the atmospheric precipitation.By comparing the short-term climatic characteristics of the 1-way and 2-way hydrological-atmospheric coupling experiments over the Upper Huaihe River Basin,the 2-way coupling has increased the soil moisture entirely against the 1-way coupling due to feedback from the sub-grid hydrological process.In horizontal space,the soil moisture increase mainly locates in the center and upper stream regions.Meanwhile,the moisture flux and latent heat flux from the land surface also increased,and the sensible heat flux decreased with high temporal and spatial correlations to the soil moisture deviations.The precipitation frequency of the 2-way coupling has a positive feedback on the soil moisture changes,while the atmospheric stability shows a negative correlation.In horizontal space,the precipitation increase is mainly distributed in the mountainous areas at the edge of the basin,where the runoff processes are stronger than other areas due to steeper landscape.Since the locations of precipitation changes are not correlated with the locations of soil moisture changes,it is considered that the effects from the sub-grid hydrological process and the surface topography are the main factors on the precipitation changes.4.The sub-grid land surface-hydrological process has limited impacts on the precipitation event dominated by large-scale weather systems,while it has significant impacts on the convective precipitation caused by orographic uplift.In the case of the large-scale weather system dominated precipitation,the effects of the sub-grid hydrological process on the precipitation generation are slight,due to the strong influences of the atmospheric systems and high saturation of the water vapor sources.Compared with the water sources of the atmosphere,the moisture and energy exchanges from the land surface are just a small fraction of the precipitation system,therefore,the land surface moisture flux cannot affect the high atmospheric level significantly.In another case of the orographic dominated precipitation,the 2-way coupling has more moist soil conditions due to stronger runoff process.Moisture flux increases the water vapor content of the low atmospheric level up to the high atmospheric level,and ultimately increases the precipitation over the region.In comparison with two events,the influence of sub-grid hydrological process on an orographic precipitation event is mainly due to the orographic uplift and moisture entangling of the air flow.