Simulating Alpine Soil Thermal Regime And Cold Regions Hydrological Processes Using Modular Modeling Method

With global consistent warming, the frozen soil active layer depth has been gradually deepening and it is frozen soil degeneration that is significant feature of cold regions environmental change. Except for climatic change, the diversities of region ecological environment can affect the thermal regime of the soil frozen active layer. Existed researches indicate that snow cover and organic material existed in surface soil can render the change of frozen soil active layer. However, its mechanism is not absolutely transparent to us.Under the background of widespread water resource absence, accurately estimating water resource quantity of river upstream is not only precondition of basin water resource reasonable programming but also the most important mission of cold region hydrological processes simulation. The cold region hydrological processes simulation is more complex because of ice, snow and frozen soil presence so that the hydrological model must take energy transport in water phase change into account except for precipitation distribution, confluence, infiltration, subsurface flow and ground water flow. How to accurately simulate ice/snow and frozen soil hydrological processes is the key to cold region hydrological processes simulation when the region or global climatic condition changes in the future. The cultural hydrological model seldom considers cold region hydrological processes factor, such as ice, snow and frozen soil, or simplify the processes as one or more parameters. What’s more, the lack of monitored data because of hard approach to cold regions and hard work conditions contradicts the abundant data needed by hydrological processes simulation. How to simulate hydrological processes under a limited data condition has become a important science focus in cold region hydrological simulation.Designed and founded on the basis of modular modelling environment, the modular modelling method, having object and forcing data feature, can be utilized flexibly to stimulate cold region water-thermal transport and hydrological processes. This research objectives are1) to simulate energy transport between space and surface soil and quantitatively analyze the influence of seasonal snow cover and organic existence in surface on the active layer thermal regime by CoupModel,2) to simulate snow hydrological processes and frozen soil hydrological processes and estimate the impact of seasonal snow cover and frozen soil on cold region hydrological processes, used the monitoring datasets in Binggou station, Fenghuoshan station and Tanggula station by Chinese Academy of Sciences (CAS). The following is the main conclusion.I. CoupModel is able to accurately simulate Qinghai-Tibet Plateau energy transport processes between space and surface soil and active layer temperature state change when the seasonal snow or organic material is present. In the Fenghuoshan basin, surface temperature as model input, soil temperature at different soil layers as verifiable indexes, calibrate parameters relative to energy transport processes utilized Bayesian theory and the result indicates the model can estimate parameters. The Nash coefficient (NSE) values between simulated soil temperature by CoupModel and observed soil temperature in Fenghuoshan basin vary from0.952to0.984in model calibration period and from0.902to0.967in model verification period. In Binggou basin, where the seasonal snow cover is present, the NSE value between simulated and observed snow depth is0.76and the NSE values between simulated soil temperature by CoupModel and observed soil temperature vary from0.937to0.962in model calibration period and from0.915to0.948in model verification period. The NSE values between simulated soil temperature by CoupModel and observed soil temperature vary from0.914to0.951in model calibration period and from0.894to0.951in model verification period at Tanggula station. Comparing soil temperature simulation results at Fenghuoshan station, Binggou Station and Tanggula station, we can conclude that the model displays the best result at Fenghuoshan station and the maximum NSE value is0.984at40cm soil depth in model calibration; moreover, the CoupModel simulated accuracy of upper layer soil is better than subsoil.2. Energy budget, distribution and transport between space and surface soil are changed by snow cover presence because of its low thermal conductivity, high melting and sublimation latent heat and higher albedo than bare soil and then induce the change of frozen soil existence and development environment, affect the thermal regime of the frozen soil active layer. The snow cover depth is controlled by changing precipitation from October to May of the next year and the result indicates the presence of shallow snow cover from0cm to20cm in winter render decrease of total radiation reaching surface owing to high albedo of snow cover and low temperature of contact surface between snow cover and soil layer because of energy consumed by snow cover melting and sublimation. Therefore, soil freezing is easy to happen and the temperature of whole active layer trend low. The snow cover which depth varies from0cm to20cm avail frozen soil development. Following snow cover depth from20cm to80cm, the snow cover insulation becomes decisive gradually and restrains energy exchange between space and soil layer. The energy stored by soil layer can’t release timely so that the freezing process of active layer changes. The variation of active layer mainly displays freezing start time delay and soil temperature rise at whole soil layer and the freezing rate decrease. As a whole, the shallow snow cover varying from0cm to20cm benefits frozen soil development and gradually become opposite with snow cover depth increasing.3. More soil thermal capacity and lower thermal conductivity than common mineral substance, the organic soil can affect thermal regime of frozen soil active layer. Based on successfully calibrated CoupModel model, research the effect of organic soil on the thermal regime of active layer by controlling the organic soil distribution at whole soil profile. The results indicate that under the background of existing climate system, the melting depth of active layer in summer decreases from about280cm to100cm and reduced proportion is over60%following organic distribution depth from0cm to80cm. The temperature condition of active layer displays adverse trend in cold season and warm season of a year:the temperature rises in winter and the temperature decreases in summer. The temperature amplitude of variation decreases all year and the response of soil temperature to air temperature is not so strong. Comparing active layer melting depth and temperature condition, we can conclude that the change of frozen soil active layer becomes steady with the organic soil increasing. The organic soil presence makes for frozen soil development at Tanggula station under existed climate system, annual average air temperature below0℃.4. The cold region hydrological model (CRHM) designed and found by modular model platform can simulate snow hydrological processes. At Binggou snow basin, the CRHM can be utilized to estimate the effect of different modules and algorithms including degree day algorithm and energy budget algorithm on snow cover accumulation and ablation, to compute snow cover material balance or water budget by degree day algorithm and energy budget algorithm and to stimulate cold region hydrological processes and discharge through flexible modular transition. The results indicate that both degree day algorithm and energy budget algorithm are able to simulate snow cover accumulation and ablation processes and the energy budget algorithm performance is better that the R2between observed snow cover depth and simulated by degree day algorithm and energy budget algorithm are0.64,0.78, respectively and the NSE values are-0.57and0.75respectively. Comparing snow cover budget simulated by degree day model with by energy budget model and using observed wind speed at the same time, we can conclude that snow cover sublimation caused by blowing snow process calculated by degree day model and energy budget model is40%and50%of total snow sublimation, respectively. The melting snow is about50%of total snow cover. However, the snow loss in blowing snow process is relatively little, about2%. To simulate day average discharge caused by the seasonal snow cover at the basin outlet, choose energy budget algorithm model because of its better performance than degree day algorithm model. The NSE value between simulated and observed discharge is0.64and the model is able to illustrate the flood processes induced by seasonal snow cover melt.5. CRHM is able to simulate hydrological processes including frozen soil freezing and melting processes. At Fenghuoshan basin of the headwater of Yangtze River in core region of Qinghai-Tibet plateau, the frozen soil is chose to estimate the effectiveness that CRHM simulates frozen soil hydrological processes and the influence of frozen soil on cold region hydrological processes. The main simulated processes include infiltration in frozen soil and unfrozen soil, surface flow, interflow, etc. At last, quantify the amount of surface flow and interflow. The results indicate comparing with simulated discharge variation by CRHM model which the frozen soil module is neglected, the performance of CRHM including frozen soil module is better and the NSE values is0.67. The model can capture flood in spring caused by frozen soil melt. The drastic effect of frozen soil on runoff happens in spring flood process and the slender effect happens in summer flood process by analyzing two simulated results with observed value at2007. The discharge component division by model is fit to isotope and catchment area observation and the ground water flow proportion can reach approximately70%.All in all, the presence of both seasonal snow cover and organic soil is able to affect the thermal regime of frozen soil active layer and frozen development environment by different ways. The research results will be used to guide frozen soil protection at a certain extent. The existence and change on timing and extent of snow cover and frozen soil can alter tempestuously cold region hydrological processes. Accurately estimating the change of snow cover and frozen soil is first step for cold region hydrological processes simulation and cold region water resource assessment under the background of global climate change in the future. The modular model construction method with particular superiority, can apply to developing climatic model, frozen soil model, snow cover model, glacier model and cold region hydrological model at Qinghai-Tibet plateau.