Efficiency and Feasibility of an Integrated Algorithm for Distributed Hydrological M odel Calibration

Increasing complexity of distributed hydrological model(DHM)has lowered the efficiency of convergence.In this study,global sensitivity analysis(SA)was introduced by combining multiobjective(MO)optimization for DHM calibration.Latin Hypercube-once at a time(LH-OAT)was adopted in global parameter SA to obtain relative sensitivity of model parameter,which can be categorized into different sensitivity levels.Two comparative study cases were conducted to present the efficiency and feasibility by combining SA with MO(SA-MO).WetSpa model with non-dominated sorting genetic algorithm-Ⅱ(NSGA-Ⅱ)algorithm and EasyDHM model with multi-objective sequential complex evolutionary metropolis-uncertainty analysis(MOSCEM-UA)algorithm were adopted to demonstrate the general feasibility of combining SA in optimization.Results showed that the LH-OAT was globally effective in selecting high sensitivity parameters.It proves that using parameter from high sensitivity groups results in higher convergence efficiency.Study case I showed a better Pareto front distribution and convergence compared with model calibration without SA.Study case II indicated a more efficient convergence of parameters in sequential evolution of MOSCEM-UA under the same iteration.It indicates that SA-MO is feasible and efficient for high dimensional DHM calibration.

Hydrological characteristics and changes in the Nu-Salween River basin revealed with model-based reconstructed data

The Nu-Salween River(NSR),the longest free-flow river in Southeast Asia,plays an irreplaceable role in social development and ecological protection.The lower NSR region is particularly valuable as it is inhabited by approximately 6.7 million people.The basin has limited hydraulic conservancy infrastructure and insufficient ability to cope with climate change risks.Studying the hydrological characteristics and changes in the basin provides the scientific basis for rational protection and development of the basin.However,owing to the limitation of observation data,previous studies have focused on the local area and neglected the study of the lower reaches,which is not enough to reflect the spatial characteristics of the entire basin.In this study,the ECMWF 5th generation reanalysis data(ERA5)and Multi-Source Weighted-Ensemble Precipitation(MSWEP)were applied to develop a geomorphology-based hydrological model(GBHM)for reconstructing hydrological datasets(i.e.GBHM-ERA5 and GBHM-MSWEP).The reconstructed datasets covering the complete basin were verified against the gauge observation and compared with other commonly used streamflow products,including Global Flood Awareness System v2.1,GloFAS-Reanalysis dataset v3.0,and linear optimal runoff aggregate(LORA).The comparison results revealed that GBHM-ERA5 is significantly better than the other four datasets and provides a good reproduction of the hydrological characteristics and trends of the NSR.Detailed analysis of GBHM-ERA5 revealed that:(1)A multi-year mean surface runoff represented 39%of precipitation over the basin during 1980–2018,which had low surface runoff in the upstream,while areas around the Three Parallel Rivers Area and the estuary had abundant surface runoff.(2)The surface runoff and discharge coefficient of variations in spring were larger than those in other seasons,and the inter-annual variation in the downstream was smaller than that in the upstream and midstream regions.(3)More than 70%of the basin areas showed a decreasing trend in the surface runoff,except for parts of Nagqu,south of Shan State in Myanmar,and Thailand,where surface runoff has an increasing trend.(4)The downstream discharge has dropped significantly at a rate of approximately 680 million cubic metresper year,and the decline rate is greater than that of upstream and midstream,especially in summer.This study provides a data basis for subsequent studies in the NSR basin and further elucidates the impact of climate change on the basin,which is beneficial to river planning and promotes international cooperation on the water-and eco-security of the basin.

A distributed hydrological forecast system and its application in predicting the flood caused by Mangkhut

The currently used hydrological forecast system in China is mainly focused on flood,and the flood forecasting frameworks are typically based on point discharge measurements and predictions at discrete locations,hence they can’t provide spatio-temporal information of various hydrological elements,such as surface runoff,soil moisture,ground water table,and flood inundation extents over large scales and at high spatial resolutions.The use of distributed hydrological model has recently appeared to be the most suitable option to bridge this gap.An open source GIS-based distributed hydrological forecast system was established recently,and the watershed delineation and hydrological modelling were integrated together seamlessly.The time and human consuming work of processing the spatial data in building distributed hydrological model could be reduced significantly,and the spatial distribution of hydrological information could be quickly simulated and predicted using this system.The system was applied successfully to forecast the flood caused by super strong typhoon"Mangkhut"which attacked the south China in2018.

An improved modeling of precipitation phase and snow in the Lancang River Basin in Southwest China

Precipitation phase(e.g., rainfall and snowfall) and snow(e.g., snowpack and snowmelt runoff) in high-mountain regions may largely affect runoff generation, which is critical to water supply, hydropower generation, agricultural irrigation, and ecosystems downstream. Accurately modeling precipitation phase and snow is therefore fundamental to developing a better understanding of hydrological processes for high-mountain regions and their lower reaches. The Lancang River(LR, or the Upper Mekong River)in China, among the most important transboundary rivers originating from the Tibetan Plateau, features active dam construction and complex water resources allocation of various stakeholders in Southeast Asian countries under climate change. This study aims to improve precipitation phase and snow modeling for the LR basin with a hydrological model and multisource remotely sensed data. Results show that joint use of the Moderate Resolution Imaging Spectroradiometer(MODIS) land surface temperature product with high spatial resolution(1 km×1 km) and an air temperature product can more precisely distinguish precipitation phase than air and wet-bulb temperature products in the LR basin. Snowfall and snowmelt were found to be controlled primarily by rainfall and snowfall temperature thresholds in snow modeling. The rainfall and snowfall temperature thresholds derived from the hydrological model through calibration with remotely sensed snowpack at basin scales were considerably lower than those derived from in situ observations. Rainfall and snowfall temperature thresholds derived from in situ observations could lead to the overestimation of snowmelt runoff due mostly to the lack of representation of point-based measurements at basin scales. This study serves as a basis for better modeling and predicting snow for the LR basin and potentially other similar basins globally.

Development of an integrated modeling system for improved multi-objective reservoir operation

Reservoir is an efficient way for flood control and improving all sectors related to water resources in the integrated water resources management.Moreover,multiobjective reservoir plays a significant role in the development of a country’s economy especially in developing countries.All multi-objective reservoirs have conflicts and disputes in flood control and water use,which makes the operator a great challenge in the decision of reservoir operation.For improved multi-objective reservoir operation,an integrated modeling system has been developed by incorporating a global optimization system(SCE-UA)into a distributed biosphere hydrological model(WEB-DHM)coupled with the reservoir routing module.The new integrated modeling system has been tested in the Da River subbasin of the Red River and showed the capability of reproducing observed reservoir inflows and optimizing the multi-objective reservoir operation.First,the WEB-DHM was calibrated for the inflows to the Hoa Binh Reservoir in the Da River.Second,the WEB-DHM coupled with the reservoir routing module was tested by simulating the reservoir water level,when using the observed dam outflows as the reservoir release.Third,the new integrated modeling system was evaluated by optimizing the operation rule of the Hoa Binh Reservoir from 1 June to 28 July 2006,which covered the annual largest flood peak.By using the optimal rule for the reservoir operation,the annual largest flood peak at downstream control point(Ben Ngoc station)was successfully reduced(by about 2.4 m for water level and 2500 m^(3)·s^(-1) for discharge);while after the simulation periods,the reservoir water level was increased by about 20 m that could supply future water use.

Analysis of catchment evapotranspiration at different scales using bottom-up and top-down approaches

Physically-based hydrological models are used to predict catchment water balance through detailed simulation of hydrological processes at small temporal and spatial scales.However,annual catchment water balance can also be easily and simply predicted using lumped conceptual model.Comparison between physically-based hydrological models and lumped conceptual models can help us understand the dominant factors on catchment water balance at different scales.In this paper,a distributed physically-based hydrological model(i.e.,bottom-up approach)and a simple water-energy balance model(i.e.,top-down approach)are used to predict actual evapotranspiration in nine sub-catchments,and the whole basin of the Luan River in northern China.Both simulations give very close values of annual evapotranspiration and show the same complementary relationship between actual and potential evapotranspiration at annual time scale.From the analysis at different time scales through comparison of the top-down and the bottom-up methods,it is shown that the annual catchment evapotranspiration is controlled mainly by annual precipitation and potential evapotranspiration,and the variability of soil water and vegetation becomes more important at a smaller time scale in the study areas.It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale but can be simplified to a linear relationship at hourly temporal and hillslope scales,which is commonly used in the physicallybased hydrological models.

A rain-on-snow mixed flood forecast model and its application

Based on summarizing the rule of rainstorm and snowmelt mixed flood,the structure of rain-on-snow runoff-generation is discussed;and critical temperature is used to determine the form of precipitation and snowmelt factor,taking into account rainfall volume of snowmelt.A rain-on-snow flood forecast model is developed by combining LL-Ⅰdistributed hydrology model.The Kalangguer River,an internal river in Xinjiang Autonomous Region,is taken for example.It is indicated that the model has a higher precision of forecasting;its determinacy coefficient is greater than 0.80.