Zhu Axing

Effectiveness Assessment of Soil Erosion Critical Source Areas for Soil and Water Conservation

Abstract: Critical source areas (CSAs), characterized by severe soil erosion and high sediment yield, are considered to have a high priority for conservation. How to identify CSAs and assess the effectiveness of conservation practices is a key issue in site—specific watershed management. The Soil and Water Assessment Tool (SWAT) model is a useful tool for site-specific conservation practices design and several studies have attempted to identify CSAs based on watershed models. However, limited research has reported about the effectiveness of conservation practices targeting CSAs. The aim of this study was to assess the effectiveness of conservation pracrices targeted on CSAs using the SWAT model. CSA was firstly identified based on the 4-year average yearly erosion of each HRU. Appropriate soil conservation practices were then designed for the CSAs. A scenario with conservation practices for the whole watershed was also established as the contrasting counter parts scheme and then compared to the outcome of CSA-targeted conservation pracrices. The result shows that SWAT can accurately simulate sediment yield in the study area. CSAs were mainly located in slope farmland areas and steep gullies, coinciding with the distribution of land use and slope. The identified CSA covered 20% of the HRUs and contributed on average 44% of sediment yield. Conservation practices targeting CSAs had higher sediment reduction effectiveness (24 115 t km-2 y-1) than conservation practice covering the whole watershed (20 290 t km-2 y-1). Thus conservation practices targeting CSAs are more effective than broad conservation practices. We conclude that soil conservation practices focusing on CSAs do increase sediment reduction effectiveness. Targeting the placement of soil conservation practices based on the CSAs concept will assist water quality control in watersheds.

Representation of Agricultural Best Management Practices in a Fully Distributed Hydrologic Model: A Case Study in the Luoyugou Watershed

Abstract Agricultural Best Management Practices (BMPs) are effective ways to reduce agricultural nonpoint source pollution from their source area to receiving water bodies. Characterization of BMPs in a watershed model is a critical prerequisite for evaluating their impacts on water quantity and water quality in a complex system. However, limited research has reported about the representation of BMPs in fully distributed models. This paper presents a stepwise procedure for representation of several BMPs and assessment of their hydrologic impacts with a fully distributed model, SEIM (Spatially Explicit Integrated Modeling). A case study is conducted in the 73 km2 Luoyugou watershed located in the Loess Plateau of China, where rainstorm erosion accounts for more than 60% of annual sediment load in average. Three BMPs are selected in this study including (i) conversion from farmland to forest, (ii) terrace, and (iii) no-till farming. These management practices are represented in the model through the alteration of model parameters characterizing their physical processes in the field. The results of scenario assessment for a historical storm event showed that the maximum sediment reduction after terrace is about 97.3%, the average sediment reduction after no-till farming is about 9.5%, and the average sediment reduction after conversion from farmland to forest is 75.6%.

Review on parallel computing of distributed hydrological models

High resolution distributed hydrological simulations over large watersheds require very large amounts of computations,which cannot be provided by sequential computation techniques on which existing hydrological models were developed.So parallel computing of distributed hydrological models is needed.In this paper,we first analyzed the parallelizability of distributed hydrological models from three angles(spatial,temporal and sub-process) and pointed out that spatial domain decomposition is the preferred approach to parallel computing of distributed hydrological models.According to spatial relationships among simulation units,distributed hydrological models,as well as simulation methods for hydrological processes,are classified into different types.Then,current studies on parallel computing of distributed hydrological models were introduced.For most current studies on parallel computing using spatial domain decomposition methods,sub-basin was adopted as the basic scheduling unit for parallel computing.The temporal-spatial discretization method proved the feasibility of parallel computing utilizing parallelization from the temporal angle.Last,the key technologies and future research directions were discussed in the following aspects: 1) parallel algorithms;2) parallel computing framework for integrated watershed system simulations;3) high performance input/output for parallel computing of distributed hydrological models.

Identification of critical source areas of soil erosion on moderate fine spatial scale in Loess Plateau in China

Critical Source Areas (CSAs) are considered as priority areas for soil conservation and it is essential to identify CSAs for effective watershed management. Soil and water assessment tool (SWAT) model is a useful tool in identifying CSAs. Previous studies that used SWAT for CSAs identification were almost carried out on the basis of sub-watershed level which was too coarse to capture spatial detail of soil erosion. This research identified CSAs of soil erosion at a moderate fine spatial detail scale in a small watershed of Loess Plateau in China using SWAT model. CSAs were identified based on the 4-year average annual sediment yield of hydrological response units (HRU). The result shows that CSAs were mainly located in steep slope farmland areas and gully dominated areas. CSAs covered 10% areas of watershed, and contributed 30% sediment yield to the watershed. Such a trend is more obvious under larger storms. This could imply that CSAs identification on HRUs level is suitable for site-specific management design. This study also confirms that CSAs identification could be a potential approach assisting water quality control.

Review on distributed hydrological modelling software systems

Distributed hydrological modelling software systems are crucial because they provide technical support to the application of distributed hydrological models.Currently,applications of distributed hydrological models have exhibited new characteristics including multi-process synthesis simulation,a wide range of users,and intensive computation.Because of these new characteristics,the existing software systems are facing great challenges with respect to flexibility,usability,and efficiency.This paper reviews existing software systems for distributed hydrological models.Firstly,we analyzed the distributed hydrological modelling applications workflow including model structure determination,parameter extraction,model running,and calibration.The characteristics of existing software systems are discussed:(1) model structure flexibility of the existing software systems is divided into three types:no support of process and algorithm selection,only support of algorithm selection,and support of both process and algorithm selection;(2) parameter extraction methods of the existing software systems are divided into menu/command line and wizard method;(3) computing forms of the existing software systems are divided into parallel computing and serial computing;(4) computing modes of the existing software systems are divided into stand-alone and network mode.Secondly,we summarized the limitations of existing software systems with respect to their flexibility,usability,and efficiency.The limitations include the following:(1) contradiction between model structure flexibility and user knowledge dependence-the more flexible the model structure is,the more knowledge users need to have;(2) the existing methods of parameter extraction are too fussy for non-expert users;(3) the serial and stand-alone softwares usually encounter computing bottleneck as the appliaction scenario is data and/or computing intensive.In the last part of this paper,the emerging trends of distributed hydrological modelling software systems are discussed.These include(1) Modular modelling.The modular development ensures software reuse,but it is not enough when scale or semantic is unmatched,so the ontology knowledge needs to be considered;(2) Intelligent modelling.Using expert knowledge to realize model structure determination and parameter extraction and combining expert knowledge and optimization algorithm to parameter calibration is needed in future work;(3) On-line modelling.The development of cloud computing and network techniques makes on-line modelling practical.In addition,mobile terminals with powerful computing and storage capacity could be potential application platforms.This means that special user interface and data format are needed;(4) Parallel computing.Taking full advantage of new parallel programming standards(CUDA,OpenCL) and exploring the finer granularity parallelizability is an emerging trend.In addition,virtual simulation is another important trend.