Applicability Analysis of Four Flow Accumulation Threshold Determination Methods in Loess Eroded Area

In this paper,a method based on the theory of geomorphic development and drainage fractal dimension is proposed for testing the rationality of four threshold determination methods for catchment area:drainage density method,channel branching method,minimum error in river source distance method,trial and error method.The results show that drainage density method and channel branch method have a good applicability in the watershed of the loess hilly landform and can reflect the accurate information of gully channel.River source minimum error method is controlled by the topography of the river source,which is only applicable to small watershed with little change of topography.Trial and error method is controlled by artificial vision and scale,so it is not suitable for the area with large changes of topography or the complex river network.

A novel gully network extraction method combining hydrological confluence process and surface morphology

The automatic extraction of gullies from digital elevation models(DEMs)has great application value in GIS and hydrology.Many types of algorithms have been developed to address this problem,and the well-known D8(Deterministic eight-node)algorithm has been widely applied and implemented in some commercial GIS software such as ArcGIS.However,a key parameter called flow accumulation threshold(FAT)must be determined in this process.Numerous studies focus on how to determine an optimal value for this parameter but ignore that the optimal threshold varies for different gullies,so the universality of a different optimal threshold parameter determined by different methods is poor.To address this problem,this study designs a parameter called surface concavity index(SC-index)that can describe the shape of gullies from the perspective of surface morphology.Based on this index,the positions of different gullies'heads are identified,and then the flow accumulation matrix calculated by the D8 algorithm is used as auxiliary data to extract the gully network in the research area.In this study,six small watersheds in the Loess Plateau in northern Shaanxi,China,were used as test areas to verify the validity of the proposed method in areas with various landform types.Experimental results show that gully heads in different test areas can be effectively identified by setting different SCindex thresholds that are related to the types of terrain in the test areas.Then,the entire gully network can be extracted in watersheds with the help of a D8 algorithm.The accuracy of the gully network extracted by the new method is better than the contrast method in all test areas.In test areas with a large area of flat land(e.g.,Chunhua),the difference between the total length of gullies extracted by the new method and the reference value is-2.77 km,while the corresponding value of the contrast method is 14.50 km.In test areas with large numbers of short gullies(e.g.,Jiuyuangou),the difference between the total length of gullies extracted by the new method and the reference value is-2.61 km while the corresponding value of the contrast method is-27.9km.It is pointed out that the new method can not only avoid the extraction of pseudo gullies,but also extract short gullies effectively.Further experimental analysis shows that the dependence of the new method on DEM resolution is significantly weaker than that of the contrast method.Taking Jiuyuangou test area as an example,when the DEM cell size increases from 5 m to 30 m,the total length of gullies extracted by the new method changes only about 1km,while the corresponding value of the contrast method exceeds 20 km.

Spatio-temporal analysis of flowering using Li DAR topography

Spatio-temporal patterns of flowering in forest ecosystems are hard to quantify and monitor. The objectives of this study were to investigate spatio-temporal patterns（e.g. soilssimple slope classesslope aspectand flow accumulation） of flowering around Lake IssaqueenaSouth Carolina（SCUSA） using plant-flowering database collected with GPS- enabled camera（stored in Picasa 3 web albums and project website） on a monthly basis in 2012 and Li DAR-based topography. Pacolet fine sandy loam had the most flowering plantsfollowed by Madison sandy loamboth dominant soil types around the lake. Most flowering plants were on moderately steep（17%–30%） and gently sloping（4%–8%） slopes. Most flowering plants were on west（247.5°–292.5°）southwest（202.5°–247.5°）and northwest（292.5°–337.5°） aspects. Most flowering plants were associated with minimum and maximum flows within the landscape. Chi-square tests indicated differences in the distributions of the proportions of flowering plants were significant by soil typeslopeaspectand flow accumulation for each month（February-November）for all months（overall）and across months. The Chi-square test on area-normalized data indicated significant differences for all months and individual differences by each month with some months not statistically significant. Cluster analysis on flowering counts for nine plant families with the most flowering counts indicated no unique separation by clusterbut implied that the majority of these families were flowering on strongly sloping（9%–16%） slopeson southwest（202.5°–247.5°） aspectsand low flow accumulation（0–200）. Presented methodology can serve as a template for future efforts to quantify spatio-temporal patterns of flowering and other phenological events.

Short-term desalination of Pulicat lagoon(Southeast India)due to the 2015 extreme flood event:insights from Land-Ocean Interactions in Coastal Zone(LOICZ)models

Introduction:We investigated the magnitude and duration of desalination of Pulicat—a coastal lagoon ecosystem connected to the Bay of Bengal on the South-eastern coast of India—during the 2015 South India flood event which was a period of high-magnitude precipitation and riverine flooding.Methods:We estimated freshwater runoff into the lagoon using flow accumulation models for a period of 55 days(November 1 to December 25,2015)using daily gridded precipitation data from the Global Precipitation Measurement and a digital elevation model.Using the estimates of freshwater runoff,direct precipitation and observed salinities,we simulated water and salinity fluxes of the lagoon using the Land-Ocean Interactions in the Coastal Zone model.Further,we also used Monte Carlo simulation to estimate the uncertainty in system salinity,the residual salinity at the boundary and the freshwater residence times in the lagoon.Results:We estimated that a high volume(~760×10^(6) m^(3))of relatively low salinity waters(residual salinity=23.47 psu)had been exported from the lagoon to the Bay of Bengal during the period which is likely to have caused a strong dip in the daily salinity profile of the coastal sea.We contend that the lagoon experienced~40%desalination due to the 2015 event with a freshwater residence time of 18.5 days.Conclusions:The study highlighted the short-term,high-magnitude desalination undergone by Pulicat lagoon during the 2015 South India floods.Considering the high residual and exchange volumes obtained from the study,we conclude that Pulicat could be a major exporter of relatively low salinity waters to the Bay of Bengal during monsoons.