Henrique G. Momm

Effect of Topographic Characteristics on Compound Topographic Index for Identification of Gully Channel Initiation Locations

Abstract. Sediment loads from gully erosion contribute to water quality problems, reduction in crop productivity by removal of nutrient-rich topsoil, and damage to downstream ecosystems. The identification of areas with high potential for gully channel development is often performed using spatially derived stream power estimates from second-order topographic indices, such as the compound topographic index (CTI). The utilization of CTI to identify where gullies develop is affected by field and local topographic characteristics and DEM resolution. In this study, the effect of overall terrain slope, local relief variance, and raster grid cell size on CTI cumulative distribution values was investigated using theoretical and observed catchment methodology. In the theoretical analysis, stochastic methods were used to generate simulated catchments to quantify the influence of overall terrain slope, local relief variance, and raster grid cell size (each considered individually). The observed methodology used three sites with distinct topographic characteristics, measured gully channels, and high-resolution topographic information. Raster grids for the three observed study sites were generated at varying raster grid cell sizes. Critical CTI values were determined through comparison of measured gully thalwegs with threshold CTI raster grids of the observed watersheds at different resolutions. Results from the theoretical investigation indicate that CTI values were linearly influenced by changes in relief variance and overall slope, while variations in raster grid cell size caused an inverse power variation in CTI values. In addition, variations in raster grid cell size, produced changes in cumulative distributions of the top 0.1% CTI values. The use of normalized CTI values (CTI n ) produced merged cumulative distribution curves when varying overall slope, terrain relief variance, and to a lesser degree DEM resolution. Similar findings were obtained from the analysis of observed catchments. When DEM resolution varied, the differences in critical CTI n values in the same field were significantly reduced when compared to original critical CTI values, although differences were not fully eliminated. Normalization of the CTI cumulative distributions improved comparisons between different sites with distinct drainage area sizes and topographic characteristics, providing a possible alternative for investigations of large watersheds with more than one topographic characteristic. Results suggest that a normalized critical CTI between 1 and 2 could be used for the identification of areas with high potential for gully development. Knowing where gullies develop is important in understanding the effect of conservation practices on soil erosion through the use of field-scale and watershed-scale simulation models. Effective watershed management plans depend on this information to target the placement of conservation practices for the efficient use of available resources.

Spatial Characterization of Riparian Buffer Effects on Sediment Loads from Watershed Systems

Understanding all watershed systems and their interactions is a complex, but critical, undertaking when developing practices designed to reduce topsoil loss and chemical/nutrient transport from agricultural fields. The presence of riparian buffer vegetation in agricultural landscapes can modify the characteristics of overland flow, promoting sediment deposition and nutrient filtering. Watershed simulation tools, such as the USDA-Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model, typically require detailed information for each riparian buffer zone throughout the watershed describing the location, width, vegetation type, topography, and possible presence of concentrated flow paths through the riparian buffer zone. Research was conducted to develop GIS-based technology designed to spatially characterize riparian buffers and to estimate buffer efficiency in reducing sediment loads in a semiautomated fashion at watershed scale. The methodology combines modeling technology at different scales, at individual concentrated flow paths passing through the riparian zone, and at watershed scales. At the concentrated flow path scale, vegetative filter strip models are applied to estimate the sediment-trapping efficiency for each individual flow path, which are aggregated based on the watershed subdivision and used in the determination of the overall impact of the riparian vegetation at the watershed scale. This GIS-based technology is combined with AnnAGNPS to demonstrate the effect of riparian vegetation on sediment loadings from sheet and rill and ephemeral gully sources. The effects of variability in basic input parameters used to characterize riparian buffers, onto generated outputs at field scale (sediment trapping efficiency) and at watershed scale (sediment loadings from different sources) were evaluated and quantified. The AnnAGNPS riparian buffer component represents an important step in understanding and accounting for the effect of riparian vegetation, existing and/or managed, in reducing sediment loads at the watershed scale.

Methods for Gully Characterization in Agricultural Croplands Using Ground-Based Light Detection and Ranging

Soil erosion has long been recognized as the primary cause of soil degradation in agricultural fields (Wells et al., 2010). Overland flow in agricultural fields is the main process associated with soil erosion, which is often grouped into categories of: sheet erosion, rill erosion, and gully erosion (Smith, 1993). Traditionally, research has focussed on understanding and modelling sheet and rill erosion processes (Poesen et al., 1996). Recent research has begun to focus on addressing gully issues such as the understanding of the formation of gullies, their contribution to overall soil loss, development of tools to locate channel initiation, and appropriate measuring techniques (Poesen et al., 2003). The increased focus on gully research can be partially attributed to recent studies demonstrating that gully formation is very common on cropland, especially in conventional tillage systems (Gordon et al., 2008) and can be as significant as sheet and rill erosion in terms of sediment yield (Bingner et al., 2010). Without a good understanding of gully processes, technology cannot be developed that can provide information needed by watershed managers when evaluating and implementing effective conservation practice plans. Gullies can be generally classified as ephemeral, classical, or edge-of-field. The Soil Society of America (2001) defines ephemeral gully as “small channels eroded by concentrated flow that can be easily filled by normal tillage, only to reform again in the same location by additional runoff events”. As the headcut migrates upstream and the channel gets wider, faster than the interval between farming tilling operations, farming equipment is forced to operate around the gully and as result the gully becomes permanent (classical gully). Finally, as the name suggests, edge-of-field gullies are defined by channels where concentrated flow crosses earth bank (Poesen et al., 2003). New methodologies are being researched to understand gully formation and estimate sediment yield (Souchere et al., 2003, Cerdan et al., 2002, and Woodward, 1999). Studies use Digital Elevation Models (DEMs) as the basis to formulate theories explaining the relationship between field topography and gully occurrence (Woodward, 1999, Parker et al., 2007, and Cerdan et al., 2002). These efforts greatly benefit from accurate and detailed topographic information which can aid in the understanding of where and when gullies form and how these features evolve over time (headcut migration). Despite the availability of DEMs at regional and local scales (spatial resolution ranging from 1 to 30 meters), these datasets often do not offer the necessary spatial and/or temporal

Automated watershed subdivision for simulations using multi-objective optimization

ABSTRACT A qualitative trial-and-error approach is commonly used to define watershed subdivisions through varying a single topographic threshold value. A methodology has been developed to quantitatively determine spatially variable threshold values using topography and a user-defined landscape reference layer. Optimization and topographic parameterization algorithms were integrated to create solutions that minimize the number of sub-watersheds and maximize the agreement between the discretized watershed and the reference layer. The system was evaluated using different reference datasets such as soil type, land management, and landscape form. Comparison of simulated results indicated that the scenario using land management as the reference layer yielded results closer to the scenario subdivided using a constant topographic threshold but with approximately 10 times more sub-catchments and therefore indicating customization of the watershed subdivision to the user-defined reference layer. The proposed optimization technology could be used in adequately applying watershed modeling technology in developing conservation practice implementation plans.

Exploring individual- to population-level impacts of disease on coral reef sponges: using spatial analysis to assess the fate, dynamics, and transmission of Aplysina Red Band Syndrome (ARBS).

Background Marine diseases are of increasing concern for coral reef ecosystems, but often their causes, dynamics and impacts are unknown. The current study investigated the epidemiology of Aplysina Red Band Syndrome (ARBS), a disease affecting the Caribbean sponge Aplysina cauliformis, at both the individual and population levels. The fates of marked healthy and ARBS-infected sponges were examined over the course of a year. Population-level impacts and transmission mechanisms of ARBS were investigated by monitoring two populations of A. cauliformis over a three year period using digital photography and diver-collected data, and analyzing these data with GIS techniques of spatial analysis. In this study, three commonly used spatial statistics (Ripley’s K, Getis-Ord General G, and Moran’s Index) were compared to each other and with direct measurements of individual interactions using join-counts, to determine the ideal method for investigating disease dynamics and transmission mechanisms in this system. During the study period, Hurricane Irene directly impacted these populations, providing an opportunity to assess potential storm effects on A. cauliformis and ARBS. Results Infection with ARBS caused increased loss of healthy sponge tissue over time and a higher likelihood of individual mortality. Hurricane Irene had a dramatic effect on A. cauliformis populations by greatly reducing sponge biomass on the reef, especially among diseased individuals. Spatial analysis showed that direct contact between A. cauliformis individuals was the likely transmission mechanism for ARBS within a population, evidenced by a significantly higher number of contact-joins between diseased sponges compared to random. Of the spatial statistics compared, the Moran’s Index best represented true connections between diseased sponges in the survey area. This study showed that spatial analysis can be a powerful tool for investigating disease dynamics and transmission in a coral reef ecosystem.

Characterizing ponds in a watershed simulation and evaluating their influence on streamflow in a Mississippi watershed

ABSTRACT Small water bodies, such as ponds and wetlands, are common landscape features, but often are not simulated within a watershed modeling framework. The wetland modeling tool, AgWET, uses a GIS framework to characterize the features of ponds and wetlands so that they can be incorporated into watershed simulations using the Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model. AgWET was used to characterize farm ponds on the Goodwin Creek Experimental Watershed in northwest Mississippi and AnnAGNPS simulated watershed hydrology. Monthly streamflow was validated at four watershed gauges with Nash-Sutcliffe efficiency values between 0.91 and 0.94. Ponds influenced watershed hydrology at various scales, with a decrease in average streamflow by 4% at the watershed outlet, 8% at the sub-watershed scale, and an average of 56% immediately downstream of the ponds. AgWET can be used to simulate ponds in watershed assessments for improved results and evaluation of future scenarios.

Disaggregating soil erosion processes within an evolving experimental landscape

Soil-mantled landscapes subjected to rainfall, runoff events, and downstream base level adjustments will erode and evolve in time and space. Yet the precise mechanisms for soil erosion also will vary, and such variations may not be adequately captured by soil erosion prediction technology. This study sought to monitor erosion processes within an experimental landscape filled with packed homogenous soil, which was exogenically forced by rainfall and base level adjustments, and to define the temporal and spatial variation of the erosion regimes. Close-range photogrammetry and terrain analysis were employed as the primary methods to discriminate these erosion regimes. Results show that (1) four distinct erosion regimes can be identified (raindrop impact, sheetflow, rill, and gully), and these regimes conformed to an expected trajectory of landscape evolution; (2) as the landscape evolved, the erosion regimes varied in areal coverage and in relative contribution to total sediment efflux measured at the outlet of the catchment; and (3) the sheetflow and rill erosion regimes dominated the contributions to total soil loss. Disaggregating the soil erosion processes greatly facilitated identifying and mapping each regime in time and space. Such information has important implications for improving soil erosion prediction technology, for assessing landscape degradation by soil erosion, for mapping regions vulnerable to future erosion, and for mitigating soil losses and managing soil resources.

An Experimental Study of Gully Sidewall Expansion

Soil erosion, in its myriad forms, devastates arable land and infrastructure and strains the balance between economic stability and viability. Gullies may form in existing channels or where no previous channel drainage existed. Typically, gullies are a result of a disequilibrium between the eroding force exerted by concentrated flowing water and the resistance of the earth materials in which it is flowing, caused by either an increase in erosional forces (related to a concentration of flow, constriction of flow, an increase in discharge, or decrease in sediment load) or decreased erosional resistance (related to a decrease in cover or some surface disturbance causing decreased cohesion). A gully is a complicated system as its evolution is controlled by water erosion at the gully head and bed, which triggers gravitational mass-movement on gully sidewalls. Gully erosion usually, but not always, includes one or more headcuts that migrate upslope over time. These are step changes in bed surface elevation where intense, localized erosion takes place, and thus are commonly associated with significant increases in sediment load. Reported experimental data shows that actively migrating gully headcuts display a self-similar organization with migration rates dependent on upstream flow depth and discharge, tailwater depth, and soil properties. The depth of gullies is often limited by the presence of a non-erodible or impervious soil layer. When erosion reaches such a layer, the gully typically widens, creating a wide shallow cross section. Once a gully is initiated, transport and deposition of the eroded soil and widening of the gully channel, further govern its evolution. Our knowledge of these processes in shallow concentrated flows within agricultural soils, however, is still quite limited and largely scaled down from river hydraulics. Experiments were conducted to examine channel sidewall expansion due to overland flow discharge. Packed soil beds were subjected to simulated rainstorms and clear-water overland flow. During overland flow, the flow rate was systematically increased to induce widening within the channel. Within these channels, equilibrium must be maintained between potential scour depth and potential channel width. Channel expansion and peaks in sediment discharge occurred episodically, linked directly to increases in upstream discharge.

Analysis of Topographic Attributes for Identification of Ephemeral Gully Channel Initiation in Agricultural Watersheds

Watershed-scale modeling tools, such as USDA’s watershed management planning tool AnnAGNPS, already have been developed with components necessary for evaluation of the effect of ephemeral gullies on farming and conservation practices; however, the identification of potential downstream gully channel initiation locations (mouths) is critical. On a watershed scale this represents a time consuming task where users may not accurately locate and describe all the existing and potential ephemeral gully mouths. Alternatively, the identification of ephemeral gully mouths are often empirically related to stream power, which is recognized as a surrogate to flow intensity and sediment carrying capacity, through the use of secondary topographic attributes. Several topographic indices have been proposed to describe stream power and thus to characterize sediment transport in overland flow specifically due to channel initiation and headcut migration. Using high spatial resolution digital elevation models (25 to 200 cm) of an agricultural field located in Kansas, USA, four topographic indices were evaluated to identify potential ephemeral gullies. Results indicated similar ability of these four indices of predicting the location of channel initiation, with slightly better spatial distribution and less number of points yielded by CTI.