H53D-01
Investigating Hillslope-Scale Connectivity Through Soil Moisture Dynamics and Hydrological Threshold Events
The existence of several preferential states associated with various antecedent moisture conditions is often advanced to explain the connection or disconnection of runoff sources to the stream network. The issue of hydrological connectivity was addressed here by studying the temporal dynamics of precipitation, soil moisture and streamflow during storm events. A small headwater forested catchment, the Hermine, located in the Laurentians near Montreal, Quebec, was monitored across a 3-month period covering a gradient of meteorological conditions (dry, wet, very wet). The objective of the study is to understand how runoff at the catchment outlet is generated under various conditions of soil moisture and of connectivity between the hillslopes and the stream. The temporal and spatial patterns of soil moisture were monitored on two facing hillslopes on a North-South transect crossing the stream using a set of three equally-spaced automatic TDT (Time Domain Transmission) probes on each hillslope. The instruments measured moisture of the soil developed on a glacial till at a depth of 30 cm and a frequency of 15 minutes. Discharge data was automatically recorded every 15 minutes at a small V-notched weir located 100 meters downstream from the hillslopes. The relationships between precipitation, spatially-variable soil moisture and streamflow were assessed using cross-correlation techniques for each hydrological event identified. Regression trees were also used to partition the discharge data into dissimilar subgroups on the basis of soil moisture threshold values. Regardless of the hillslope and of the location of the TDT probe, soil water contents above 40 % were needed for significant peak discharges to be observed at the catchment outlet. A flashier response to precipitation was observed on the south-facing, flatter slope while the drainage of soil water was much longer on the north-facing, steeper slope, also characterized by a more organic soil matrix. Soil saturation did not always occur from the valley bottom upwards, especially under low antecedent wetness conditions, thus causing a disruption of hydrological connectivity. The various shapes of the soil water-streamflow hysteretic relationships can also be seen as an emergent property of hillslope-scale connectivity. This work sheds some light on the time-varying nature of hydrological connection between various points on a hillslope and the controls exerted by local landscape features.
H53D-02
TI: * Smith, M W
(mark.smith@durham.ac.uk), Durham University, Department of Geography
Durham University Science Laboratories
South Road, Durham, DH1 3LE, United Kingdom
Bracken, L J
(l.j.bracken@durham.ac.uk), Durham University, Department of Geography
Durham University Science Laboratories
South Road, Durham, DH1 3LE, United Kingdom
Cox, N J
(n.j.cox@durham.ac.uk), Durham University, Department of Geography
Durham University Science Laboratories
South Road, Durham, DH1 3LE, United Kingdom
In regions where Hortonian overland flow dominates, soil surface morphology will demonstrate systematic changes as the volume of overland flow increases with distance downslope. Overland flow gathers in depressions and eventually begins to form flow concentrations and rills. Distinct zones of similar soil surface morphology may be easily identified on a hillslope where vegetation cover is limited (Bracken and Kirkby, 2005). Each zone will differ in hydraulic resistance to overland flow and the degree of flow concentration. This in turn will determine the velocity at which flow is routed downslope. The spatial configurations of such zones across a hillslope will influence transfer of runoff from hillslope to river channel. This paper presents results from a study which uses laser-scanned surfaces to examine the influence on soil surface roughness on flow concentration and velocity at both the plot and hillslope scale. In semi-arid environments, where flood events are often both flashy and localised (Bull et al., 2000), this approach offers an appreciation of the gradual development of connected flowpaths over a hillslope during a rainstorm. This develops our ability to predict water inputs into ephemeral channels and, therefore, can potentially be applied to help predict the generation of flood waves through semi-arid catchments.
H53D-03
Quantifying Hydrologic Pathways in Depressional Wetlands Through a Water Budget Approach: Implications for Determining Landscape-scale Nutrient Treatment Potential
Depressional wetland hydrologic dynamics in the Okeechobee basin, FL are not well understood. Management of these wetlands is related to landscape-scale reduction of nutrient loads to Lake Okeechobee, where efforts are being made to attenuate the rate of hyper-eutrophication. Part of the total nutrient load to the lake originates from cow-calf operations in the lake catchment, which are managed to promote surface water drainage, because of shallow water table conditions. Four depressional wetlands in the Lake Okeechobee catchment were instrumented and monitored for three years to estimate the components of the water budget. The objectives of this study were 1) quantify wetland hydrologic inflows and outflows, 2) compare wetland inflows and outflows between four isolated wetlands, and 3) explore the potential role of these wetlands as a means to reduce nutrient export from agricultural settings. During the three monitoring years, the majority of water exported from the wetlands was due to surface water drainage through ditch networks (62%), followed by recharge to groundwater (21%), and then evapotranspiration (17%). Rainfall on the wetland water surface and runoff (overland flow) were similar in magnitude and incorporated 96% of the total inflow to the wetlands, with the remaining 4% originating from groundwater discharge to the wetland. While recharge to groundwater from the wetland was only 21% of the total outflow, it occurred more frequently than surface export through ditches (67% and 25% respectively of total days with standing water). The relative amount of ditch flow across three wetlands was similar, however, the extent of ditching at the fourth wetland appeared to limit surface water outflow (84% reduction). These wetlands are hydrologically connected to the upland pastures, primarily through runoff, and might be used as natural nutrient filters in these landscapes. The potential for wetlands to attenuate nutrient loads from pasture runoff is likely a function of the extent of ditching. By removing ditches or controlling surface water outflow from wetlands, surface water recharge to groundwater becomes an important pathway for nutrient transport.
H53D-04
Linking slope, aspect and river network probabilistic structures
Hydrologic basins are shaped by their river network, the structure of which has been characterized in some detail. Less attention has been devoted to the corresponding properties of other landscape features, such as slope, aspect and ridge statistics, despite their role on several hillslope processes, vegetation patterns, and basin- scale CO2 and water fluxes. We employ Digital Elevation Models (DEMs) from basins with different features and compare the results with those of simple statistical models of the network, hillslope, and ridge system. In particular, we explore the relationship between reach order and the shape of their corresponding sub-basins, with a focus on the basin-scale probabilistic structure of aspect and slope.
H53D-05
Impact of Uncertainty in Runoff and Routing Processes on the Estimation of Non-parametric Unit Hydrographs for the Cypress Creek Watershed, TX
Remote sensing data and GIS tools have opened the door to simplify the parameterization of distributed watershed models. However, decisions about the spatial homogeneity of model parameters should also be based on the actual response of a basin to rainfall. For the last 75 years, hydrologists have relied on the unit hydrograph (UH) as a key tool for analyzing watersheds because its shape is directly related to important attributes of the drainage basin controlling runoff (e.g., topography, land use, soil properties, stream network, etc.). Deconvolution of excess rainfall from direct runoff can provide non-parametric estimates of the UH that capture the effects of sub-basin heterogeneity, thereby making these hydrographs particularly useful tools for comparing and classifying watersheds. Due to the mathematical instability of deconvolution, it is unclear whether meaningful UH estimates can be obtained for the purpose of inter-basin comparisons, particularly when processes controlling excess precipitation and direct runoff within the watershed are uncertain. This study evaluates the sensitivity of non-parametric UH's to uncertainty in watershed properties for six gauged sub-basins of the Cypress Creek Watershed, TX. We have used MATLAB to conduct a rainfall-runoff analysis of the Cypress Creek Watershed, TX over a 17 day period during Tropical Storm Allison in 2001. For the six basins analyzed, discharges for Cypress Creek are available at the outflow of each sub-basin and NEXRAD rainfall data are available throughout the watershed. To determine the direct runoff contributed by each sub-basin, incoming upstream flows were routed by simple advection and then subtracted from the downstream discharge record. Excess precipitation was calculated by applying the Green & Ampt infiltration model to the rainfall record for each basin after accounting for initial abstractions and direct losses due to impervious surfaces. In each step of this procedure, the parameters controlling routing and runoff (i.e., stream velocity, soil conductivity and suction, moisture deficit, watershed impervious area and initial abstraction) were highly uncertain, but could be constrained by a range of likely values (e.g., hydraulic conductivity values spanning soils ranging from sands to clays) or limited to a physically plausible range (e.g., stream velocity). By randomly drawing sets of parameters from within their plausible ranges, we were able to assess the impact of uncertainty on our ability to reproduce consistent UH's. We have found that while the magnitude of the UH can vary substantially for different parameter combinations, primarily due to mass balance considerations, the shape of the UH is largely insensitive to the changes in parameter values. This analysis suggests that the UH's found by deconvolution in the Cypress Creek Watershed are useful tools for performing inter-basin comparisons of rainfall-runoff behavior.
H53D-06
Physically-Based Distributed-Parameter Hydrologic Modeling of the Bull Creek Watershed, Austin, Texas
Recent advances in computing power, data storage and the increased availability of spatially distributed data sets have encouraged research into the benefits and potential applications of physically-based, distributed hydrologic models. Physically-based, distributed parameter, structured grid models simulate watershed processes using physics-based equations, such as energy, momentum and continuity. Hydrologic parameters are specified for each grid cell within the model domain in an effort to best represent the spatial variability of watershed characteristics. The intent of this study is to contribute to the ongoing effort to evaluate the physically-based, distributing modeling approach for hydrologic study, flood forecasting and other applications. The hydrology of a partially urbanized watershed located in Austin, Texas is simulated using the physically-based, distributed parameter model Surface Subsurface Hydrologic Analysis (GSSHA). GIS-based data sets, collected from publicly available sources and the City of Austin Watershed Protection Division, were processed using ArcGIS version 9.1 and the Watershed Modeling System graphical modeling environment. NEXRAD precipitation data for three significant rain events were processed and quality-controlled using rain-gauge observations. Model-generated hydrographs for these events were compared to observed flow data at a USGS flow gage located at the basin outlet.
H53D-07
Downslope Unsaturated Throughflow
The hydrologic science community lacks a consensus on the role of lateral unsaturated throughflow in soil-water cycling. For example, some studies indicate that lateral unsaturated flow may contribute to stormflow, while others suggest this is not the case. Some analyses require soil heterogeneity for its occurrence and others do not. Some researchers indicate that lateral unsaturated flow may support baseflow while others report that it is not likely to contribute much flow at all. These disparate findings preclude the development of a conceptual model for the initiation and stability of lateral (also called downslope or slope parallel) unsaturated flow processes. This shortcoming is particularly acute given that lateral unsaturated flow is often invoked to explain the spatial and temporal variability in root zone soil-water dynamics, a critical facet of ecohydrology. Few studies, however, have addressed the temporal variability and controls on the development and stability of downslope unsaturated flow. Here we present data and analyses from the Coos Bay experiments that document the dynamics of unsaturated zone flow paths, we characterize the conditions necessary for its initiation, and we present estimates of annual downslope flux.