H31F-1352
Application of Electromagnetic Induction Sensors for Mapping the Subsurface in Small Watersheds.
The development of an integrated approach to characterizing small watersheds is crucial to understanding the complex links and feedback mechanisms within them. High spatial resolution soil texture data is well correlated to soil hydraulic properties. We present preliminary work using electromagnetic induction (EMI) to map subsurface properties in small watersheds. In this work we used both the Geonics EM-38 and the Dualem EMI sensors which were integrated with a GPS receiver and handheld computer to obtain geo-referenced bulk electrical conductivity (ECa) measurements. In the vertical orientation the sensors respond to the ECa of the top meter of soil. The ECa depends on the solution EC, soil water content, clay / rock content and soil depth. Data obtained from EMI in the form of ECa maps, can provide supplementary information for assessing flow pathways and locating monitoring instrumentation without soil-specific calibration. With ECa calibration, soil texture maps can be generated. This work may be more suited to semi-arid climates where seasonal wet and dry periods can be exploited in data analysis. Current work is looking at methods of developing the best survey and calibration methodology to interpret the measured ECa response for hydrological application.
H31F-1353
Modelling the Spatio-temporal Process Variability of Soil Water Balance in a Forest
In terrestrial forest ecosystems the partitioning of precipitation into evapotranspiration, runoff (recharge) and soil moisture change is influenced by the forest canopy. At the scale of a forest stand, factors such as tree species composition, crown architecture, and different canopy interception results in spatial variable evapotranspiration and soil moisture patterns. Both processes directly affect the recharge and runoff in space and time. To accurately predict the runoff from forested ecosystems, we need to know how much of this spatial process variability needs to be represented in our models. In this study, we model runoff and recharge from a forest stand based on spatial uniform values of precipitation, evapotranspiration and compare it to model results that include spatial variation and variation plus spatial correlation. In a mixed European beech-Norway spruce stand spatio-temporal variation of volumetric water content (VWC) was measured by TDR (time domain reflectometry) at 194 locations across 0.5 ha plots with permanently installed 30 cm vertical wave guides. Measurements were repeated 28 times before and after rewetting periods during the vegetation seasons in 2000 and 2001. Additionally, the locations of all trees within the plots were recorded. Geostatistics was used to describe spatial correlation between VWC measurements stochastically and to interpolate recharge patterns in space. Spatial patterns of recharge were analyzed according to antecedent soil moisture content, tree species locations, and canopy throughfall. We used measured surface and subsurface runoff from sprinkling experiments (50 m) at the same site to calibrate and validate hydrological parameters in our model. Runoff for the entire site was modeled in three different ways: first by spatially uniform precipitation, soil water content and evapotranspiration, second by incorporating the spatial variation and third by also considering the spatial correlation in the input variables. The model performance was tested by comparing model results with measured spatio-temporal change in soil water storage and evapotranspiration. Results of the geostatistical analysis show for example that spatial correlation and variation (expressed by the coefficient of variation) is not constant in time but shows a hysteresis that depends on the wetting and drying history of the site. Spatial patterns of transpiration are reflecting the spatial distribution of tree species (beech and spruce have different strategies in water consumption). Including these spatial patterns in the soil water content as a measure to describe the connectivity of wetter areas events improves the model performance (in respect to runoff) in particular during periods of high precipitation. Consideration of spatial process variability not only improves the overall model performance but also helps to better understand the interaction of soil science, plants and hydrology.
H31F-1354
Temporal Stability of Soil Moisture Spatial Distribution in a Forested Catchment
Soil moisture spatial distribution is a key component to characterize and model water movement at the catchment scale. The general pattern of spatial variability in soil moisture is much more stable over time than would be expected from random processes. However, such temporal persistence may depend on the scale of observation. Soil moisture, integrated with soils information and landforms, was monitored at 77 sites at the Shale Hills Catchment in central Pennsylvania over one and half years. Daily and weekly soil moisture records were used to investigate the temporal stability of moisture distribution in the catchment.
H31F-1355
Impact of Soil Depths on the Catchment-Scale Geomorphic Hydrologic Response
Catchment modelling in areas dominated by geomorphologic active processes is often hampered by the lack of reliable and easily applicable models of sediment production and transfer at the basin scale (e.g. erosion and landsliding). One of the major problems in distributed soil erosion and landsliding models is the correct evaluation of the spatial distribution of the soil thickness (i.e. distance to bedrock). The soil thickness may strongly vary within a single catchment as a function of the vegetation cover, lithology, climate, gradient, hillslope curvature, upslope contributing area and land use. In this paper we focus on the prediction of soil thickness by means of topographic and geomorphologic attributes, such as local topographic gradients, slope curvatures and physiographic types (e.g. patterns of soil catenas and toposequences). The application of the model to a relevant case study (the Terzona creek catchment, central Italy) provides results which are in substantial agreement with observed field data, confirming the capability of the model to enhance the prediction of soil thickness in cases of practical interest. To assess the overall influence of computed soil depths on the ensuing hydrologic response at the basin-scale, the Green-Ampt approach for shallow soils has been incorporated into a geomorphic model of the hydrologic response, exploiting key information (e.g. the spatial distribution of soil uses and textures) obtained from remotely sensed images. The basic geomorphologic assumptions pertain to the different dynamics of runoff production in topographically convergent or divergent sites, with a view to the effect of soil depths as seen through different models of soil production. As such, this class of models represents a useful tool for estimating the hydrologic response of a river basin in a spatially distributed framework. A discussion follows on the application of the model to the Brenta river basin (Northern Italy) and on a comparative analysis between the geomorphic approach described above (where the soil saturation dynamics are linked to the soil thickness) and other classical models of runoff production.
H31F-1356
Soil-type based prediction of contributing areas for diffuse losses of agrochemicals
The lack of adequate spatial data often limits the prediction of contributing areas for diffuse losses of agrochemicals using distributed hydrological models. This data problem is often amplified by the use of highly parameterized simulation tools. In order to overcome the problem of over-parameterization we present a parsimonious rainfall-runoff model that was adapted to the spatial soil data available in the study region. The approach is based on the hydrologic responses of two soil types and includes effects of topography. It requires 10 parameters that were obtained by simultaneously fitting the model to discharge from four neighboring catchments in the area of Lake Sempach in the Swiss Plateau. They differ significantly with respect to soil composition and hydrological behavior. The higher the percentage of poorly drained soils in a catchment, the more responsive is the discharge behavior. Monte Carlo simulations resulted in 8100 accepted parameter sets yielding satisfactory calibrations during the simultaneous fitting procedure. Under the soil and climate conditions prevailing in the study area saturation-induced flow processes (overland flow, preferential flow to tile drains) dominate the transport of agrochemicals like Phosphorus to surface waters. Conceptually, these processes were represented in the model by a single fast flow component. It originated from all areas of a given soil type with a topographic index above a saturation-dependent critical value. Hence, the model allowed for predictions of soil-type specific delineation of contributing areas that obey the spatial distribution of soil types in different catchments and are consistent with the respective overall hydrological responses.
H31F-1357
The Effect of Anthropogenic Disturbance in the Ecohydrology of Pinyon Juniper Woodlands with Soil Biocrust
The canopy and intercanopy in pinyon-juniper woodlands are intrinsically related and constitute an ecosystem of great importance in the arid lands in the United States. The integrity of this ecosystem is continually challenged by anthropogenic disturbances as oil exploration and recreation activities in these environments. An important feature in the intercanopy of pinyon juniper drylands is the soil biological crust or biocrust. Biocrust is important to the nutrient cycles and hydrology and sediment production of this system. To quantify the effect of disturbances in the soil physical properties in the interspace of a pinion-juniper woodland we measured soil bulk density, water content, and hydraulic conductivity in undisturbed and disturbed areas of sandy soil. The disturbance had minimal impact on the bulk soil properties in the interspace soils. We also analyzed chlorophyll and other pigments related with bacterial activity in top 4mm of the soil and found chlorophyll activity to be almost zero in the disturbed areas while the undisturbed ones showed several orders of magnitude higher concentrations. Subsequent analysis of the soil surface properties indicates that a drastic change in the surface roughness and structure at the surface due to disturbances can alter the microenvironment characteristics critical for the bacterial survival. Since cyanobacteria colonization is the first step in the formation of biocrust, we conclude that surface properties are critical for the preservation and establishment of biocrust in the intercanopy space of the pinyon-juniper woodlands ecosystems.
H31F-1358
Contrasting Particle Clogging in Soils and Granular Media Filters
Deposition of colloidal particles leads to permeability reduction (or clogging) in the soil geomembrane, which reduces fluxes, alters flow patterns, and limits both colloid-associated contaminant transport and delivery of colloidal reactants for purposes of remediation. Comparison of experimental results for soils and granular media filters reveals qualitatively different clogging phenomena with regard to (1) particle stabilization, (2) fluid velocity, and (3) the fractal dimension of particle deposits. These differences have important implications for contaminant hydrology, because the classical approach for understanding particles in natural environments is taken from the filtration literature, which is based on clean granular media. Accordingly, many of the relevant experiments have been performed with granular filters using media such as glass beads or quartz sand. In such filters, clogging is associated with destabilized particles, slower fluid velocity and deposits with smaller fractal dimensions. In contrast, in soils clogging is associated with stabilized particles, faster fluid velocity and deposits with larger fractal dimensions. With regard to these variables, soils are opposite to filters but identical to cake filtration. Numerous examples will be presented from the filtration literature and the soil science literature to illustrate these differing viewpoints. This analysis demonstrates that experiments on clean granular media filters should not be expected to predict particle clogging in soils, sandstones or other natural porous materials containing more than a few percent fines.
H31F-1359
Investigating the Influence of Clay Mineralogy on Stream Bank Erodibility
Soil scientists concerned with erosion of agricultural fields and geotechnical engineers concerned with the mechanical behavior of soils under different conditions have both examined the role of clay mineralogy in controlling soil/sediment properties. Fluvial geomorphologists studying stream channel erosion and stability have focused more on the effects of particle-size distribution, vegetation and rooting. The clay mineralogy of bed and bank sediment has the potential to influence cohesiveness and erodibility, however. The goal of this study is to determine the influence of clay mineralogy on the erodibility of natural stream bank sediment, utilizing techniques drawn from pedology and soil mechanics. Bank samples were collected from eleven sites in small watersheds in central and western Virginia. To obtain sediment containing a range of different clay minerals, watersheds with different types of bedrock were chosen for sampling. Rock types included mafic to felsic metamorphic and igneous rocks, shale, sandstone, and limestone. Where stream bank materials were clearly stratified, different layers were sampled separately. X-ray diffraction of the clay-fraction of the sediment indicates the presence of kaolinite, illite, vermiculite, and mixed-layer clay minerals in various abundances in the different samples. Clay content is 9-46%, as determined by the hydrometer method, and textures range from silty clay and silt loam to clay loam and sandy loam. Organic mater contents range from 1-5% by the loss-on-ignition method. Bulk density of intact sediment samples averages 1.5 g/cc. Liquid limits range from 23-41 with one sample having a value of 65; plasticity indices range from 15-22. While these tests predict that the samples would show a range of mechanical behaviors, the channel morphology at the sampling sites was not strikingly different, all having steep cut banks eroded primarily by scour with no evidence of mass movement and most having a width/depth ratio around 4.5. The ASTM pinhole test for identifying dispersive clay soils is being adapted to measure erodibility of intact and remolded sediment samples in the laboratory to look for more subtle differences in behavior under erosive conditions. Factors such as the extent and method of sample compaction are being taken into account in order to standardize the method.
H31F-1360
Phosphorus Transport at a Wastewater Infiltration Site; Mines Park, CO
Phosphorus (P) from onsite wastewater systems (OWSs) is often considered to be a water resources problem because it contributes to eutrophication of surface-water bodies. In mountainous watersheds, OWSs are commonly used, yet the P transport mechanisms are poorly understood. This study focuses on P transport directly below the OWS infiltrative surface at the Mines Park Test Site, a wastewater infiltration experimental site on the Colorado School of Mines campus. At this site, six soil samples were taken from a two-depth, 30ft long trench. These samples were analyzed by sorption batch tests to determine isotherm constants. The batch test results, combined with effluent composition data from the site, were used to create a 1-D reactive transport model using Geochemist's Workbench. By using the reactive transport model, both chemical precipitation of P compounds and P sorption onto soil were modeled. The resulting P breakthrough in the model was then compared to P concentrations in lysimeter samples from 2ft and 4ft below the infiltrative surface, taken after approximately two years of wastewater infiltration at the site.
H31F-1361
Air Entrapment, Soil Water Retention and Distributed Hydrologic Response in a Semi Arid Catchment
Numerical hydrologic models are beginning to incorporate more sophisticated representations of soil physical properties. A remaining obstacle in the implementation of this approach is that the hydraulic properties of soil do not scale well, limiting the application of pedotransfer functions. To address this knowledge gap, the relationships among soil water inputs, distributed soil state variables and streamflow were investigated for a zero order catchment in a semi-arid system. Soil water potential and soil moisture content measurements for several locations along hillslope transects were made over a hydrologic year. In addition, pedotransfer functions were developed from spatially distributed soil cores at the surface and from various depths. These data are used to support hypotheses of the spatial variability in hydrologic flowpaths in this system and the relationships among soil water retention curves, vertical and lateral hydraulic connectivity in this system. Specifically, we propose the hypothesis that air entrapment is typical for this system and has a dramatic influence on the delivery of snowmelt and rain to streamflow. Phenomenological evidence includes observations of pipe flow near the surface during snowmelt, in the absence of any measured soil saturation above the base of the soil column. Secondly, we propose the hypothesis that hillslope sensor locations demonstrate hydraulic connection to the stream through wetting and drainage timing synchronous with the rising and falling limbs of the hydrograph. These two hypotheses support the concept of hydrograph generation above base flow driven by a pressure wave from hydraulically connected locations, which may vary with volume of entrapped air, but is largely controlled by the distribution of water inputs, soil depth and extent of the macropore drainage network. Testing the posed hypotheses of hydrologic process in the context of pedotransfer functions will allow advances in field hydrology to inform modeling efforts.