D. G. Chandler

WIND EROSION AND FUGITIVE DUST FLUXES ON AGRICULTURAL LANDS IN THE PACIFIC NORTHWEST

With recent emphasis of agricultural wind erosion and associated dust emissions impacting downwind air quality, there is an increased need for a prediction method to estimate dust emissions and ambient particle concentrations on a wind event basis. Most current wind erosion methods predict average annual or seasonal erosion amounts, and only very approximate estimates of suspended dust emissions are available. A project in the Columbia Plateau of eastern Washington State was initiated to develop an empirical method to estimate dust emissions for this region. Field measurements, wind tunnel tests, and laboratory analyses were combined to provide an empirical wind erosion equation and a related vertical flux dust emission model. While based on measured data, the model has not been independently verified. When combined with a transport-dispersion model and calibrated, estimates of downwind particulate concentrations compared reasonably with those measured.

Runoff responses among common land uses in the uplands of Matalom, Leyte, Philippines

Changing tropical hydrologic regimes have been attributed to conversion from tropical forest to agricultural use and are a threat to many tropical upland ecosystems. A series of experiments were conducted in headwater catchments of Matalom, Leyte, Philippines, to quantify the effect on the near-surface hydrology of land uses common to the steep slopes and thin, calcareous soils. Overland and subsurface runoff were collected to compare the surface hydrologic response of forested, tilled, slash/mulch, and pasture catchments. The forest site demonstrated the lowest annual runoff response, at less than 3% of rainfall, and the highest rainfall threshold which initiated runoff. Conversely, the pasture site demonstrated the greatest annual runoff response (76%) with the lowest thresholds. A pasture with contour-hedgerows demonstrated greater infiltration than the pasture without this conservation practice, generating runoff at 31% of annual rainfall. The plowed and slash/mulch sites had similar annual runoff responses, at 17% and 13% of rainfall, although the predominant flow pathway differed between these sites. Surface runoff accounted for the majority of runoff at the plowed and pasture-fallow sites; whereas, interflow provided the largest contribution to runoff at the slash/mulch and forest sites.

Predicting Wind Erodibility of Loessial Soils in the Pacific Northwest by Particle Sizing

ABSTRACT Wind tunnel tests on silty textured loessial soils throughout the Columbia Plateau of eastern Washington state and northern central Oregon were used to define soil erodibiltiy for wind erosion predictions. Aggregate and primary particle size distributions were determined by common techniques and correlated with the wind tunnel erosion values. Dry aerodynamic particle sizing by a laser method was not useful due to disaggregation caused by the method, although it verified the upper size of sieving cuts. Neither wet dispersed nor aerodynamic techniques provided useful results for discriminating among the wind tunnel erodibility of the tested soils. A combination of standard wire-mesh and sonic sieving of dry aggregates from 30 µm to 2000 µm proved most applicable for correlation to soil mass eroded during wind tunnel field trails. Separate correlations were made for silty soils that are readily suspendible and sandy soils that are largely moved by saltation. Application of these correlations to wind erosion predictions will rely on developing additional relationships with the natural wind erodibility for these same soil classes.

The use of alkalinity as a conservative tracer in a study of near-surface hydrologic change in tropical karst

Abstract Water shortages commonly increase in frequency following forest clearance on lands overlying karst in the tropics. The mechanism underlying this hydrologic change is likely to depend on the land use which follows forest cover. To determine the flow paths which prevail for a progression of land uses common to the uplands of Leyte, Philippines, samples of interflow were collected during the rainy season and titrated to determine their alkalinities. The ratio of the measured alkalinity to the value predicted by equilibrium calculations for each sample was used as an indication of the contact time of the water with the limestone. The responses of the alkalinity saturation ratio and the runoff depth to increasing rainfall depth were used to substantiate the hypothesis that epikarst infilling and changing soil structure create throttles to percolation and infiltration. The forest site was found to generate interflow primarily as pipe flow, with the infiltration and percolation throttles rarely exceeded. Similarly, infiltration was not limiting for the slash/mulch site; however, the level of soil disturbance was adequate to initiate a throttle at the epikarst which increased the volume of interflow generated. The total percolation was similar for the plowed and slash/mulch sites; however, the interflow was decreased at the plowed site by reduced infiltration at the soil surface. The throttles to surface infiltration and epikarst percolation were even greater at the pasture sites, resulting in high runoff generation. However, comparatively greater infiltration was observed in the pasture having contour-hedgerows.

Modeling Interannual Variability in Snow-Cover Development and Melt for a Semiarid Mountain Catchment

Observed changes in midelevation snow cover and duration have raised concerns over future impacts of global warming on snowmelt-dependent water resources and ecosystems. However, predictions of future changes in snow hydrology and water supply from mountain basins are complicated by natural variability in climate and interactions among topography, vegetation structure, wind and radiation energy, and snow deposition. In this study, interannual variability in snow-cover development, snow melt, and runoff is assessed for a range of precipitation and temperature conditions typical of a mountain catchment, the Reynolds Mountain East (RME) basin, in Idaho. A spatially distributed energy and mass balance snow model, Isnobal, coupled with a windfield and snow redistribution model, is used to continuously simulate snow accumulation and melt for five individual snow seasons (1984, 1986, 1987, 2001, and 2006), representing the historic range of climatic variance. The modeling results compare well with the field meas...

Infiltration performance of engineered surfaces commonly used for distributed stormwater management.

Engineered porous media are commonly used in low impact development (LID) structures to mitigate excess stormwater in urban environments. Differences in infiltrability of these LID systems arise from the wide variety of materials used to create porous surfaces and subsequent maintenance, debris loading, and physical damage. In this study, the infiltration capacity of six common materials was tested by multiple replicate experiments with automated mini-disk infiltrometers. The tested materials included porous asphalt, porous concrete, porous brick pavers, flexible porous pavement, engineered soils, and native soils. Porous asphalt, large porous brick pavers, and curb cutout rain gardens showed the greatest infiltration rates. Most engineered porous pavements and soils performed better than the native silt loam soils. Infiltration performance was found to be related more to site design and environmental factors than material choice. Sediment trap zones in both pavements and engineered soil rain gardens were found to be beneficial to the whole site performance. Winter chloride application had a large negative impact on poured in place concrete, making it a poor choice for heavily salted areas.

Distribution of surface imperviousness in small urban catchments predicts runoff peak flows and stream flashiness

Urban growth is a global phenomenon and the associated impacts on hydrology from land development are expected to increase, especially in peri-urban catchments. It is well understood that greater peak flows and higher stream flashiness are associated with increased surface imperviousness and storm location. However, the effect of the distribution of impervious areas on runoff peak flow response and stream flashiness of peri-urban catchments has not been well studied. In this study, a new geometric index, Relative Nearness of Imperviousness to the Catchment Outlet (RNICO), is defined to correlate imperviousness distribution of peri-urban catchments to runoff peak flows and stream flashiness. Study sites include 21 suburban catchments in New York representing a range of drainage area from 5 to 189 km2 and average imperviousness from 10 to 48%. Based on RNICO, all development patterns are divided into three classes: upstream, centralized, and downstream. Results showed an obvious increase in runoff peak flows and decrease in time to peak when moving from upstream to centralized and downstream urbanization classes. This indicates that RNICO is an effective tool for classifying urban development patterns and for macro-scale understanding of the hydrologic behavior of small peri-urban catchments, despite the complexity of urban drainage systems. We also found that the impact of impervious distribution on runoff peak flows and stream flashiness decreases with catchment scale. For small catchments (A 40 km2), the impact of impervious surface distribution on peak flows and stream flashiness was negligible due to the complex drainage network and great variability in travel times. This study emphasizes the need for greater monitoring of discharge in small peri-urban catchments to support flood prediction at the local scale.

A Generalized Additive Soil Depth Model for a Mountainous Semi-Arid Watershed Based Upon Topographic and Land Cover Attributes

Soil depth is an important input parameter in hydrological and ecological modeling. Presently, the soil depth data available in national soil databases (STATSGO, SSURGO) is provided as averages within generalized map units. Spatial uncertainty within these units limits their applicability for spatially distributed modeling. This work reports a statistical model for prediction of soil depth in a semi-arid mountainous watershed that is based upon topographic and other landscape attributes. Soil depth was surveyed by driving a rod into the ground until refusal at geo-referenced locations selected to represent the range of topographic and land cover variations in Dry Creek Experimental Watershed, Boise, Idaho, USA. The soil depth survey consisted of a model calibration set, measured at 819 locations over 8 sub-watersheds, and a model testing set, measured at 130 locations randomly distributed over the remainder of the watershed. Topographic attributes were derived from a Digital Elevation Model. Land cover attributes were derived from Landsat TM remote sensing images and high resolution aerial photographs. A Generalized Additive Model was developed to predict soil depth over the watershed from these attributes. This model explained about 50% of the soil depth spatial variation and is an important improvement towards solving the need in distributed modeling for distributed soil depth input data.

Hydrologic processes that govern stormwater infrastructure behaviour

Using water budget data from published literature, we demonstrate how hydrologic processes govern the function of various stormwater infrastructure technologies. Hydrologic observations are displayed on a Water Budget Triangle, a ternary plot tool developed to visualize simplified water budgets, enabling side-by-side comparison of green and grey approaches to stormwater management. The tool indicates ranges of hydrologic function for green roofs, constructed wetlands, cisterns, bioretention, and other stormwater control management structures. Water budgets are plotted for several example systems to provide insight on structural and environmental design factors, and seasonal variation in hydrologic processes of stormwater management systems. Previously published water budgets and models are used to suggest appropriate operational standards for several green and grey stormwater control structures and compare between conventional and low-impact development approaches. We compare models, characterize and quantify water budgets and expected ranges for green and grey infrastructure systems, and demonstrate how the Water Budget Triangle tool may help users to develop a data-driven approach for understanding design and retrofit of green stormwater infrastructure.

Inference of Soil Hydrologic Parameters from Electronic Soil Moisture Records

Soil moisture is an important control on hydrologic function, as it governs vertical fluxes from and to the atmosphere, groundwater recharge and lateral fluxes through the soil. Historically, the traditional model parameters of saturation, field capacity and permanent wilting point have been determined by laboratory methods. This approach is challenged by issues of scale, boundary conditions and soil disturbance. We develop and compare four methods to determine values of field saturation, field capacity, plant extraction limit and initiation of plant water stress from long term in-situ monitoring records of TDR-measured volumetric water content (. The monitoring sites represent a range of soil textures, soil depths, effective precipitation and plant cover types in a semi-arid climate. The  records exhibit attractors (high frequency values) that correspond to field capacity and the plant extraction limit at both annual and longer time scales, but the field saturation values vary by year depending on seasonal wetness in the semi-arid setting. The analysis for five sites in two watersheds is supported by comparison to values determined by a common pedotransfer function and measured soil characteristic curves. Frozen soil is identified as a complicating factor for the analysis and users are cautioned to filter data by temperature, especially for near surface soils.