A Simple Deterministic Model for Steady Evaporation under Shallow Water Table Conditions

Water Resources - Evaporation from soil surface plays a significant role in water resources management. Evaporation is an important component of hydrological cycle and is needed for most soil,...     

Water Table Depth Estimates over the Contiguous United States Using a Random Forest Model

Water table depth (WTD) has a substantial impact on the connection between groundwater dynamics and land surface processes. Due to the scarcity of WTD observations, physically-based groundwater models are growing in their ability to map WTD at large scales; however, they are still challenged to represent simulated WTD compared to well observations. In this study, we develop a purely data-driven approach to estimating WTD at continental scale. We apply a random forest (RF) model to estimate WTD over most of the contiguous United States (CONUS) based on available WTD observations. The estimated WTD are in good agreement with well observations, with a Pearson correlation coefficient (r) of 0.96 (0.81 during testing), a Nash-Sutcliffe efficiency (NSE) of 0.93 (0.65 during testing), and a root mean square error (RMSE) of 6.87 m (15.31 m during testing). The location of each grid cell is rated as the most important feature in estimating WTD over most of the CONUS, which might be a surrogate for spatial information. In addition, the uncertainty of the RF model is quantified using quantile regression forests. High uncertainties are generally associated with locations having a shallow WTD. Our study demonstrates that the RF model can produce reasonable WTD estimates over most of the CONUS, providing an alternative to physics-based modeling for modeling large-scale freshwater resources. Since the CONUS covers many different hydrologic regimes, the RF model trained for the CONUS may be transferrable to other regions with a similar hydrologic regime and limited observations.     

Temporal Hyporheic Zone Response to Water Table Fluctuations

Expansion and contraction of the hyporheic zone due to temporal hydrologic changes between stream and riparian aquifer influence the biogeochemical cycling capacity of streams. Theoretical studies have quantified the control of groundwater discharge on the depth of the hyporheic zone; however, observations of temporal groundwater controls are limited. In this study, we develop the concept of groundwater‐dominated differential hyporheic zone expansion to explain the temporal control of groundwater discharge on the hyporheic zone in a third‐order stream reach flowing through glacially derived terrain typical of the Great Lakes region. We define groundwater‐dominated differential expansion of the hyporheic zone as: differing rates and magnitudes of hyporheic zone expansion in response to seasonal vs. storm‐related water table fluctuation. Specific conductance and vertical hydraulic gradient measurements were used to map changes in the hyporheic zone during seasonal water table decline and storm events. Planar and riffle beds were monitored in order to distinguish the cause of increasing hyporheic zone depth. Planar bed seasonal expansion of the hyporheic zone was of a greater magnitude and longer in duration (weeks to months) than storm event expansion (hours to days). In contrast, the hyporheic zone beneath the riffle bed exhibited minimal expansion in response to seasonal groundwater decline compared to storm related expansion. Results indicated that fluctuation in the riparian water table controlled seasonal expansion of the hyporheic zone along the planar bed. This groundwater induced hyporheic zone expansion could increase the potential for biogeochemical cycling and natural attenuation.     

Relationships Between Ground-Water Silica, Total Dissolved Solids, and Specific Electrical Conductivity

Specific electrical conductivity (SEC), total dissolved solids (TDS), and silica (SiO₂) are ground-water quality parameters routinely measured in a laboratory. Electrical conductivity measurements are made quickly and are less costly than TDS measurements. Once the relationship between the parameters is determined by regression analysis, TDS can be estimated quickly from the SEC and SiO₂ measurements. Water quality data from 25 city wells in Fresno, California, and historical ground-water quality data from the adjacent San Joaquin River/Kings River alluvial interfan (central San Joaquin Valley, California), the Kaweah River alluvial fan, and the Kern River alluvial fan (southern San Joaquin Valley) were used in this investigation. For the specific hydrologic areas studied, the model's TDS predictive ability is improved when SiO₂ is included with SEC as the independent variables.     

In situ Permeability Measurements Based on a Radial Gas Advection Model: Relationships Between Soil Permeability and Diffuse CO2 Degassing in Volcanic Areas

In this paper we have developed a new method for measuring in situ soil permeability, which is based on the theory of radial gas advection through an isotropic porous medium. The method was tested in the laboratory and at several locations on the island of Vulcano (Aeolian Islands, Italy). It consists of a special device which generates a gas source at a depth of 50 cm and it permits measurement of the relative induced pressure in nearby soil at different depths. The characteristic error of the method was less than 10%. Furthermore, soil permeability measurements were carried out in the island of Vulcano during different periods of the year (between May 2000 and June 2001). A strong decrease in permeability in the upper layers of the soil during and after rainfall was noted, with very poor correlations between the spatial distributions of soil CO₂ flux and shallow soil permeability.     

水—气间的合成化合物分布

Production and application of large quantities of synthetic organic compounds have resulted in a widespread contamination of the water environment. Many synthetic organic compounds are found toxic. Some are mutagenic and carcinogenic even at trace level concentration. The impact of these pollutants has become one of the major issues of environmental concern in recent years.The exchange of organic chemicals across the air-water interface is a significant process affecting the fate of organic compounds released into rivers and lakes. Benzene, toluene, trichloroethylene and tetra-chloroethylene were selected as the model chemicals in this study. These four chemicals have been classified as priority pollutants by the U.S. Environmental Protection Agency and designated as hazardous waste chemicals by the Ontario Ministry of Environment, Canada. The equilibrium distribution of the selected volatile organic compounds between air and water was simulated in the laboratory at the University of Saskatchewan. A gas chromatograph headspace auto sampler system was used to analyze the chemical concentrations. Partition coefficients were determined from the experimental results. The effect of ionic strength on the equilibrium partitioning of selected organic compounds was also investigated in this study. Concentrations of sodium chloride from 100 mol·m^-3 to 1 000 mol·m^-3 were used to simulate various ionic strengths in water at temperature of 20℃. Typical sea-water was simulated with 3.5% sodium chloride and tested at temperatures from 15℃ to 45℃. The salting-out coefficients for the selected organic compounds were determined.     

Predicting Water Table Response to Rainfall Events,Central Florida

A rise in water table in response to a rainfall event is a complex function of permeability, specific yield, antecedent soil‐water conditions, water table level, evapotranspiration, vegetation, lateral groundwater flow, and rainfall volume and intensity. Predictions of water table response, however, commonly assume a linear relationship between response and rainfall based on cumulative analysis of water level and rainfall logs. By identifying individual rainfall events and responses, we examine how the response/rainfall ratio varies as a function of antecedent water table level (stage) and rainfall event size. For wells in wetlands and uplands in central Florida, incorporating stage and event size improves forecasting of water table rise by more than 30%, based on 10 years of data. At the 11 sites studied, the water table is generally least responsive to rainfall at smallest and largest rainfall event sizes and at lower stages. At most sites the minimum amount of rainfall required to induce a rise in water table is fairly uniform when the water table is within 50 to 100 cm of land surface. Below this depth, the minimum typically gradually increases with depth. These observations can be qualitatively explained by unsaturated zone flow processes. Overall, response/rainfall ratios are higher in wetlands and lower in uplands, presumably reflecting lower specific yields and greater lateral influx in wetland sites. Pronounced depth variations in rainfall/response ratios appear to correlate with soil layer boundaries, where corroborating data are available.     

Stratigraphic Trapping of Spilled Jet Fuel Beneath the Water Table

Environmental conditions and the initial attempt to recover JP-4 jet fuel from a shallow aquifer at a tank farm in Hanahan, South Carolina, in 1975. allowed the jet fuel to become stratigraphically trapped below the water table. The trapped jet fuel remained an undetected source of dissolved hydrocarbon contamination in shallow ground water in the area for 17 years. The trapped jet fuel was located when a variety of chemical, hydrologic. geologic, and historical evidence led investigators to install and sample deeper wells. These findings emphasize the need to use an integrated approach lo evaluating the data when determining the extent of contamination and planning fuel recovery operations in a lithologically heterogeneous aquifer.     

The Hierarchy Model of Seismicity:Its Empirical Relationships and Fractal Natures

Starting from a simple hierarchy model of seismicity,some empirical relationships of seismicity and the fractal natures of its time and space and of the source size of earthquakes have been derived in this paper.From the results,some conclusions can be drawn as follows.First,the coefficient b in the Gutenberg-Richter relationship is equal to 2bt where bt is the coefficient in the empirical relationship between the magnitude and average epicentral distance,and b is also equal to the coefficient bi in the empirical relationship between the magnitude and average time interval.Second,the fractal dimension values of the average epicentral distance,the average time interval,and the source size are 2,1,and 26,respectively.