Sustainable Root Zone Salinity and Shallow Water Table in the Context of Land Retirement

This study uses five years of field data from the Land Retirement Demonstration Project located in western Fresno County of California to develop a comprehensive theoretical and numerical modeling framework to evaluate the specific site conditions required for a sustainable land retirement outcome based on natural drainage. Using field data, principles of mass balance in a control volume, the HYDRUS-1D software package for simulating one-dimensional movement of water, heat, and multiple solutes in variably-saturated media, and a model-independent parameter optimizer, the processes of soil water and solute movement in root zone and deep vadose zone were investigated. The optimization of unsaturated soil hydraulic parameters and downward flux (natural drainage) from the control volume against observed vadose zone salinity levels and shallow groundwater levels yield difficult to obtain natural drainage rate as a function of water table height within the control volume. The results show that the unsaturated soil hydraulic properties and the downward flux from the soil profile are the critical parameters. A “natural drainage approach” to sustainable land management for drainage-impaired land is proposed. With this approach it is feasible to design a sustainable land use regimen for drainage-impaired lands in general and retired lands in particular.     

Simulating Root Water Uptake from a Heterogeneous Vegetative Cover

A small body of knowledge exists on the root water uptake (RWU) and evapotranspiration in humid environments having a mixture of natural vegetative cover. In this paper, we assess the impacts of atmospheric conditions and land cover on RWU from a natural vegetative cover. An intensive field investigation was carried out to monitor water table fluctuations along two flow transects comprising an upland grass area and a lowland riparian zone. Calibration and validation of the soil hydraulic parameters using the two-dimensional variably saturated ground water flow model, HYDRUS-2D, confirms the reliability of the model to simulate satisfactorily the large-scale daily fluctuation of RWU. Simulation results revealed that the actual RWU during the wet season is about 40% higher than RWU in the dry season due to high water table levels and temperatures prevailing throughout the wet season. Simulation results using HYDRUS-2D, which was modified to accommodate variable surface boundary conditions and heterogeneous root distribution, showed that the RWU from the riparian zone was 38 and 56% higher than RWU from the pasture land during the dry and wet seasons, respectively.     

Impact of Shallow Groundwater on Evapotranspiration Losses from Uncultivated Land in an Irrigated River Valley

In many agricultural regions of the West, decades of intensive irrigation have produced shallow water tables under not only cultivated fields but also the nearby uncultivated land. It is possible that the high water tables under the uncultivated lands are substantially increasing evapotranspiration (ET) rates, which would represent an unnatural and potentially nonbeneficial consumptive use. The objective of this paper is to quantify loss of water that occurs from uncultivated lands in a semiarid irrigated river valley (the Lower Arkansas River Valley in southeastern Colorado). A remote-sensing algorithm is used to estimate actual ET rates on 16 dates on the basis of Landsat satellite images. On the same dates, water table depths, soil moisture values, and soil water salinities are measured at up to 84 wells distributed across three study sites. On the basis of a water balance of the root zone, it is estimated that 78% of the ET is supplied by groundwater upflux at these sites. It is also observed that the ET and groundwater upflux decrease with increasing water table depth. A regression analysis indicates that the spatial variations in ET are most closely related to variations in vegetation-related attributes, whereas soil moisture and water table depths also explain substantial amounts of the variation. Valley-wide implications for reducing nonbeneficial ET through water table control also are discussed.     

Maximization of Water Storage in Backfilled and Lined Channels and Dimples Subject to Evaporation and Leakage

A channel or axisymmetric dimple filled with a coarse porous material and aimed at a temporary storage of infiltrated water as a perched water table aquifer is studied. The bottom shape is varied based on the criterion of maximal water storage after a certain period of drainage and evaporation. Leakage into the vadose zone through a thin liner occurs with a specific discharge proportional to the pressure drop across the liner. Evaporation through a horizontal shrinking water table is spatially uniform. An ordinary differential equation, which follows from the mass balance condition, is solved either explicitly or numerically. The class of triangular, polynomial, and conical sections is studied. The shape of maximal water retention is calculated for a given initial stored water volume or water table width, evaporation intensity, liner thickness and conductivity, vadose zone pressure beneath the liner, and selected time interval between two sequential infiltration events.     

Water Table Fluctuation in the Presence of a Time-Varying Exponential Recharge and Depth-Dependent ET in a Two-Dimensional Aquifer System with an Inclined Base

An analytical solution is presented for water table fluctuation between ditch drains in presence of exponential recharge and depth-dependent evapotranspiration (ET) from groundwater table in a two-dimensional gently sloping aquifer. The groundwater head above the drain is small compared to the saturated thickness of the aquifer. A sound mathematical transformation is devised to transform the two-dimensional groundwater flow equation into a simple form, which makes possible to obtain an analytical solution. The transient midpoint water table variations from the proposed solution compare well with the already existing solutions for horizontal aquifer. A numerical example is used to illustrate the combined effect of depth-dependent ET coupled with a time-varying exponential recharge on the water table fluctuation. The inclusion of a depth-dependent ET in the solution results in water table decline at a faster rate as compared to the case when ET is not considered. With an increase in slope of the aquifer base, water table profiles become asymmetric and the water table divide shifts towards the lower drain. The height of the water table profiles increases on moving away from the boundary of the aquifer and the highest level of the ground water table is obtained in the central portion of the aquifer basin due to the presence of drainage ditches on the aquifer boundary. When the effect of ET is incorporated in combination with recharge, the analytical solution results in accurate and reliable estimates of water table fluctuations under situations subjected to a number of controlling factors. This study will be useful for alleviation of drainage problems of the aquifers receiving surface recharge and surrounded by streams.     

稳定流与非稳定流情况下非饱和—饱和区域重金属运移模拟

为定量研究稳定流与非稳定流情况下非饱和带—饱和区域内水流及溶质迁移过程,在非饱和带及饱和区域流动理论及溶质运移理论基础上,建立了非饱和带—饱和区域内水流方程与污染物运移模型,采用Van Genuchten-Mualem模型来描述相对渗透率函数,基于Toughreact软件对模型进行模拟计算并给出模拟结果。模拟结果得出:在稳定流情况下,非饱和带及饱和区域流速均逐渐减小,最后趋于稳定;而非稳定流情况下,流速初始阶段逐渐减小,后期随着各季度降水量的不同流速大体呈周期性变化。在总降水量相同及其他条件相同的情况下,非稳定流相对于稳定流对溶质运移存在促进作用,即其运移速率更快,但影响尺度不大;随着时间的推移,污染物将透过包气带进入地下水中,其中钠离子最快,六价铬次之,铀出现最晚。     

Analytical Modeling of Contaminant Transport through Vadose and Saturated Soil Zones

Screening level analyses for risk-based corrective actions and preliminary remediation investigations and feasibility studies for contaminated sites require simple and conservative models for the transport of contaminants to potential receptor locations. A quasi-analytical method is presented to analyze the transport of contaminants originating at a source located near the ground surface separated from the water table by a relatively thick vadose zone. The method includes one segment to compute source decay rate, a second to simulate vertical transport through the vadose zone, and a third to simulate saturated zone transport to the receptor. Practical application of the method is demonstrated by an illustrative example. The method may be useful and convenient for situations where the objectives of the analysis do not warrant the use of sophisticated numerical models.     

SIMGRO, a GIS-Supported Regional Hydrologic Model in Irrigated Areas: Case Study in Mendoza, Argentina

The SIMGRO hydrologic simulation model was extended to include irrigation practice. It could then be used to evaluate the effect of hydrologic changes in an irrigated area in the province of Mendoza, Argentina where, given an average annual rainfall of approximately 200?mm, irrigation is crucial for agriculture. A storage dam was recently constructed in the Mendoza River to control the fluctuating river flow and to guarantee that the demand for water is met throughout the year. The dam will impact on parts of the irrigation system where groundwater levels are already high and salinization occurs. To evaluate these changes and possible mitigation measures, two performance indicators that consider groundwater and surface water were used: Relative evapotranspiration and the depleted fraction. Scenario runs revealed that the irrigation water losses from the canals affect the groundwater levels in the downstream part of the irrigated area; an increase in salinity was also revealed.     

Using Modified Bellani Plate Evapotranspiration Gauges to Estimate Short Canopy Reference Evapotranspiration

Modified Bellani plate atmometer has been offered as an alternative and simpler technique to combination-based equations to estimate evapotranspiration (ET) rate from green grass surface. However, there is a lack of information on its’ accuracy in humid climates. The evaporation rate (EA) from one type of atmometer marketed under the brand name ETgage? (or ETG) with a Number 30 green canvas cover that simulates the ET rate from a green grass surface was tested against the reference ET of a short green grass canopy (ETo) computed using the Food and Agriculture Organization of the United Nations Paper No. 56 Penman–Monteith (FAO56-PM) equation in two sites in north-central Florida. The ETG underestimated the ETo as much as 27%. The root mean square error (RMSE) of daily estimates of EA ranged from 1.03 to 1.15?mm. Data analyses indicated that the most of the poor performances and underestimations of the ETG occurred on rainy days. Using only the nonrainy day EA versus ETo relationship, the daily RMSE was as low as 0.47?mm and r2 was as high as 0.89, and the underestimations were within 3% of the ETo. Averaging daily ETG readings over 3 and 7 day periods considerably improved (lower RMSE and percent error, %E, and higher r2) ETo estimates. The ETG performed quite well on nonrainy days. The adjustment factors were developed and tabulated as a function of rainfall amount to adjust the EA values on rainy days. Results showed that an average adjustment factor of 0.84?(EA/0.84 = ETo) can be used as a practical number if rainfall observations are not available. The underestimations of the ETG on rainy days were attributed, in part, to the wetting of the green canvas cover due to the rainwater accumulations on it and to the lower diffusivity (higher resistance) value of the canvas cover (112–294?s?m?1) compared to the diffusivity of a green grass surface used in the ETo definition (70?s?m?1). Although it is found that the ETG is feasible and practical device, the EA values measured on rainy days require careful interpretation in humid and rainy climates such as Florida. The rainy day EA values should be used cautiously with the proper regression equation and adjustment factors to estimate ETo for irrigation scheduling if the input variables are not available to use the FAO56-PM equation for ETo estimates.     

Case Study of a Full-Scale Evapotranspiration Cover

The design, construction, and performance analyses of a 6.1?ha evapotranspiration (ET) landfill cover at the semiarid U.S. Army Fort Carson site, near Colorado Springs, Colo. are presented. Initial water-balance model simulations, using literature reported soil hydraulic data, aided selection of borrow-source soil type(s) that resulted in predictions of negligible annual drainage ( ? 1?mm/year). Final construction design was based on refined water-balance simulations using laboratory determined soil hydraulic values from borrow area natural soil horizons that were described with USDA soil classification methods. Cover design components included a 122?cm thick clay loam (USDA), compaction ? 80% of the standard Proctor maximum dry density (dry bulk density ～ 1.3?Mg/m3), erosion control measures, top soil amended with biosolids, and seeding with native grasses. Favorable hydrologic performance for a 5?year period was documented by lysimeter-measured and Richards’-based calculations of annual drainage that were all <0.4?mm/year. Water potential data suggest that ET removed water that infiltrated the cover and contributed to a persistent driving force for upward flow and removal of water from below the base of the cover.     

Effects of Stormwater Infiltration on Quality of Groundwater Beneath Retention and Detention Basins

Infiltration of storm water through detention and retention basins may increase the risk of groundwater contamination, especially in areas where the soil is sandy and the water table shallow, and contaminants may not have a chance to degrade or sorb onto soil particles before reaching the saturated zone. Groundwater from 16 monitoring wells installed in basins in southern New Jersey was compared to the quality of shallow groundwater from 30 wells in areas of new-urban land use. Basin groundwater contained much lower levels of dissolved oxygen, which affected concentrations of major ions. Patterns of volatile organic compound and pesticide occurrence in basin groundwater reflected the land use in the drainage areas served by the basins, and differed from patterns in background samples, exhibiting a greater occurrence of petroleum hydrocarbons and certain pesticides. Dilution effects and volatilization likely decrease the concentration and detection frequency of certain compounds commonly found in background groundwater. High recharge rates in storm water basins may cause loading factors to be substantial even when constituent concentrations in infiltrating storm water are relatively low.     

Modeling BMPs to Optimize Municipal Wastewater Land Treatment System

Nutrient simulations of a municipal wastewater land treatment system were performed using the ground-water loading effects of agricultural management systems model. A total of 33 best management practices (BMPs) were simulated and compared to a baseline simulation of the current reed canary grass management practice. BMPs consisted of various combinations of crop types and cropping practices. Effectiveness of BMPs was compared based on the amount of nitrogen leached through the root zone and the harvested market value of each crop. Based on the model predictions and local market conditions, recommendations for the facility were given as the following: reed canary grass cut four times per year, orchard grass cut three or four times per year, or silage corn with a winter cover crop under low wastewater loading. Overloading the reed canary grass and orchard grass, especially for short periods of time, was simulated to have no adverse effects on the ground water. The ground-water loading effects of agricultural management systems model was demonstrated as a useful tool for the optimization of a municipal wastewater land treatment facility.     

Simple Expert System for Evaluation of Nutrient Pollution Potential in Groundwater from Manure Application

An expert system for the evaluation of groundwater pollution potential due to leaching of nutrients from land application of manure is developed in this study. The expert system consists of five modules: surface loading, sorption, vadose zone transport, saturated zone transport, and final module. The expert system evaluates the pollution potential in two steps. In the first step, rating values are evaluated for the selected modules. A set of expert system rules is assigned by the writers to evaluate the rating value of each selected module. In the second step, the pollution potential is evaluated from calculating the weighted average rating of the selected modules. Finally, the expert system is validated against a site assessment done by a group of experts for a concentrated animal feeding operation (CAFO) facility in Oklahoma. The advantages of the expert system are its user-friendliness, requirement of easily available input data, and faster evaluation time. The expert system is recommended to the farmers for improving their management practices and for selecting sites suitable for building new CAFOs. It is also recommended to the regulatory agencies as a screening level tool to identify the most vulnerable sites.     

Approach Using Active Groundwater Storage for Hydrologic Model Calibration in West-Central Florida

Hydrologic model calibration is always a challenging and tedious process especially for the calibration of complex models, which includes continuous hydrograph models, requires sophisticated calibration methods. The Hydrologic Simulation Program-FORTRAN (HSPF) is one of the popular and powerful time variable hydrologic models. However, in order to improve the assessment of hydrologic activities in shallow ground water settings, the model needs to be reliably calibrated for ground water contribution. Little guidance is provided in the literature concerning the manner of this contribution. In fact, the most common calibration of HSPF uses subjective parameter fitting and focuses on the attainment of statistical goodness of fit of runoff fluxes and water levels, ignoring ground water components. The goal of this research is using a different approach to calibrate HSPF with observed water table records. In this study, HSPF is applied on a small area in west-central Florida and calibrated by comparing active ground water storage to well elevation records in range land and forested land covers. The Nash-Sutcliffe efficiency and correlation coefficient computed using observed and simulated daily flows are 0.91 and 0.96 at Peace River, respectively, also with good fair results for other stations in the model domain. The study shows that improved calibration of the model can be achieved if active ground water storage and well records are compared for timing and magnitude of fluctuations.     

Reference and Crop Evapotranspiration in South Central Nebraska. II: Measurement and Estimation of Actual Evapotranspiration for Corn

In planning, designing, and managing of surface and groundwater supply, it is essential to accurately quantify actual evapotranspiration (ETc) from various vegetation surfaces within the water supply areas to allow water management agencies to manipulate the land use pattern alternatives and scenarios to achieve a desired balance between water supply and demand. However, significant differences among water regulatory agencies and water users exist in terms of methods used to quantify ETc. It is essential to know the potential differences associated with using various empirical equations in quantifying ETc as compared with the measurements of this critical variable. We quantified and analyzed the differences associated with using 15 grass (ETo) and alfalfa-reference (ETr) combination, temperature and radiation-based reference ET (ETref) equations in quantifying grass-reference actual ET (ETco) and alfalfa-reference actual ET (ETcr) as compared with the Bowen ratio energy balance system (BREBS)-measured ETc (ETc-BREBS) for field corn (Zea mays L.). We analyzed the performance of the equations for their full season, irrigation season, peak ET month, and seasonal cumulative ETc estimates on a daily time step for 2005 and 2006. The step-wise Kc values instead of smoothed curves were used in the ETc calculations. The seasonal ETc-BREBS was measured as 572 and 561?mm in 2005 and 2006, respectively. The root-means-quare difference (RMSD) was higher for the full season than the irrigation season and peak ET month estimates for all equations. The standardized ASCE Penman-Monteith (PM) ETco had a RMSD of 1.37?mm?d?1 for the full growing season, 1.05?mm?d?1 for the irrigation season, and 0.76?mm?d?1 for the peak month ET. The ASCE-PM, 1963 and 1948 Penman ETc estimates were closest to the ETc-BREBS. The FAO-24 radiation and the HPRCC Penman ETc estimates also agreed well with the ETc-BREBS. Most combination equations performed best during the peak ET month except the temperature and radiation-based equations. There was an excellent correlation between the ASCE-PM ETco and ETcr with a high r2 of 0.99 and a low RMSD of 0.34?mm?d?1. The difference between the ETcr and ETco was found to be larger at the high ETc range (i.e., >8?mm), but overall, the ETcr and ETco values were within 3%. Significant differences were found between the cumulative ETco-METHOD and ETcr-METHOD versus ETc-BREBS. Most combination equations, including the standardized ASCE-PM ETco and ETcr underestimated ETc-BREBS during the early periods of the growing season where the soil evaporation was the dominant energy flux of the energy balance and in the late season near and after physiological maturity when the transpiration rates were less than the midseason. The underestimations early in the season can be attributed to the lack of ability of the physical structure of the ETref×crop coefficient approach to “fully” account for the soil surface conditions when complete canopy cover is not present. The results of this study can be used as a reference tool by the water resources regulatory agencies and water users and can provide practical information on which method to select based on the data availability for reliable estimates of daily ETc for corn.     

Impacts of Unsaturated Zone Soil Moisture and Groundwater Table on Slope Instability

The combined effect of soil moisture in unsaturated soil layers and pore-water pressure in saturated soil layers is critical to predict landslides. An improved infinite slope stability model, that directly includes unsaturated zone soil moisture and groundwater, is derived and used to analyze the factor of safety’s sensitivity to unsaturated zone soil moisture. This sensitivity, the change in the factor of safety with respect to variable unsaturated zone soil moisture, was studied at local and regional scales using an active landslide region as a case study. Factors of safety have the greatest sensitivity to unsaturated zone soil moisture dynamics for shallow soil layers (<2?m) and comparatively deep groundwater tables (1 m). For an identical groundwater table, the factor of safety for a 1 m thick soil mantle was four times more sensitive to soil moisture changes than a 3-m thick soil. At a regional scale, the number of unstable areas increases nonlinearly with increasing unsaturated zone soil moisture and with moderately wet slopes exhibiting the greatest sensitivity.     

Soil Salinity Modeling over Shallow Water Tables. II: Application of LEACHC

Root zone salinity is one of the major factors adversely affecting crop production. A saline shallow water table can contribute significantly to salinity increases in the root zone. A soil salinity model (LEACHC) was used to simulate the effects of various management alternatives and initial conditions on root zone salinity, given a consistently high water table. The impact of water table salinity levels, irrigation management strategies, soil types, and crop types on the accumulation of salts in the root zone and on crop yields was evaluated. There were clear differences in soil salinity accumulations depending upon the depth and salinity of the water table. In general, increasing water table depth reduced average soil profile salinity, as did having lower salinity in the water table. Among the four irrigation strategies that were compared, the 14-day irrigation interval with replenishment of 75% of evapotranspiration (ET) resulted in the lowest soil salinity. With a 4-day interval and 50% ET replenishment, a wheat yield reduction of nearly 40% was predicted after three years of salt accumulation. Soil type and crop type had minimal or no impact on soil salinity accumulation. Under all conditions, soil water average electrical conductivity increased during the 3-year simulation period. This trend continued when the simulation period was extended to 6 years. Under the conditions shown to develop the highest average soil salinity (high water table, low irrigation), an annual presowing irrigation of 125 mm caused a nearly 50% reduction in soil salinity at the end of the 6-year simulation period, as compared with the soil salinity given no presowing irrigation.     

Simulating the Effect of Sulfate Addition on Methylmercury Output from a Wetland

The watershed analysis risk management framework (WARMF) model was applied to Wetland S6 of the Marcell Experimental Forest, using the data from a field experiment, conducted to investigate the effect of sulfate additions on mercury methylation in the wetland. The wetland was modeled as interconnected land catchments. Actual meteorology data and mercury and sulfate concentrations of precipitation were input to the model. To simulate the sulfate sprinkling, the experimental section of the bog was irrigated with sulfate water on the actual dates of sulfate additions. The model simulated wetland outflows that matched the measured outflows with an R-square of 0.856. WARMF also simulated other phenomena observed in the experiment: higher sulfate and MeHg levels at the wetland outlet after every sulfate addition, and higher sulfate and MeHg levels in the pore water of the bog after only the May addition, not the July and September additions. According to WARMF, the low groundwater table in May allowed the sprinkled sulfate to percolate to the soil stratum 10–30 cm below the ground level of the bog, where the pore water was sampled. In July and September, the sulfate could not reach that zone because the percolation was blocked by high groundwater tables. The sampled soil stratum was not the site of methylation that contributed MeHg to the wetland outlet. The saturated zone of the top 10 cm of bog was the site that produced MeHg, which was flushed to the outlet after all sulfate additions. WARMF predicted that quadrupling the sulfate deposition would increase the MeHg output by 216%, which might become lower with more data and better model calibration.     

Contaminated Vadose Zone Characterization Using Partitioning Gas Tracers

This paper describes laboratory research conducted to investigate the performance of partitioning tracers for the detection of nonaqueous-phase liquids (NAPLs) in vadose zones. Once evaluated, the tracers may be used for volume estimation of NAPLs and remediation performance assessment of vadose zones. These laboratory studies used glass chromatography columns packed with: (1) Ottawa sand; and then (2) in a separate experiment, soil extracted from the Chemical Waste Landfill at Sandia National Laboratories. The columns were prepared in a manner that resulted in a three-phase system of air, water, and NAPL in the columns. Conservative and partitioning gas tracers were injected into the columns, and their elution concentrations were analyzed. The method of moments was used to estimate partition coefficients between the air and NAPL phases for each of the tracers. The partition coefficients and retardation factors, also estimated during the study, are used to select appropriate tracers for NAPL detection. This research identified several suitable perfluorocarbon tracers and demonstrated the feasibility of using partitioning tracers as a tool for NAPL detection in the vadose zone.