Saltcedar is an invasive shrub that has moved into western United States riparian areas and is continuing to spread. Saltcedar is a phreatophyte that can utilize a saturated water table for moisture once established and is also highly tolerant of saline soil and water conditions. Literature has indicated that depth to the water table and salinity have a significant effect on growth and water use by saltcedar. Several studies were initiated to help develop a simulation model of saltcedar growth and water use based on the EPIC9200 simulation model. A study was initiated at the USDA-ARS Blackland Research Center Temple, Texas in the summer of 2002 to better understand the effects of water table depth and salinity on (1) saltcedar above and below ground biomass, root distribution, leaf area and (2) water use. Five different salinity levels (ranging from 0 ppm to 7500 ppm) and three different water table depths (0.5m, 1.0m, and 1.75m) were studied. Results indicated that increasing depth to the water table decreased saltcedar water use and growth. For the 0.5m water table depth, saltcedar water use during the 2002 growing season averaged 92.7 ml d-1 while the 1.75m depth averaged 56.6 ml d-1. Both root and shoot growth were depressed by increasing water table depth. Salinity had no effect on saltcedar growth or water use except at the 1250 ppm level, which used 110 ml of H2O d-1. This salinity had the highest water use indicating that this may be near the ecological optimum level of salinity for saltcedar. A predictive equation was developed for saltcedar water use using climatic data for that day, the previous day's climatic data, water table depth and salinity that included: previous day total amount of solar radiation, water table depth, previous day average wind speed, salinity, previous day total precipitation, previous day average vapor pressure, minimum relative humidity, previous day average wind direction, and maximum air temperature. Data from the field study and a potential growth study were integrated into the model. The model was parameterized for the Pecos River near Mentone, Texas. Predicted saltcedar water use was slightly lower than results reported by White et al. 2003.

Estimating the quantity of water that reaches the water table following an infiltration event is vital for modeling and management of water resources. Estimating the time scale of groundwater recharge after a rainfall event is difficult because of the dependence on nonlinear soil characteristics and variability in antecedent conditions. Modeling the flow of water through the variably saturated zone is computationally intensive since it requires simulation of Richards' equation, a nonlinear partial differential equation without a closed-form analytical solution, with parametric relationships that are difficult to approximate. Hence, regional scale coupled (surface water - groundwater) hydrological models make simplistic assumptions about the quantity and timing of recharge following infiltration. For simplicity, such models assume the quantity of recharge to be a fraction of the total rainfall and the time to recharge the saturated groundwater is scaled proportionally to the depth to water table, in lieu of simulating computationally intensive flow in the variably saturated zone. In integrated or coupled (surface water - groundwater) regional scale hydrological models, better representation of the timing and quantity of groundwater recharge is required and important for water resources management. This dissertation presents a practical groundwater recharge estimation method and relationships that predict the timing and volume accumulation of groundwater recharge to moderate to deep water table settings. This study combines theoretical, empirical, and simulation techniques to develop a relatively simple model to estimate the propagation of the soil moisture wetting front through variably saturated soil. This model estimates the time scale and progression of recharge following infiltration for a specified depth to water table, saturated hydraulic conductivity and equilibrium moisture condition. High-resolution soil moisture data from a set of experiments conducted in a laboratory soil column were used to calibrate the HYDRUS-1D model. The calibrated model was used to analyze the time scale of recharge by varying soil hydraulic properties and simulating the application of rainfall pulses of varying volume and intensities. Modeling results were used to develop an equation that relates the non-dimensional travel time of the wetting front to excess moisture moisture content above equilibrium. This research indicates that for a soil with a known retention curve, the wetting front arrival time at a given depth can be described by a power law, where the power is a function of the saturated hydraulic conductivity. This equation relates the non-dimensional travel time of the wetting front to excess moisture content above the equilibrium moisture content. Since the equilibrium moisture content is dependent on the water retention curve, the powers in the equation governing the timing of recharge depend on the saturated hydraulic conductivity for a large variation in water retention curve. Also, the power law relates recharge (normalized by applied pulse volume) to time (normalized by the time of arrival of wetting front at that depth). The resulting equations predicted the model simulated normalized (relative) recharge with root mean square errors of less than 14 percent for the tested cases.

This dissertation focuses on a variety of vadose zone processes that impact water table fluctuations. The development of vadose zone process conceptualization has been limited due to both the lack of recognition of the importance of the vadose zone and the absence of suitable field data. Recent studies have, however, shown that vadose zone soil moisture dynamics, especially in shallow water table environments, can have a significant effect on processes such as infiltration, recharge to the water table, and evapotranspiration. This dissertation, hence, attempts to elucidate approaches for modeling vadose zone soil moisture dynamics. The ultimate objective is to predict different vertical and horizontal hydrological fluxes. The first part of the dissertation demonstrates a new methodology using soil moisture and water table data collected along a flow transect. The methodology was found to be successful in the estimation of hydrological fluxes such as evapotranspiration, infiltration, runoff, etc. The observed dataset was also used to verify an exponential model developed to quantify the ground water component of total evapotranspiration. This analysis was followed by a study which analyzed the impact of soil moisture variability in the vadose zone on water table fluctuations. It was found that antecedent soil moisture conditions in the vadose zone greatly affected the specific yield values, causing a broad range of water table fluctuations for similar boundary fluxes. Hence, use of a constant specific yield value can produce inaccurate results. Having gained insight into the process of evapotranspiration and specific yield, a threshold based model to determine evapotranspiration and subsequent water table fluctuation was conceptualized and validated. A discussion of plant root water uptake and its impact on vadose zone soil moisture dynamics is presented in the latter half of this dissertation. A methodology utilizing soil moisture and water table data to determine the root water uptake from different sections of roots is also described. It was found that, unlike traditional empirical root water uptake models, the uptake was not only proportional to the root fraction, but was also dependent on the ambient soil moisture conditions. A modeling framework based on root hydraulic characteristics is provided as well. Lastly, a preliminary analysis of observed data indicated that, under certain field conditions, air entrapment and air pressurization can significantly affect the observed water table values. A modeling technique must be developed to correct such observations.

A new methodology is proposed for estimation of evapotranspiration (ET) flux at small spatial and temporal scales. The method involves simultaneous measurement of soil moisture (SM) profiles and water table heads along transects flow paths. The method has been applied in a shallow water table field site in West-Central Florida for data collected from January 2002 through June 2004. Capacitance shift type moisture sensors were used for this research, placed at variable depth intervals starting at approximately 4 in. (10 cm) below land surface and extending well below the seasonal low water table depth of 59 in. (1.5 m). Vegetation included grassland and wetland forested flatwoods. The approach includes the ability to resolve multiple ET components including shallow and deep vadose zone, surface interception capture and depression storage ET. Other components of the water budget including infiltration, total and saturation rainfall excess runoff, net runoff, changes in storage and lateral groundwater flows are also derived from the approach. One shortcoming of the method is the reliance on open pan or other potential ET estimation techniques when the water table is at or near land surface. Results are compared with values derived for the two vegetative covers from micrometeorological and Bowen ratio methods. Advantages of the SM method include resolving component ET.

Detailed soil and water data are essential to ensure the optimum long-term management of fields. The objective of this study was to estimate water table depths, spatially variable and layered soil properties using electromagnetic induction methods. Soil samples were collected and analyzed within two wild blueberry, a soybean-barley and a pasture fields. Observation wells were installed. The DualEM-2 was calibrated to predict the soil properties and groundwater depths. The apparent ground conductivity (ECa) and water table depths were measured simultaneously from each well, before and after every significant rainfall for three consecutive days. Comprehensive surveys were conducted in selected fields to measure ECa with DualEM-2. Survey data were imported in C++ program to estimate layered soil properties using mathematical models. Regression models were developed to predict soil properties and groundwater depths. The predicted soil properties and groundwater table maps were generated. This information can help to develop variable rate technologies.

I investigated the effects of multi-decadal water table drawdown on carbon (C) exchange, as well as functional relationships between C fluxes and environmental controls, in a boreal rich fen. Drainage increased ecosystem respiration of CO2 and decreased CH4 fluxes, but did not affect understory primary productivity. Drainage altered plant responses to light availability, as well as the responses of ecosystem respiration and CH4 flux to water table position. In a laboratory experiment, I found that drainage reduced potential CO2 and CH4 fluxes suggestive of decreased peat substrate quality. Together, these results indicate that long-term drying increases C losses and that both environmental conditions and substrate quality influenced C fluxes post-drainage. My findings suggest that C losses from increased ecosystem respiration may not be mitigated by increases in plant productivity or decreases in substrate quality, and that long-term drainage reduced C storage capacity in this boreal fen. / The Natural Sciences and Engineering Research Council of Canada

The North American cranberry plant (Vaccinium macrocarpon Aiton) is a wetland crop species grown commercially in natural or constructed peat or sandy soil basins. Since production is highly water dependent, water requirements are very significant and have prompted growers to explore new water management practices to improve irrigation efficiency and protect water resources. One way of conserving and better managing water, given the infrastructure in place, would be to develop sub-irrigation. / The design of a subsurface irrigation system requires the evaluation of various soil properties. This was undertaken at four established cranberry production sites, situated near Saint-Louis-de-Blanford, Quebec. Two of the fields contained a sandy soil, and the other two were peat soils. Soil physical properties measured included: saturated hydraulic conductivity, bulk density, porosity, soil moisture characteristic curves and particle size distribution.

Increasing levels of nitrate (NO3-) in groundwater have become a major environmental and health concern. In situations where NO3-concentrations in the soil-water system pose an environmental hazard, water table management may be a desirable practice to reduce such pollution. Careful management of N applications is also believed to reduce NO3- levels. / A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Quebec, about 30 km South-West of Macdonald Campus of McGill University, to investigate the effect of water table management (WTM) and N fertilizer combinations on potentially leachable NO3- and denitrification rates in the top soil layer (0--0.15 m). The field was planted with monocrop corn (Zea mays. L) in both years. Treatments consisted of a factorial combination of two water table managements, free drainage (FD) and subirrigation (SI) (about 1.0 m and 0.6 m, respectively, below the soil surface) and two N fertilizer rates, 200 kg ha-1 (N200) and 120 kg ha-1 (N120). / Water table management had a significant effect on reducing NO3 - concentrations in the soil profile. Subirrigation treatment reduced NO3- in the top soil layer by 41% and 15% in 1996 and 1997, respectively. Similarly, NO3 - levels were 50% and 20% lower in N120 compared to N200 treatment. / Climatic conditions (rainfall and temperature) played a large role in regulating denitrification rates. Due to drier and cooler conditions in 1997, denitrification rates were lower compared to 1996, leaving more NO3 - in the soil profile. Following harvest, this high NO 3- concentration may be subject to leaching.

A two-year field study was carried out in eastern Ontario to investigate the effects of water table management (WTM) on water quality and crop yields. Corn (Zea mays L.) and soybean (Glycine max Merr.) were planted in alternate strips across the three treatments of 50 cm controlled water table (CWT), 75 cm CWT and free drainage (FD). Drainflow volume and nitrate-N concentration of the drainage water were measured. Soil samples were collected and analysed for total N, P, K, available N, soil moisture and organic matter levels. Chlorophyll-meter readings and plant harvest parameters were also measured. Rainfall, soil and air temperatures were recorded throughout the growing seasons. / The obtained data show that in 1995, the CWT plots significantly increased total drainflow, as compared to FD. In 1996, overall drainflow and nitrate concentrations were significantly reduced. Both the corn and soybean yields were higher with WTM than with FD for both years. (Abstract shortened by UMI.)

A field lysimeter experiment was conducted during 1993 and 1994 using 4 water table depths (WTD) (0.3, 0.6, 0.8, and 1.0 m), 13 treatment levels of calcium (0, 1500, and 2500 kg/ha) and potassium (0, 160, and 400 kg/ha), to determine their effects on tomato quality and yield. Plant parameters measured included: yield (fruit/plant), fruit height, maximum and minimum equatorial width, degree of catfacing (scale of 1 to 5), and sunscald (scale of 0 to 2). / Water table treatment was usually highly significant for the parameters measured at harvest. Largest height, equatorial width and yield of tomato fruit occur with 0.6 to 0.8 m WTD. Fertilizer treatments were rarely significant by WTD, but if they were, they tended to be in the 0.3 or 1.0 m WTD. Maintaining a WTD of 0.6 to 0.8 m and fertilizing with 160 kg/ha of K can improve quality and total yield of tomatoes.