Gary W. Parkin

Role of the riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario

Abstract Uncultivated riparian areas can play an important role in reducing nutrient loading to streams in agricultural watersheds. Groundwater flow and geochemistry were monitored in the riparian zone of a small agricultural watershed in southern Ontario. Hydraulic and geochemical measurements were taken along a transect of monitoring wells extending across the riparian area into an agricultural field. Chloride and nitrate concentrations in groundwater samples collected from the agricultural field were much higher than in samples from the riparian area. A sharp decline in both nitrate and chloride concentrations was observed near the field–riparian zone boundary. It appears that increased recharge within the riparian zone, as compared to the artificially drained field, caused nitrate-rich groundwater from the field to be diverted downward beneath the riparian zone, thus limiting the input of agrochemicals to the riparian area and consequently protecting the stream from potential contamination. Geochemical data also indicated that nitrate was attenuated in the downward moving groundwater. Patterns of dissolved oxygen concentrations and redox potential in the subsurface coincided with the pattern defined by groundwater nitrate. These patterns indicated that conditions within the riparian zone and at depth near the field–riparian zone boundary were conducive to denitrification. A linear relation between the δ 15 N and δ 18 O values of nitrate from the monitored transect also supported denitrification as the primary nitrate removal mechanism. This study provides a new conceptual model of how riparian zones may prevent nitrate contamination of streams, and highlights the need for a complete understanding of both groundwater flow and geochemistry in riparian environments.

Measurement of soil water content below a wastewater trench using ground‐penetrating radar

Ground-penetrating radar (GPR) was used to measure the distribution of soil water content below a wastewater trench. Two household septic system trenches were installed in a sandy soil. Vertical time domain reflectometry (TDR) probes were installed through the bottom of one of the trenches. The TDR probes were used to measure soil water content for comparison to the GPR data. The GPR measurements were performed in zero-offset gather (ZOG) and multiple-offset gather (MOG) survey modes with the antennas in horizontal boreholes located on opposite sides and below the wastewater trench. The ZOG survey mode gave an estimate of the average water content in the horizontal plane below the trench at a prescribed interval along the trench. The MOG survey mode produced a tomographic image of the distribution of soil water content in the same horizontal plane below the trench. A similar range of values of water contents was measured by TDR and GPR methods.

The non-invasive characterization of pumping-induced dewatering using ground penetrating radar

Abstract Ground penetrating radar (GPR) profiling is a non-invasive geophysical technique that has been used by Endres et al. [Ground Water 38 (2000) 566] to successfully image pumping-induced drainage in an unconfined aquifer. However, the drained water volume calculated from the GPR data was significantly less than the actual pumped volume. To investigate the reasons for this discrepancy, a seven-day pumping test and five-day recovery test was performed at Canadian Forces Base Borden in Ontario, Canada. A dense spatial coverage of GPR profiles was used to better quantify variations in drainage due to small-scale aquifer heterogeneity. In addition, a neutron moisture content probe was used to directly observe drainage and the soil moisture profile at a sealed well near the pumping well. Neutron logging indicated that the transition zone translated downward during pumping without undergoing significant extension. Comparison of the GPR- and neutron-derived transition zone drawdowns show nearly equal responses. Both of these observations support the hypothesis that the behaviour of the GPR reflection is an accurate measure of the transition zone response. In contrast, transition zone drawdown obtained from both GPR and neutron logging are significantly delayed relative to potentiometric drawdown, resulting in an extended capillary fringe. The drained water volume was determined from the GPR-derived transition zone drawdown data using a number of different approaches. Methods that incorporated information about spatial variations in drainage gave larger estimates of drained water volume; however, those estimates were still lower than the actual pumped volume. The unaccountable volume of water could be a result of several factors—aquifer heterogeneity may still not be adequately represented by the increased GPR coverage, and/or leakage from the underlying aquitard may be providing a significant volume of water.

Unsaturated Hydraulic Conductivity Measured by Time Domain Reflectometry Under a Rainfall Simulator

We used time domain reflectometry (TDR) probes installed vertically at the soil surface beneath a constant-rate rainfall simulator to measure cumulative water storage and the soils unsaturated hydraulic conductivity. The slope from linear regression of water storage on time before any applied water infiltrates to the bottom of the TDR probe gives an estimate of the local infiltration rate. Local infiltration rates measured by TDR in the field were plotted against the corresponding local steady state water contents to give an estimate of the soils unsaturated hydraulic conductivity over a range in water content of 20% using only two applied rainfall rates. The spatial variability in local infiltration rates may be the result of infrequent high-intensity pulses of rainfall leading to temporary ponding and redistribution of water at the soil surface. Nonlinear optimization was used to estimate the saturated hydraulic conductivity and inverse capillary length scale from TDR data.

Using automated soil water content measurements to estimate soil water budgets

The distribution of precipitation into the components of a soil water budget has a profound impact on crop growth, groundwater recharge, soil erosion, and groundwater and surface water contamination levels. The main objectives of this study were to develop a new method of measuring soil water balances and to demonstrate the use of the method in examining differences between partitioning of water in conventional tillage (CT) and no-tillage (NT) management systems. Hourly precipitation, evapotranspiration, and changes in soil water storage data were collected automatically over a 3-yr period at a field site near Elora, Ontario. Runoff and interception were calculated as the difference between measured increases in soil water storage and total rainfall during each significant rain event when the soil was not frozen. Drainage was then calculated, as it was the only component of the soil water balance not measured. The amount of soil water stored in the NT system was greater than the CT system during the latte...

Evaluating the Impact of Assimilating Soil Moisture Variability Data on Latent Heat Flux Estimation in a Land Surface Model

Accurate specification of the soil moisture in land-surface models has the potential to improve the evapotranspiration estimates from these models. However, soil moisture is highly variable in space and time due to variability in climatic, topographic, vegetative, and soil properties. It is anticipated that including information on soil moisture variability into a land-surface model will improve model estimates of evapotranspiration. In this experiment, the spatial variability of soil moisture was measured over an agricultural field in southern Ontario over a 70 m × 70 m area ten times during the growing season. These data were used to update the Canadian Land Surface Scheme (CLASS) using three assimilation techniques. The techniques evaluated included two versions of ensemble Kalman filter (EnKF), and direct insertion of soil moisture data into the model. The results showed that assimilating observed soil moisture variability into CLASS improves model latent heat flux estimates by up to 14%. The amount of improvement depends on the method and timing of assimilation. The effect was largest at the beginning of the growing season, while it was smallest at peak growth. Application of EnKF, considering both instrumental error and field variability, resulted in greater improvement in latent heat flux estimates compared to the other two methods. This study showed that assimilation of soil moisture variability into CLASS can result in greater improvement in modelled ET comparing to assimilating of the mean of the sampling area.

Transformations and losses of swine manure 15N as affected by application timing at two contrasting sites.

An improved understanding of the fate of manure N is necessary for developing efficient manure management plans that ensure adequate crop nutrition and minimum environmental problems. This study quantified the fate of 15N-labelled liquid swine manure applied at three different times (late-fall, spring pre-plant and side-dress) on two soil types (a well-drained fine sandy loam and an imperfectly drained silt loam). Manure N uptake by corn (Zea mays L.) was significantly lower with fall application than with two spring applications (14-18% vs. 30-38% of applied N) in both soil types. Manure application increased total N leaching (30-43 vs. 27 kg N ha-1 yr-1 in the control), especially with fall application. Manure N contributed 18-25% of the total N leached in the fine sandy loam and 8-10% of the total N leached in the silt loam. Application timing did not affect manure N leaching in the silt loam, which ranged between 3 and 5% of applied N. In the fine sandy loam, fall application resulted in significantly...

Borehole GPR measurement of soil water content during an infiltration experiment

Borehole ground penetrating radar (GPR) measurements of water content within the unsaturated zone were performed during infiltration experiments using both a uniform and point source of water at the soil surface. Horizontal GPR antenna access tubes were installed at a depth of 1 m to measure the water content within the horizontal plane underlying the surface water sources. The zero offset profile and multiple offset gather modes of data collection were used to produce velocity profiles and tomograms that were converted to water content. Time domain reflectometry (TDR) probes provided near-surface measurements of water content to complement the deeper borehole GPR data. For the point source, the water content tomograms and profiles showed that the zone of increased water content was restricted to a 1 m diameter zone, centered below the point source. For the uniform source, significant variability in water content within the studied area was observed possible due to preferential flow of water through the soil profile.

Infiltration under constant head and falling head conditions.

Prediction of the infiltration of water into field soils requires knowledge of the field-saturated hydraulic conductivity and a second parameter, such as the matric flux potential (corresponding to field saturation), or the Green and Ampt wetting-front pressure head, or the alpha parameter. Analytical solutions of 1-D infiltration under both constant and falling-head conditions are reviewed and several new solutions are developed based on the Green and Ampt assumptions. A laboratory experiment using the falling head technique is analyzed using several approximate analytical solutions.

Predicting nitrogen fertilizer requirements for corn by chlorophyll meter under different N availability conditions

Nitrogen management strategies that enhance fertilizer use efficiency and maximize profitability in corn require a rapid and accurate method to determine the crop N needs of current hybrids. The objective of this study was to evaluate the potential of a portable chlorophyll meter for predicting N fertilizer requirements for corn grown under varying levels of N availability. Several crop management treatments were imposed in an attempt to create conditions ranging from low N availability (oily food waste application in spring and fall, application at different rates in spring) to high N availability (continuous fertilized corn, winter wheat cover crop). Different corn hybrids were sown at different sites (21 site yr) and varying N fertilizer application rates were applied. Chlorophyll meter readings (CMR) were taken at the 5th to 6th leaf stage (V6) using a SPAD-502 chlorophyll meter. The crop management treatments, corn hybrids and their interaction significantly affected the chlorophyll meter readings. A...