Mean Water Table Research Articles

Recording relative water table depth using PVC tape discolouration: Advantages and constraints in fens

Abstract Question: Recognizing water table dynamics in wetlands is crucial for understanding species-environment relationships, ecosystem function and changes during restoration. The PVC tape discolouration method enables spatially and temporally extensive studies of reductive conditions associated with long-term water table dynamics in peat soils. The reliability of the method has been verified only for ombrotrophic bogs, even though wide usage can be expected in minerotrophic fens. Location: Třeboň basin, Czech Republic. Methods: Using data from 49 plots in six poor and moderately rich fens, we correlated the directly measured lowest, highest and mean water table depths and the same variables indicated by discolouration of PVC tape attached to green bamboo stakes installed vertically in the soil profile. Results: The depth to the first sign of PVC discolouration was highly correlated with the directly measured position of the highest water table, the correlation between the depth of complete discolourat...

Plant biomass and species composition along an environmental gradient in montane riparian meadows.

In riparian meadows, narrow zonation of the dominant vegetation frequently occurs along the elevational gradient from the stream edge to the floodplain terrace. We measured plant species composition and above- and belowground biomass in three riparian plant communities-a priori defined as wet, moist, and dry meadow-along short streamside topographic gradients in two montane meadows in northeast Oregon. The objectives were to: (1). compare above- and belowground biomass in the three meadow communities; (2). examine relations among plant species richness, biomass distribution, water table depth, and soil redox potential along the streamside elevational gradients. We installed wells and platinum electrodes along transects (perpendicular to the stream; n=5 per site) through the three plant communities, and monitored water table depth and soil redox potential (10 and 25 cm depth) from July 1997 to August 1999. Mean water table depth and soil redox potential differed significantly along the transects, and characterized a strong environmental gradient. Community differences in plant species composition were reflected in biomass distribution. Highest total biomass (live+dead) occurred in the sedge-dominated wet meadows (4311+/-289 g/m(2)), intermediate biomass (2236+/-221 g/m(2)) was seen in the moist meadow communities, dominated by grasses and sedges, and lowest biomass (1403+/-113 g/m(2)) was observed in the more diverse dry meadows, dominated by grasses and forbs. In the wet and moist communities, belowground biomass (live+dead) comprised 68-81% of the totals. Rhizome-to-root ratios and distinctive vertical profiles of belowground biomass reflected characteristics of the dominant graminoid species within each community. Total biomass was positively correlated with mean water table depth, and negatively correlated with mean redox potential (10 cm and 25 cm depths; P <0.01) and species richness ( P <0.05), indicating that the distribution of biomass coincided with the streamside edaphic gradient in these riparian meadows.

The short-term impact of managed realignment on soil environmental variables and hydrology

In July 2000 a flood defence embankment protecting abandoned agricultural land in the estuary of the River Torridge, Devon, UK, was breached as part of a managed realignment (MR) project. The objective was to restore the agricultural land, reclaimed from saltmarsh some 200 years previously, to its former habitat. Changes to the soil hydrological regime and alteration of the soil environment at the site were studied. The most significant observed impact at the MR site was the change in flooding regime, with regular tidal inundation occurring to a maximum depth of 52 cm during spring tides. Prior to the realignment, soil water table fluctuations were linked to patterns of spring and neap tides. Post-realignment, a change in mean water table height of more than 50 cm was observed at the MR site, and soil redox potential at 5 cm depth was reduced by over 700 mV immediately following reflooding, changing the soil environment from an oxidised to a reduced environment. Topsoil water (collected from 10 cm depth) demonstrated large, short-term reductions in pH. Prior to realignment topsoil water pH ranged from 6.6 to 8.7, but following realignment remained below 5 for approximately nine weeks, reaching a minimum of 3.3. Short-term changes in conductivity and NH 4 + concentrations in topsoil water also occurred, conductivity rising from <2000 μS to >40,000 μS following realignment, while NH 4-N concentrations rose from 0.10 mg l −1 to 10.05 mg l −1. Cotton tensile strength loss (CTSL) reflected these changes, exhibiting large decreases in decomposition rates at 5–10 cm depth immediately following the realignment. These results have implications for the management of MR projects, and for the health and quality of the estuary in general.

Integrating groundwater dynamics in regional hydrological modelling

The paper presents an integrated catchment model and a method with which it is possible to analyse local water table dynamics inside subbasins along with river flow on the regional scale. A simple but comprehensive mechanistic groundwater module coupled with the eco-hydrological model SWIM (Soil and Water Integrated Model), which integrates hydrological processes, vegetation, erosion and nutrient dynamics at the watershed scale, was used in the study. The reliability of the model results was tested under well defined boundary conditions by comparing the results with those from a two dimensional numeric groundwater model under steady-state and transient conditions as well as with observed data for two meso-scale basins, using contour maps of the long term mean water table, observed groundwater level data in wells and observed river discharge. Especially in lowland catchments, where the water table is relatively shallow, the dynamics of river discharge are mainly influenced by changes in groundwater contribution to river flow. However, a correct reproduction of river discharge by hydrological models does not guarantee the adequacy of simulated spatio-temporal dynamics of soil moisture, water fluxes and groundwater in the basin. But even though the primary purpose of distributed hydrological models is to reproduce river discharge and water fluxes in the entire catchment, they are often validated using only the observed river discharge at the basin outlet for comparisons. The additional use of groundwater observations for model validation can serve as a measure to overcome the problem. The study area is located in the lowland part of the Elbe river basin, which is representative for semi-humid landscapes in Europe, where water availability during the summer season is the main limiting factor for plant growth and crop yields. The importance of adequate reproduction of the groundwater dynamics is illustrated in an investigation of a decreasing trend in regional groundwater level.

Water table waves in an unconfined aquifer: Experiments and modeling

Comprehensive measurements are presented of the piezometric head in an unconfined aquifer during steady, simple harmonic oscillations driven by a hydrostatic clear water reservoir through a vertical interface. The results are analyzed and used to test existing hydrostatic and nonhydrostatic, small‐amplitude theories along with capillary fringe effects. As expected, the amplitude of the water table wave decays exponentially. However, the decay rates and phase lags indicate the influence of both vertical flow and capillary effects. The capillary effects are reconciled with observations of water table oscillations in a sand column with the same sand. The effects of vertical flows and the corresponding nonhydrostatic pressure are reasonably well described by small‐amplitude theory for water table waves in finite depth aquifers. That includes the oscillation amplitudes being greater at the bottom than at the top and the phase lead of the bottom compared with the top. The main problems with respect to interpreting the measurements through existing theory relate to the complicated boundary condition at the interface between the driving head reservoir and the aquifer. That is, the small‐amplitude, finite depth expansion solution, which matches a hydrostatic boundary condition between the bottom and the mean driving head level, is unrealistic with respect to the pressure variation above this level. Hence it cannot describe the finer details of the multiple mode behavior close to the driving head boundary. The mean water table height initially increases with distance from the forcing boundary but then decreases again, and its asymptotic value is considerably smaller than that previously predicted for finite depth aquifers without capillary effects. Just as the mean water table over‐height is smaller than predicted by capillarity‐free shallow aquifer models, so is the amplitude of the second harmonic. In fact, there is no indication of extra second harmonics (in addition to that contained in the driving head) being generated at the interface or in the interior.

The Sensitivity of Global Climate Model Simulations to the Representation of Soil Moisture Heterogeneity

Improving the treatment of subgrid-scale soil moisture variations is recognized as a priority for the next generation of land surface schemes. Here, the impact of an improved representation of subgrid-scale soil moisture heterogeneity on global climate model (GCM) simulations of current and future climates is carried out using Version three of the Hadley Centre Atmospheric Climate Model (HadAM3) coupled to the Met Office Surface Exchange Scheme (MOSES). MOSES was adapted to make use of the rainfall runoff model TOPMODEL algorithms, which relate the local water table depth to the grid box mean water table depth, assuming that subgrid-scale topography is the primary cause of soil moisture heterogeneity. This approach was also applied to produce a novel model for wetland area, which can ultimately be used to interactively model methane emissions from wetlands. The modified scheme was validated offline by forcing with near-surface Global Soil Wetness Project (GSWP) data, and online within the HadAM3 global climate model. In both cases it was found to improve the present-day simulation of runoff and produce realistic distributions of global wetland area. (Precipitation was also improved in the online simulation.) The new scheme results in substantial differences in the modeled sensitivity of runoff to climate change, with implications for the modeling of hydrological impacts.

In situ long‐term redox potential measurements in a dyked marsh soil

AbstractThe long‐term measurement of soil redox potential (EH) by permanently installed Pt electrodes may be restricted by electrode breakdown (electrode rupture and resin leakage) and contamination, especially under wet and strongly reducing soil conditions. The EH of a slightly alkaline (pH 7.1 to 7.3) Calcaric Gleysol developed from marine sediment in the dyked marsh of Schleswig‐Holstein, Northern Germany, was monitored weekly during a 4‐year period using permanently installed Pt electrodes. Measurements were performed in fivefold at 10, 30, 60, 100, and 150 cm. Furthermore, water table level was recorded. Sulfide occurred in 150 cm as a heritage of the previous marine environment. Mean water table level was 84 cm below the soil surface but was characterized by both short‐term and seasonally strong fluctuations. Levels of water table ranged from 33 to &gt;200 cm below soil surface. In consistence with water table level, the EH continually decreased with soil depth. Mean redox conditions were oxidizing at 10 (550 mV) and 30 cm (430 mV), weakly reducing at 60 cm (230 mV), and moderately reducing at 100 (120 mV) and 150 cm depth (–80 mV). Soil hydrology differed markedly during the study as expressed by periods of water saturation for each depth. This was reflected by Pt electrodes response, since period of water saturation and EH were significantly negatively correlated as calculated for each year and depth (rs = –0.971; n = 20; P &lt; 0.01). The 60‐cm depth was most frequently influenced by water table fluctuations, showed the largest EH range (920 mV) and the most distinct seasonal pattern in EH. Good function of the electrodes in this depth can be concluded even after long time of installation in soil. Although established in a sulfide‐bearing environment, three of five electrodes at 150 cm showed a substantial increase (+500 mV) in EH during summer of the third and fourth years of investigation, which had low water tables. It is not clear whether the non‐response of two electrodes was due to electrode contamination or spatial variation in EH. When operating in reducing systems, this problem can be circumvented by installing a large number of electrodes or by a regular replacement of electrodes. Using properly constructed and permanently installed Pt electrodes, soil EH can be monitored for extended periods under wet and reducing soil conditions.

Methane emissions from natural peatlands in the northern boreal zone in Finland, Fennoscandia

Methane (CH 4) emissions were measured over two snow-free seasons from ten natural minerotrophic peatlands in the northern boreal zone of Finland using a static chamber technique. The daily CH 4 fluxes at the sites ranged from −2.9 to 790 mg m −2 d −1. At nine wet fens, representing three ecological peatland classes, the seasonal mean CH 4 emissions were 8.1–250 mg m −2 d −1 during a dry summer, and 15–330 mg m −2 d −1 during a wet summer. At these sites, the mean CH 4 emission correlated positively with the mean water table level ( r 2=0.64). The driest site (herb-grass spruce mire) showed the lowest CH 4 emissions, with the seasonal averages being below 1 mg m −2 d −1. The CH 4 emissions from these natural northern boreal peatlands in the main aapa-mire zone were similar or higher than those reported from other boreal peatlands in Finland and elsewhere, probably attributable to the high fertility and moisture of the northern boreal sites, characteristics typical of peatlands in the aapa-mire zone. Thus, the aapa mires in the boreal zone in Fennoscandia, Canada and Russia should be a larger natural source for atmospheric CH 4 than estimated in the global inventories for wetland CH 4 emissions.

Influence of the tide on the mean watertable in an unconfined, anisotropic, inhomogeneous coastal aquifer

For an isotropic, homogeneous coastal aquifer, previous studies [Aust J Phys 26 (1973) 513; Water Resour Res 17 (4) (1981) 1222] have found that the mean watertable is influenced by the sea tide and will stand considerably above the mean sea level even in the absence of net inland recharge. This paper investigates the influence of the tide on the mean watertable for the case that the unconfined coastal aquifer is inhomogeneous and anisotropic. A two-dimensional free surface model is considered under the following assumptions: (a) the principle directions of the hydraulic conductivities are horizontal and vertical; (b) the horizontal hydraulic conductivity K x ( y) depends on the depth of the aquifer only and the vertical hydraulic conductivity K y ( x, y) is arbitrary; (c) the water–land boundary is vertical and there is no seepage face; (d) the specific storage is constant wherever the free surfaces are located; and (e) there is no net inland recharge. An integral equation satisfied by the asymptotic watertable as the landward distance approaches infinity is derived for multi-sinusoidal-component sea tide. This equation suggests that the asymptotic watertable is independent of K y ( x, y) and higher than mean sea level for arbitrary K x ( y). The asymptotic watertable is solved explicitly from the integral equation for three common patterns of K x ( y): constant, piecewise constant and linearly decreasing with the depth. Compared with the first two patterns, the third pattern has significant enhancing effect on the asymptotic watertable. Several previously published analytical solutions and the experiment data are in line with the analytical solution of this paper.

A 7500-year peat-based palaeoclimatic reconstruction and evidence for an 1100-year cyclicity in bog surface wetness from Temple Hill Moss, Pentland Hills, southeast Scotland

Analyses of plant macrofossils, peat humification and testate amoebae were used to reconstruct a proxy climate record spanning the last 7500 years from an ombrotrophic bog, Temple Hill Moss, in southeast Scotland. The plant macrofossil data were subjected to detrended correspondence analysis (DCA) which modelled effectively the significant wet shifts within the record. A mean water table depth transfer function was applied to the testate amoebae data to provide quantifiable changes. The three proxy records show coherent phase changes which are interpreted as variability in past effective precipitation. Two tephra horizons (Glen Garry and Lairg A) were used in conjunction with radiocarbon dates to construct an age/depth model, producing a robust geochronology from which a time series was calculated. The palaeoclimatic reconstruction identified major wet shifts throughout the Holocene, with specific events occurring around cal. 6650, 5850, 5300, 4500, 3850, 3400, 2800–2450, 1450–1350 and 250–150 BP. Spectral analysis of the plant macrofossil DCA and colorimetric humification data produced a millennial scale periodicity of 1100 years. The same periodicity has also been found in a palaeoclimatic reconstruction from a site in Cumbria (Walton Moss), and may be linked with millennial scale periodicities found in oceanic palaeoclimatic records.

Effect of climatic variability from 1980 to 1997 on simulated methane emission from a boreal mixed mire in northern Sweden

The objective of this study was to evaluate the interannual variation during 1981–1997 in methane emission from an oligotrophic lawn plant community in a boreal mire. Daily mean air temperatures and daily accumulated precipitation figures were used as forcing variables for a mechanistic soil physical model, to generate estimates for the depths of snow, soil frost, and water table, together with soil temperature profiles. Methane emissions were then simulated with a mechanistic mixed mire methane model, described in this paper. The methane model simulated total methane emission rates separated into diffusion, ebullition, and plant‐mediated transport. The climate record for 1981–1997 represented almost the total range in growing season temperature sums in northern Sweden during the twentieth century. The average temperature sum for 1980–1997 was 735 (range 553–981) degree days. The average accumulated annual precipitation during the same period was 283 (range 140–397) mm. The integrated simulated annual methane emission during 1981–1997 varied almost threefold among years, with a mean of 17.4 ±1.1 (SE) and a range of 11 to 27 g m−2. The simulated annual emissions for the three calibration years 1995–1997 were 18, 14, and 22 g m−2, respectively, slightly higher than the integrated measured emissions over the growing season (May‐September), which were 16, 13, and 18 g m−2 y−1, respectively. Given the model formulation and parameterization, the single most important climatic predictor of simulated annual methane emission was mean water table position (r2 = 0.58). Adding annual soil temperature sum at 26 cm in a multiple regression solution significantly increased the explained variance (R2 = 0.85). The study emphasizes that interannual variability in methane emission may be large, and therefore extrapolations of annual methane emissions must be based on time series that adequately span the interannual variability in the local climate.

Soils and Hydrology of a Wet‐Sandy Catena in East‐Central Minnesota

Soil properties are strongly related to the retention and movement of water within the soil system. The purposes of this study were to document the near‐surface hydrology of a wetland–upland hillslope on a sandy glacial outwash plain in east‐central Minnesota and to describe the patterns of soil morphology with respect to observed hydrology. Water levels, soil temperature, soil‐water tension, and redox potential were monitored at seven points along a 41‐m hillslope transect composed of Psamments, Aquents, Aquods, and Saprists. In addition to standard field descriptions, particle‐size distribution, percentage of organic C, and citrate‐dithionate and ammonium‐oxalate extractable Fe (Fed and Feo, respecitively) were determined for profiles at each transect point. During the study period, the 30‐yr mean annual precipitation (MAP) was exceeded in 4 of 5 yr. Mean water levels were highest in spring and water levels typically rose from September through May. The depth to redoximorphic features increases with elevation above the peatland and the upper extent of redoximorphic features is 15 to 60 cm above the measured mean water table level. In the upper landscape positions, the redoximorphic features were located 12 cm above the maximum recorded water level. The distribution characteristics of Fe throughout the soil system indicate that Fe has been removed from the upslope soils and reconcentrated in organic‐rich horizons in the lower landscape positions. The combination of Fe distribution and the location of redoximorphic features well above the mean water table suggest that the regional water table has been lowered.

Attenuation of Petroleum Hydrocarbons in Smear Zones: A Case Study

Soil and ground water in a shallow aquifer were contaminated by petroleum hydrocarbons, mostly comprised of toluene. The degrees of soil and ground-water contamination were closely correlated. The vertical profile of soil contamination showed an exponential or semilinear decrease both upward and downward from the mean water table, and this was closely related to water table fluctuation. Vertical and seasonal variations in the concentrations of petroleum hydrocarbon contaminants and their degradation products were analyzed in detail. Total natural attenuation rate and the contribution of biodegradation to the rate were estimated. Biodegradation was found to be an effective attenuation process at this site.

Factors affecting the distribution of hydrocarbon contaminants and hydrogeochemical parameters in a shallow sand aquifer

The distributions of hydrocarbon contaminants and hydrogeochemical parameters were investigated in a shallow sand aquifer highly contaminated with petroleum hydrocarbons leaked from solvent storage tanks. For these purposes, a variety of field investigations and studies were performed, which included installation of over 100 groundwater monitoring wells and piezometers at various depths, soil logging and analyses during well and piezometer installation, chemical analysis of groundwater, pump tests, and slug tests. Continuous water level monitoring at three selected wells using automatic data-logger and manual measuring at other wells were also conducted. Based on analyses of the various investigations and tests, a number of factors were identified to explain the distribution of the hydrocarbon contaminants and hydrogeochemical parameters. These factors include indigenous biodegradation, hydrostratigraphy, preliminary pump-and-treat remedy, recharge by rainfall, and subsequent water level fluctuation. The permeable sandy layer, in which the mean water table elevation is maintained, provided a dominant pathway for contaminant transport. The preliminary pump-and-treat action accelerated the movement of the hydrocarbon contaminants and affected the redox evolution pattern. Seasonal recharge by rain, together with indigenous biodegradation, played an important role in the natural attenuation of the petroleum hydrocarbons via mixing/dilution and biodegradation. The water level fluctuations redistributed the hydrocarbon contaminants by partitioning them into the soil and groundwater. The identified factors are not independent but closely inter-correlated.

Topographic Influence on the Seasonal and Interannual Variation of Water and Energy Balance of Basins in North America

A large area basin-scale (LABs) hydrologic model is developed for regional, continental, and global hydrologic studies. The heterogeneity in the soil-moisture distribution within a basin is parameterized through the statistical moments of the probability distribution function of the topographic (wetness) index. The statistical moments are derived using GTOPO30 (30 arc sec; 1-km resolution) digital elevation model data for North America. River basins and drainage network extracted using this dataset are overlaid on computed topographic indices for the continent and statistics are extracted for each basin. A total of 5020 basins with an average size of 3255 square kilometers, obtained from the United States Geological Survey HYDRO1K data, is used over the continent. The model predicts runoff generation due to both saturation and infiltration excess mechanisms along with the baseflow and snowmelt. Simulation studies are performed for 1987 and 1988 using the International Satellite Land Surface Climatology Project data. Improvement in the terrestrial water balance and streamflow is observed due to improvements in the surface runoff and baseflow components achieved by incorporating the topographic influences. It is found that subsurface redistribution of soil moisture, and anisotropy in hydraulic conductivities in the vertical and horizontal directions play an important role in determining the streamflow and its seasonal variability. These enhancements also impact the surface energy balance. It is shown that the dynamics of several hydrologic parameters such as basin mean water table depth and saturated fraction play an important role in determining the total streamflow response and show realistic seasonal and interannual variations. Observed streamflow of the Mississippi River and its subbasins (Ohio, Arkansas, Missouri, and Upper Mississippi) are used for validation. It is observed that model baseflow has a significant contribution to the streamflow and is important in realistically capturing the seasonal and annual cycles.

Effect of the 1997-1998 ENSO-Related Drought on Hydrology and Salinity in a Micronesian Wetland Complex

The potential effects of global climate change on coastal ecosystems have attracted considerable attention, but the impacts of shorter-term climate perturbations such as ENSO (El Nino-Southern Oscillation) are lesser known. In this study, we determined the effects of the 1997–1998 ENSO-related drought on the hydrology and salinity of a Micronesian mangrove ecosystem and an adjacent freshwater swamp. A network of 9 piezometer clusters installed at the study site served as sampling points for continuous and manual measurements of salinity and water level. During the drought period from January through April 1998, mean water table levels in the mangroves and freshwater swamp were approximately 12 and 54 cm lower, respectively, than during May through December when precipitation returned to near normal levels. At the peak of the drought (February 1998), the most dramatic result was a reversal in groundwater flow that sent groundwater from the mangroves upstream toward the freshwater swamp. Flow nets constructed for this period and immediately after illustrate the strong hydrological linkage between the two systems. This linkage was also illustrated by measurements of groundwater salinity in the piezometer network. Ninety-six percent of the salinity measurements taken in the mangroves during the study were at least 10‰ less than the salinity of sea water, indicating that the mangroves were consistently receiving freshwater flows. An analysis of variance of groundwater salinity measurements during and after the drought showed that salinity levels in the 0.5 and 1.0 m depth piezometers were greater during than after the drought. In a comparison of salinity values in 0.5-m wells during low tide, mean salinity was approximately twice as high during the drought than after (14.7‰ versus 6.2‰, respectively). This study demonstrates that short-term climate perturbations such as ENSO can disrupt important coastal processes. Over repeated drought cycles, such perturbations have the potential to affect the structure and function of mangrove forests and upstream ecosystems.

Scaling net ecosystem CO2 exchange from the community to landscape‐level at a subarctic fen

SummaryLandscape‐ and community‐level CO2 measurements were made at a subarctic sedge fen near Churchill Manitoba during the 1997 growing season. Climatic conditions were warmer and drier than the 30‐y normal. Landscape‐scale micrometeorological measurements indicated that the wetland gained 49 g CO2 m−2 during the growing season. Chamber‐scale measurements from the main vegetation community types showed that small hummocks (Carex spp. sites) dominated the CO2 exchange, yielding an effective scaling factor of 70%. Scaled parameters of two algorithms describing photosynthesis and respiration for each community type show strong similarity to those derived at the landscape level. Scaling photosynthesis, respiration, and net ecosystem CO2 exchange from the community to landscape‐level over the season is within the maximum probable error of each methodological approach and helps substantiate the 1997 CO2 budget. We explore the equilibrium response of net ecosystem CO2 exchange of this fen to climatic change by examining the feedback of water table position on vegetation distribution and nitrogen availability. Based on the effective scaling factors computed for each community type, we hypothesize that a small decrease in mean water table position could nearly triple the net uptake of CO2 at this wetland.

Groundwater-surface water interaction and the climatic spatial patterns of hillslope hydrological response

Abstract. A transient, mixed analytical-numerical model of hillslope hydrological behaviour is used to study the patterns of infiltration, evapotranspiration, recharge and lateral flow across hillslopes. Computational efficiency is achieved by treating infiltration and phreatic surface movement analytically. The influence of dynamic coupling of the saturated and unsaturated zones on the division of hillslopes into units of distinct hydrological behaviour is analyzed. The results indicate the importance of downhill groundwater flow on the lateral distribution of soil moisture and hydrological fluxes; unsaturated lateral flow is shown to be of relatively minor importance. For most conditions, the hillslope organizes itself into three distinct regions; an uphill recharge and a downhill discharge zone separated by a midline zone over which there is, on average, no recharge or discharge. A temporal perturbation analysis of the phreatic surface, made to quantify the deviations between the equivalent-steady water table derived by Salvucci and Entekhabi (1995) and the long-term mean water table, shows that the equivalent-steady water table effectively couples the unsaturated and saturated zone dynamics across storm and interstorm periods and divides the hillslope into distinct hydrological regions. The second order closure terms in the perturbation analysis, expressed as the gradient of water table variance, quantify the deviations and tend to make the hydrological zones relatively less distinct.

Spatial patterns of aerobic limit depth and oxygen diffusion rate at two peatlands drained for forestry in Alberta

The spatial variability of soil aeration (aerobic limit depth and oxygen diffusion rate (ODR)) among different drainage ditch spacings and at various distances from drainage ditches was examined at two peatlands drained for forestry in north-central Alberta. Drainage lowered mean water table levels (p &lt; 0.001) at both peatlands. Lower water table levels within drained areas were associated with greater aerobic limit depths (p &lt; 0.001) and greater ODR (p &lt; 0.001 at Saulteaux River; p &lt; 0.027 at Wolf Creek) compared with undrained areas of both peatlands. Spatial patterns of aerobic limit depth and ODR indicated strong spatial variability of soil aeration in the immediate vicinity of drainage ditches. However, little sensitivity of soil aeration to different ditch spacings was evident. Potential effects of post-drainage subsidence on soil aeration was evident as increased distance of both the aerobic limit, and near-zero oxygen diffusion rates above the water table surface at both peatlands. One effect of post-drainage peatland subsidence may be to increase the thickness of the capillary zone above the water table. However, reduction of aeration within the rooting zone of peatland trees due to subsidence was not observed because of low water tables within the drained areas of both peatlands.

RIPARIAN ECOSYSTEM MANAGEMENT MODEL (REMM): I. TESTING OF THE HYDROLOGIC COMPONENT FOR A COASTAL PLAIN RIPARIAN SYSTEM

The Riparian Ecosystem Management Model (REMM) was used to simulate shallow groundwater movement,water table depths, surface runoff, and annual hydrologic budgets for a Coastal Plain riparian buffer system near Tifton,Georgia, USA. The riparian buffer consisted of zone 3 (grass downslope from a row-crop field); zone 2 (mature pineforest downslope from zone 3); and zone 1 (mature hardwood forest downslope from zone 2, adjacent to stream).Measured surface runoff and shallow groundwater movement from the adjacent agricultural field were used as thehydrologic input to REMM. Uncalibrated simulation results for a five-year period were compared to measured values forthe same time period. The overall error in zone 2 and zone 1 mean water table depths was about 0.07 m, althoughabsolute errors were higher. The water table dynamics simulated by REMM were similar to observed although lags wereobserved in the response of the simulated water table to large rainfall events. Mixed results were obtained from observedversus simulated surface runoff comparisons, primarily due to large variability in observed runoff depths along theriparian transect. Simulated surface runoff depths for zone 3 were within one standard deviation for four out of the fiveyears. For zone 2, surface runoff depths could only be simulated within one standard deviation for two out of the fiveyears. Simulated seasonal total depths of surface runoff did not always agree with observed values but usually followedboth similar temporal and spatial patterns. Annual hydrologic budgets produced total streamflow comparable to thoseestimated for the riparian buffer site. These results provide an adequate basis for subsequent testing of other REMMmodel components including water quality and nutrient cycling.