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a Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299 USA
b Dep. of Forest Ecol. and Manage., Univ. of Wisconsin, Madison, WI 53706 USA
krbrye{at}students.wisc.edu
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Abbreviations: CP, chisel plow • CV, coefficient of variation • Et, evapotranspiration • ETL, equilibrium-tension lysimeter • NT, no tillage • RO, runoff
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Several field studies have quantified the annual hydrologic budget (McGowan et al., 1980; Hanna et al., 1983; Luxmoore, 1983; Maule and Chanasyk, 1987; Moreno et al., 1996; Roman et al., 1996). Several studies use estimates for either drainage or evapotranspiration to construct water budgets for various ecosystems (Chopart and Vauclin, 1990; Gabrielle et al., 1995; Akinremi et al., 1996; Maraux and Lafolie, 1998). Independent measurements of water-budget components are essential to validate simulation models and empirical equations. Water loss through runoff, soil-surface evaporation, plant transpiration, and soil water–storage changes have been studied (Clark, 1940). However, drainage and canopy-intercepted precipitation are two components of the water budget that can also be important and have received less emphasis in the past.
Previous water-balance studies frequently did not directly measure the drainage component of the water budget. Drainage rates are often inferred from measuring soil water–storage changes and matric potential gradients and applying Darcy's law (McGowan and Williams, 1980; Hanna et al., 1983; Luxmoore, 1983; Moreno et al., 1996; Roman et al., 1996). These methods are valid; however, large uncertainties in soil water properties and potential gradients suggests that direct drainage measurements may be preferred (Logsdon and Jaynes, 1996; Bosch and West, 1998).
Direct field measurements of drainage are difficult to perform and reliable data sets are challenging to obtain. However, indirect methods demonstrate the importance of the drainage component in the water balance. Maule and Chanasyk (1987) reported 32 to 34% of annual precipitation as drainage calculated by the measured hydraulic gradient method of McGowan and Williams (1980) for fallow and barley (Hordeum vulgare L.) fields in Edmonton, Alberta, Canada. Luxmoore (1983) reported the drainage fraction of the precipitation between 37 and 39% for a forested silt loam soil in eastern Tennessee between late May and early September. Hanna et al. (1983) recognized and stressed that determining water movement through a soil profile (i.e., by direct measurement) is critical to optimal crop-production management in terms of water use and nutrient loss through leaching.
Direct-drainage flux measurements commonly accompany lysimeter studies aimed at quantifying fluxes of ions in soil solution (Brye et al., 1999). One disadvantage with field measurements of downward fluxes is the potentially high replicate variability. Jemison and Fox (1992) document a CV of 43% for leachate volumes collected with zero-tension lysimeters in Hagerstown silt loam in central Pennsylvania. Brye et al. (1999) reported smaller variability among replicates of equilibrium-tension lysimeters (ETL), where CVs ranged from 8% for a prairie to 37% for a chisel-plowed agroecosystem.
Precipitation interception by residue can impact the water and chemical balances by altering the amount and characteristics of water that reach the soil surface (Seastedt, 1985). Reported affects of various types of residue differ widely, ranging from 47 to 84% interception per rainfall event for big bluestem prairie grass (Andropogon gerardii Vitman) residue in Nebraska (Clark, 1940), 10.8 to 18.1% interception of annual precipitation for curly mesquite [Hilaria belangeri (Steud.) Nash] and sideoats gramma [Bouteloua curtipendula (Michx.) Torr.] in Texas (Thurow et al., 1987), and 9% interception of annual precipitation for a no-tillage corn–soybean [Glycine max (L.) Merr.] rotation in Illinois (Savabi and Stott, 1994). Winter wheat (Triticum aestivum L.) and corn residue has been reported to possess a moisture storage capacity between 1.0 and 3.5 mm, while soybean residue moisture-storage capacity ranged between 0 and 2 mm (Savabi and Stott, 1994).
Clark (1940) reported that for precipitation events between 3.2 and 50.8 mm, mean interception was 5.9 mm and maximum interception was 25.9 mm for big bluestem prairie grass residue. However, live canopy interception is typically smaller (1–5 mm) than residue interception (Clark, 1940; Steiner et al., 1983). More recently, Seastedt (1985) studied interception of an annually burned and unburned portion of the tallgrass Konza prairie in Kansas. Seastedt (1985) reported that interception from unburned prairie was 42% of monthly precipitation. Specifically, out of an average of 76 mm of precipitation in a month period, average interception totaled 32 mm of water (Seastedt, 1985).
Previous studies document the much smaller impact of residue interception on the water balance of agroecosystems compared with a prairie's; therefore, rainfall interception by maize residue was not assessed in this study. Savabi and Stott (1994) reported that corn residues, with surface coverage between 0.3 and 1.5 kg m-2, intercept only 1 to 3 mm (note: the maize residue returned in the fall of 1996 for the tillage treatments of the present study ranged from 0.75 to 1.59 kg m-2).
In summary, crop canopies, grass canopies, and chopped crop residues tend to intercept only a few millimeters of rainfall. However, thick prairie residues can intercept more rainfall, perhaps as much as 30 mm. We hypothesized that land-use changes have altered water-budget components among natural prairies and managed agroecosystems established from prairies. Therefore, the objectives of this study were to compare independent field measurements of water-budget components for several years in three ecosystems influenced by land use common to south-central Wisconsin, namely, a restored prairie, no-tillage maize, and chisel-plow maize. Special emphasis was placed on quantifying the drainage component of the water budget.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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2.5 km northeast of the agricultural plots. The soil at both sites is a Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll). The parent material is loess overlying glacial till. The landscape slope is 3% for the prairie and 2% for the agricultural site. The three ecosystems under investigation include chisel-plow fertilized maize, no-tillage fertilized maize, and a 21-yr-old restored tallgrass prairie. Selected initial landscape and soil characteristics and chemical properties of the prairie and agricultural ecosystems are listed in Table 1
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Climatic Data
On-site weather measurements began on Day 155 (4 June) of 1995. Ambient air temperature, relative humidity, rainfall, and soil temperatures at 10, 30, 70, and 120 cm are continuously measured for each ecosystem using automated micrometeorological weather stations. Precipitation was collected manually using a funnel collection system for rainfall and a bucket collection system for snowfall. Both collection systems are similar to those used by Likens et al. (1977).
Drainage Measurements
Drainage was measured under undisturbed soil columns using replicate ETLs (0.19 m2) in the prairie, no-tillage, and chisel-plow agroecosystems (Brye, 1997; Brye et al., 1999). Two lysimeter pits were excavated straddling a chisel-plow and no-tillage treatment block (Fig. 1). Two lysimeters were placed in each lysimeter pit, one for each tillage treatment. One lysimeter pit was excavated and two lysimeters were installed in the prairie (Fig. 1). The lysimeters were located near, but not within, established prairie plots to keep the lysimeters near the micrometeorological weather station and minimize disruption of the plots. Drainage sampling began in July 1995 for the fertilized no-tillage and chisel-plow agroecosystems and in October 1995 for the restored prairie. Leachate was collected every 14 d between March and December and every 30 d during the winter months.
A portable, regulated vacuum system provided continuous suction to the 0.2-µm stainless-steel porous plate of the lysimeters (Brye et al., 1999). Heat-dissipation sensors (Reece, 1996) were located immediately above the porous plate of each lysimeter and in the surrounding bulk soil to continuously monitor the matric potential at the two locations near each lysimeter. The regulated vacuum system was adjusted manually several times a week to provide suction that is slightly more negative (2–3 kPa) than the matric potential recorded in the surrounding bulk soil with the heat-dissipation sensors. Tensions applied to lysimeters (ranging from 5–50 kPa) were set to closely mimic the tension of the bulk soil surrounding each lysimeter to avoid ponding above, and bypass flow around, the lysimeters.
Soil Water Measurements
Soil water profiles were measured every 7 d from March through October and approximately every 21 d from November through February using a neutron hydroprobe (Model 503; Campbell Pacific Nuclear, Martinez, CA). The hydroprobe was calibrated in the field during the summer of 1995 under wet and dry soil conditions using calculated volumetric water-content values obtained from gravimetric water content and bulk density samples. Separate calibration equations were generated for 0 to 20 cm (R2 = 0.92) and 30 to 140 cm (R2 = 0.82). Moisture-profile measurements were replicated four times in each ecosystem. Neutron probe counts were recorded at 10-cm increments, at a measurement interval of 16 s, and to a depth of 140 cm during the spring, summer, and fall; and at 20-cm increments during the winter months (November–February).
Winter Surface-Water–Storage Measurements
Liquid water equivalents of snowfall were measured to determine winter surface-water storage for the ecosystems. Approximately every 7 d, when snow was present, the snow depth was measured using a meter stick in 10 random locations within a 1.0-m radius around each ETL pit. A snow core of average depth was extracted from around each pair of lysimeters using a 3.9-cm (o.d.) coring cylinder. Liquid water equivalent of the snow pack was calculated from the core and liquid water volume.
Prairie Residue Rainfall Interception Measurements
Field measurements of rainfall interception by residue were conducted using plastic trays that collected rainfall transmitted through the prairie's residue layer. Two tray methods, differing in sampling area and whether the residue was disturbed (cut) or not, were used to quantify residue interception of rainfall.
In one method, five small residue rainfall interception trays (22.5 cm long x 14.9 cm wide x 5.1 cm deep) were installed at the prairie site on 19 May 1997. These shallow trays were placed beneath the standing vegetation at random locations. Within 12 to 18 h following each rainfall event, each small residue interception tray was removed, the volume of throughfall was measured, and the tray was replaced.
The second method involved three large residue interception trays (34.3 cm long x 24.1 cm wide x 7.6 cm deep) set out at the prairie site on 28 May 1997. In this method, the residue within a 0.5- by 0.5-m frame was cut and placed on top of a wire mesh screen (with square 6.0 x 6.0 mm openings), slightly smaller than the area of residue cut to avoid edge-flow affects. Residue-covered mesh screens were placed on top of the interception trays in the same locations and orientations from which the residue was cut so that standing grass stems remained standing after being placed on the screens. Volumes of rainfall transmitted through the residue were recorded at the same time as small tray measurements. Residue interception tray locations were changed after measurements from four to five rainfall events.
To ascertain the interception capacity of the residue layer, and not the canopy, rainfall was collected below the canopy and above the residue. Residue interception was determined by the difference between the rainfall, collected below the live canopy and above the residue near the interception trays using a funnel collector (80.3 cm2), and the transmitted water collected in the trays. Evaporative loss of collected rainfall may have occurred from either the residue interception trays or the rainfall collector before they could be checked. We assumed that the evaporative losses were negligible compared with the magnitude of the rainfall event because of the short elapsed time between the cessation of rain and collection. However, we recognize that this assumption may be less valid with small rainfall events.
Ecosystem Water Budgets
Components of the water budget (precipitation, interception, drainage, stored soil water changes, runoff, and winter surface-water–storage changes) were measured for 132 consecutive wk (25 June 1995 through 3 Jan. 1998). The weekly water-budget equation for the growing season was written in the form where evapotranspiration equals inputs minus outputs minus storage changes.
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Precipitation (P) was estimated from tipping-bucket rain-gauge measurements recorded hourly by each micrometeorological weather station and from manually collected precipitation measurements. Drainage (D) was estimated from ETLs. Soil water–storage changes (
Ssoil) were estimated from neutron hydroprobe measurements. Evapotranspiration (Et) was calculated as the residual difference in the water balance for each week. Cumulative measurement errors for all the other components in the water-balance equation are imbedded in the Et term. During the growing season, RO represents surface runoff of rainfall during and following major precipitation events and is estimated from
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During the winter season (i.e., first fall snowfall to spring thaw), Et is assumed to be zero, therefore the water-balance equation is given by
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Ssurface is surface-water–storage (i.e., changes in snow cover) changes measured by manual coring of snow. The winter water-balance equation takes care of three conditions that occur in the winter weeks (i) when precipitation occurs while drainage, soil water storage, and surface storage do not change (P > 0 and D + Ssoil + Ssurface = 0) and runoff equals precipitation (RO = P); (ii) when zero precipitation occurs while drainage and soil water storage remain unchanged, surface storage decreases (P = 0, D + Ssoil = 0, and Ssurface < 0), and runoff occurs from melting (RO = Ssurface); and (iii) when zero precipitation occurs while drainage and soil water storage remain unchanged, surface storage increases (P = 0, D + Ssoil = 0, and Ssurface > 0), and run-on actually occurs by drifting snow (RO = -Ssurface).
Drifting snow during the winter weeks affects both the prairie and agricultural research sites. Drifting snow has several fates once it has accumulated at a site: the snow can be blown off again, in which case the drifting snow does not impact the water balance of the given ecosystem; the snow can sublimate; the snow can melt and run off; the snow can melt, infiltrate the frozen soil, and drain through the soil profile; or finally, some combination of melting, sublimation, runoff, infiltration, and drainage may occur. Due to this seasonal phenomenon, the potential maximum extra water from drifted snow (DS) that can infiltrate into the soil can be estimated by
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Data Analysis
We assume that our comparison of the restored prairie and agricultural ecosystems is valid given substantial similarities of the soils among the three ecosystems. Similarities in landscape characteristics, soil taxonomy, particle-size distribution, bulk density, and organic matter support this assumption (Table 1). These similarities are probably enhanced by the general uniformity of tillage and fertilization practices on all of the ecosystems until prairie restoration was initiated in 1976. Because of the limited availability of natural prairies, native or restored, in Wisconsin and the Midwest, the close proximity and matching characteristics between these two study sites offers a unique opportunity to compare the impact of land-use changes on the water balance of natural and managed ecosystems. We assumed that the variability among the four prairie plots and also among the four agricultural blocks is typical of the variability expected among plots with similar soil, ecological, and climate conditions despite the different plot layouts at the two study sites.
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Evapotranspiration
Cumulative Et is plotted in Fig. 2
for the prairie and agroecosystems. Evapotranspiration timing and rates were not significantly different for chisel-plow and no-tillage plots during the experiment (within 0.2 mm d-1). The prairie began to transpire sooner than the maize (Week 1596 vs. 2196 and Week 1897 vs. 2397), had a slightly lower midsummer maximum (<10% lower for the prairie), and transpired later into the fall (Week 4196 vs. 3996 and 4297 vs. 3997). For the prairie, no-tillage, and chisel-plow maize, the average daily evapotranspiration rate was higher in 1996 (2.4, 3.0, and 3.1 mm d-1, respectively) than in 1997 (2.5, 1.8, and 1.7 mm d-1, respectively). Peak Et occurred between Weeks 27 and 33, with the prairie tending to peak about a week earlier than the agroecosystems. Although the evaporation is calculated as the residual of the water budget and thus suffers from the accumulated error of the other components, the CV for evapotranspiration was 1.6, 9.0, and 16.7% for the prairie, no-tillage, and chisel-plow ecosystems.
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An extreme summer rainfall event during Week 2596 caused the single largest weekly drainage flux for the entire 132-wk period for all three ecosystems. Peak weekly drainage occurred during Week 2596 at 84.4 mm wk-1 for the no-tillage agroecosystem, 89.6 mm wk-1 for the chisel-plow agroecosystem, and 21.1 mm wk-1 for the prairie. Peak drainage was much lower during 1997, compared with 1996, primarily because of the lack of large precipitation events. However, peak agroecosystem drainage in 1997 occurred through a layer of frozen soil >5 cm for the no-tillage ecosystem and >70 cm for the chisel-plow ecosystem during Week 297; drainage was 14.2 mm wk-1 in the no-tillage ecosystem and 23.9 mm wk-1 in the chisel-plow ecosystem. There was no drainage in the prairie during the same week.
For the entire 132-wk evaluation period, total drainage was 199 mm of water (CV = 5.7%) for the prairie ecosystem, 563 mm of water (CV = 13.6%) for the no-tillage ecosystem, and 793 mm of water (CV = 18.5%) for the chisel-plow ecosystem (Fig. 3). Standard errors of lysimeter replicates for individual sample dates averaged 1.0 (0.03–5.8) mm for the prairie ecosystem, 4.8 (0.09–41.9) mm for the no-tillage ecosystem, and 6.6 (0.01–58.7) mm for the chisel-plow ecosystem.
Soil Water–Storage Changes and Runoff
Soil water storage was consistently greater in the prairie than in the maize ecosystem (Fig. 4)
, with the greatest difference occurring in the deeper (0.8–1.4 m) portion of the soil profile (Fig. 5)
. Mean volumetric water contents (m3 m-3) (± SE) were 0.33 (0.02), 0.33 (0.02), and 0.30 (0.03) in the upper 70 cm of the soil profile and ranged from 0.23 to 0.38, 0.22 to 0.38, and 0.20 to 0.36 for the prairie, no-tillage, and chisel-plow ecosystems, respectively. Mean volumetric water contents (m3 m-3) (± SE) were 0.35 (0.02), 0.32 (0.03), and 0.30 (0.03) in the lower 70 cm of the soil profile and ranged from 0.30 to 0.38, 0.24 to 0.37, and 0.18 to 0.36 for the prairie, no-tillage, and chisel-plow ecosystems, respectively. Frequent soil moisture measurements easily tracked annual recharge and soil water withdrawal periods for these three ecosystems. However, the infiltration capacity of the Plano silt loam soil was exceeded several times during the 132-wk evaluation period in these ecosystems resulting in runoff.
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Prairie Residue Interception
During the growing season of 1997, average rainfall was 17.0 mm per event, ranging from 1.3 to 47.8 mm. Residue interception measured with the small interception trays averaged 12.3 mm (SE ± 1.17 mm), ranging from 1.3 to 32.7 mm. Residue interception measured with the large interception trays averaged 10.4 mm (SE ± 1.24 mm), ranging from 1.3 to 30.4 mm. Peak prairie residue (i.e., biomass plus necromass) density was 2.8, 2.2, and 2.5 Mg ha-1 for 1995, 1996, and 1997, respectively. However, no clear relationship exists between residue mass and rainfall interception.
Ecosystem Water Budgets
Cumulative water-budget components (i.e., precipitation, winter surface-water–storage changes, soil water–storage changes, drainage, runoff, and evapotranspiration) for the 132-wk evaluation period are plotted in Fig. 6, 7, and 8
for the prairie, no-tillage, and chisel-plow agroecosystems, respectively, and a seasonal summary for 1996 and 1997 is contained in Table 2 . Runoff events are denoted above weekly precipitation bars in the figures.
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Clearly we cannot prove that differences between the restored prairie and agroecosystems are solely caused by the vegetation and management because such a controlled experiment can no longer be conducted in Wisconsin or in most of the Midwest for that matter. However, the assumption that the agroecosystems used in this study are similar to the prairie site before restoration is reasonable. Additionally, the observation that the no-tillage maize treatment falls between the prairie and the chisel-plow maize treatment in terms of water content and drainage adds further credibility.
Equilibrium-tension lysimeter drainage measurements (Fig. 3) substantiate the conclusion that agriculture-related soil disturbance (i.e., tillage) promotes drainage. The existing tillage treatments shared the same 3-yr no-tillage history before plot establishment. The year 1996 marked the second consecutive year of mechanical disturbance for the chisel-plow-tilled plots, the third consecutive year of minimum mechanical disturbance for the no-tillage plots, and the twentieth consecutive year of zero mechanical disturbance in the restored prairie. Figure 9 depicts the decreasing drainage trend with increasing years since cultivation. Evapotranspiration is also plotted as a fraction of annual precipitation to show that all treatments had similar Et rates, so reduced drainage is not from increased Et.
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Rainfall interception was a significant component of Goose Pond prairie's water balance. Most of this interception is associated with the residue and not the live canopy because canopy interception for grasses is typically much smaller (Clark, 1940; Steiner et al., 1983). Residue interception results reported for the Goose Pond prairie were similar to interception reported by Clark (1940) for big bluestem residue in Nebraska and by Seastedt (1985) for a mix of grasses, big bluestem, little bluestem [Schizachyrium scoparium (Michx.) Nash], switchgrass, and Indiangrass, in the Konza prairie of Kansas. Between 26 Mar. 1997 and 13 Nov. 1997, 681 mm of rainfall were delivered to the prairie, while an estimated 477 mm were intercepted and ultimately evaporated, significantly lowering water inputs available for drainage from the prairie.
The ability to assess drainage and leaching potential from field measurements over a range of ecosystems is both difficult and extremely valuable. Forty percent of total precipitation measured as drainage from a chisel-plow agroecosystem is a large fraction of moisture inputs and may be viewed as excessive. However, drainage calculated indirectly with hydraulic properties gathered from tensiometers in the field and pressure-plate studies has been reported to be 33% of growing season precipitation (Maule and Chanasyk, 1987). Zero-tension lysimeter field studies have reported that drainage comprised between 30 and 40% of precipitation during a measurement period for a silt loam soil in Kentucky (Tyler and Thomas, 1977) and for sandy and clay loam soils in Sweden (Bergstrom and Johannson, 1991). The drifting-snow phenomenon experienced by the agricultural ecosystems contributes some additional water to soil profiles, but does not alter conclusions about CP drainage > NT drainage > prairie drainage.
The drainage component of the water budget is difficult to measure under field conditions. Winter drainage cannot be estimated with soil water–storage changes because wet soils can drain without storage change and because infiltration is highly variable during winter. The usefulness of the ETLs for measuring drainage is demonstrated by the reasonable agreement that was achieved for cumulative drainage recorded for replicate lysimeters (Brye, 1997; Brye et al., 1999). Lysimeter replicate drainage differences were generally smaller than treatment drainage differences. Drainage measurement variability obtained for this water-balance field study was reasonable compared with other similar studies (Brye et al., 1999).
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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Received for publication August 28, 1998.
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionsREFERENCES |
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denotes significant runoff episodes
