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Soil Science Society of America Journal 67:471-477 (2003)
© 2003 Soil Science Society of America

DIVISION S-1—SOIL PHYSICS

Long-term Movement of a Chloride Tracer under Transient, Semi-Arid Conditions

M. F. Dycka, R. G. Kachanoski*,b and E. de Jongc

a KBL Land Use Consulting Ltd., 230–323 10th Ave. SW, Calgary, AB T2R 0A5, Canada
b Office of the Vice President, Research, Univ. of Alberta, 3rd Floor University Hall, Edmonton, AB T6G 2J9, Canada
c Dep. of Soil Science, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada

* Corresponding author (gary.kachanoski{at}ualberta.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Understanding the magnitude and variability of water and solute fluxes in the vadose zone is required to assess the risk of potential contamination of ground water resources. In this study, the long-term (>30 yr) movement of a Cl tracer applied to the soil surface is quantified. In 1966, granular KCl was applied to the surface of nine 6 by 90 m plots at a field site near Saskatoon, Saskatchewan. Additional KCl was applied to two of the plots in 1970. Application rates varied between 0.11 to 2.24 kg KCl m-2. Moment analysis was used to calculate the mean travel depth (E[z]) and variance (V[z]) of depth breakthrough curves compiled from soil samples taken 1, 2, 3, 4, 28, and 34 yr after the initial KCl application from the 2.24-kg KCl m-2 plot. Spatial variability of Cl transport was assessed on 51 cores taken along a 10-m transect, 34 yr after application. Initial movement of the tracer through the root zone was relatively quick (E[z] = 1.34 m, 4 yr after application), followed by slow movement (E[z] = 1.68 m, 34 yr after application). The change in E[z] between 4 and 34 yr after application was used to calculate a velocity below the root zone of 11 mm yr-1 and a deep drainage estimate of 3 mm yr-1 ({theta} = 0.3) which is within the range of other estimates for the Canadian Prairies. Variability in E[z] along the 10-m transect, after 34 yr of transport was low (coefficient of variation [CV] = 4%), suggesting relatively uniform deep drainage. This contrasts sharply with the high variation in convective fluxes measured in other steady-state high-flow field-tracer experiments.

Abbreviations: CMB, chloride mass balance • CV, coefficient of variation • DBTC, depth breakthrough curve • EC, electrical conductivity


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
TRANSPORT through the vadose zone is a primary environmental pathway controlling the fate of a variety of potential ground water contaminants. In arid and semi-arid climates water and solute fluxes below the root zone are small and difficult to quantify. Deep drainage in semi-arid environments is typically <10 mm yr-1 (e.g., Tyler et al., 1996; Gee et al., 1994; Cook et al., 1989; Keller et al., 1988) with some estimates <1 mm yr-1 (Joshi and Maulé, 2000; Scanlon et al., 1999; Murphy et al., 1996). As a result, traditional methods of measuring water flux such as soil water balance and hydraulic gradient techniques are unreliable because the magnitude of errors associated with these techniques is high compared with the magnitude of deep drainage (Gee and Hillel, 1988). The potentially long residence times of water and solutes in the vadose zone dictate that long-term experiments are required to assess transport processes. Because of practical constraints, however, many field-tracer experiments (Butters et al., 1989; Ellsworth et al., 1991; Hills et al., 1991; van Wesenbeeck and Kachanoski, 1994) have been performed under high-flow steady-state conditions that may not be indicative of the boundary conditions realized under semi-arid environments.

Water and solute movement through the vadose zone may be compartmentalized as (Gee and Hillel, 1988): (i) movement through the root zone; and (ii) movement below the root zone. Fluxes within the root zone are transient. Infiltration, and evapotranspiration, result in spatially and temporally variable hydraulic and concentration gradients (Gee and Hillel, 1988; Tyler and Walker, 1994). Despite average annual evapotranspiration exceeding precipitation in semi-arid environments over the long-term, net downward fluxes below the root zone are possible during periods where precipitation exceeds evapotranspiration. Fluxes below the root zone may be transient or steady state (Gee and Hillel, 1988). Consistently high demand for water by plants may result in minimal deep drainage, so that water contents and matric potentials below the root zone approach a quasi-steady state in response to the long-term soil water balance. Transient fluxes below the root zone may be a result of ephemeral events such as ponding in topographic depressions during high magnitude precipitation events (Derby and Knighton, 2001).

A variety of conservative tracers have been used to aid estimation of deep drainage in semi-arid environments. Fallout of radioactive tracers such as 36Cl and 3H occurred during the aboveground nuclear tests in the 1950s and 1960s (Tyler and Walker, 1994). The peak concentration of these tracers in soil water is often assumed to be the convective front of deep drainage and that deep drainage is approximately constant. Deep drainage is estimated as the average velocity of the tracer peak multiplied by the average water content of the profile. Tyler and Walker (1994), however, noted that the velocity of tracers decreases through the root zone. As a result deep drainage estimates using the average depth of the peak concentration would be overestimated.

The Cl mass balance (CMB) technique (Allison and Hughes, 1978) has received considerable attention as a potentially quick, reliable, and economical technique for estimating deep drainage. The technique has been used in numerous vadose zone investigations (e.g., Allison and Hughes, 1978; Allison and Hughes, 1983; Allison et al., 1985; Cook et al., 1989; Tyler et al., 1996; Scanlon et al., 1999; Scanlon, 2000). In essence, the CMB technique assumes that the long-term average mass flux of Cl at the soil surface is equal to the long-term average mass flux of Cl below the root zone. The deep drainage rate is estimated as the long-term average annual Cl mass flux at the soil surface divided by the average soil water Cl concentration below the root zone. Uncertainty in the long-term average Cl flux at the soil surface, and the contribution of transport processes such as diffusion and anion exclusion result in uncertainties in deep drainage estimates obtained with CMB.

There have also been numerous applied tracer experiments such as Starr et al. (1986), Butters et al. (1989), Ellsworth et al. (1991), Roth et al. (1991), Hills et al. (1991), and van Wesenbeeck and Kachanoski (1994). The controlled boundary conditions of these experiments have allowed researchers to observe, quantify and model the variability of field scale transport processes. Whether insight in vadose zone transport processes gained from these steady-state high-flow experiments is applicable to the low-flow transient conditions realized in semi-arid environments is unknown. Quantification of transport processes and deep drainage in these environments could be realized in long-term applied tracer experiments performed under natural conditions. Therefore, the objective of this study was to quantify the long-term (>30 yr) movement of an applied conservative tracer through and below the root zone, under semi-arid transient flow conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Site Description
The field site (51°52' N lat., 107°18' W long.) was established in the fall of 1966 (Ballantyne, 1974; Ballantyne, 1980) approximately 50 km west of Saskatoon, Canada. Surficial sediments were deposited by a glacial lake during the Wisconsin deglaciation (Christiansen, 1979). The soils are classified as an Elstow association: Dark Brown Chernozems (Typic Ustolls) developed on loamy glacio-lacustrine parent material (Ellis et al., 1968). The lacustrine sediments are underlain by glacial till, which is drained by the Tessier aquifer. The water table occurs at approximately 15 m below the surface within a sand layer (Meneley, 1975). The site has been under a crop-fallow rotation dominated by wheat (Triticum aestivum L.) with some barley (Hordeum vulgare L.) since the site was established (Gordon Carr, landowner, personal communication, 2000). The long-term average precipitation at the site is 321 mm yr-1 with approximately 40% coming from September to May when crop growth and evaporative demand is low (Table 1). Annual precipitation varied considerably ranging from 225 mm yr-1 to 481 mm yr-1, and had a coefficient of variability of 64%.


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  		Table 1. Crop rotation and precipitation summary for the site at Laura, SK, Canada.

 
Sampling and Chemical Analysis
The site was originally established to test the effects of KCl fallout from potash mines on crop growth. Potassium chloride was applied at various rates in the fall of 1966 in 90 by 6 m plots oriented north-south (Fig. 1) . From 1967 to 1970 (4 yr), composite soil samples from two closely spaced soil cores (50-mm diam.) were taken from the southern ends of all of the plots. The cores were sectioned into 0.15-m increments down to 0.60 m, and into 0.30-m increments to 1.2 to 2.2 m. The 1.8- and 3.4-kg KCl m-2 strips were originally 0.67 and 1.1 kg KCl m-2, respectively. A second application of KCl took place in the fall of 1971. A follow-up soil core (70-mm diam.) was taken from the south end of the 3.4-kg KCl m-2 plot in the summer of 1994 (0.3-m increments down to 6.0 m). The samples were air-dried and ground to pass through a 2-mm sieve. Chloride concentrations in saturation extracts were determined by titration with AgNO3 (1967– 1970; Ballantyne, 1980), and colorimetrically for the 1994 samples. No manure or fertilizer material with Cl has been added to the site.



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  		Fig. 1. Topographic location of research site showing original KCl plots. (+) Location of background core. (—) Location of transect established in summer 2000.

 
In the summer of 2000, 34 yr after tracer application, 51 undisturbed cores, 53 mm in diameter, 5.5- to 6.0-m deep, were taken with a drill rig at 0.2-m intervals along a 10-m transect located 2 and 8 m from the west and south boundaries of the 3.4-kg KCl m-2 plot (Fig. 1). The general location of this transect was approximately the same as the soil cores taken between 1967 and 1970. An additional core was taken approximately 15 m outside the Cl treatments for background concentrations (Fig. 1). Land management was the same for the area of the background core. The morphology and dimensions of the major soil horizons and sedimentary layers were described and recorded for each core (Table 2). Soil horizons were described according to the Canadian System of Soil Classification (Agriculture Canada, 1987). Sedimentary layer descriptions were based on texture (by hand), appearance, and the first author's knowledge of sedimentary structures. After description, each core was sliced into 0.10-m increments (approximately 3000 total samples) that were bagged and weighed in the field. Samples were stored in polyethylene bags at room temperature until they were dried. Each sample was dried for 48 h in ovens set at 40°C. Dried samples were weighed for calculation of field water content (gravimetric and volumetric) and bulk density. Samples were ground to pass through a 2-mm sieve.


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  		Table 2. Average dimensions and morphology of pedogenic horizons and sedimentary layers.

 
Chloride and other soluble salts were extracted by adding 30 g of water to a 15-g subsample of dried, ground soil and shaking for 1 h. The extracts were collected by suction filtration and stored at 4°C. Electrical conductivity (EC) was measured on each extract using an EC bridge. Chloride concentrations in the extracts were determined colorimetrically on a autoanalyzer with a technique similar to the one described by Tel and Hesseltine (1990). Extract concentrations were used to calculate resident soil Cl concentration (kg Cl kg-1 soil; kg Cl m-3 soil).

For convenience of calculation, average background Cl concentrations were calculated for layers with similar texture from the core taken outside of the Cl treatments. For example, the background Cl concentrations for the Ah, Bm, and Cca/Ck horizons were averaged together because they had similar textures (Table 3). Background Cl concentrations were subtracted from each depth on the samples from the 3.4-kg Cl m-2 plot, depending on which layer the sample was taken from. The background Cl concentrations were at least an order of magnitude lower than samples from the tracer plots. Resident Cl concentrations (kg Cl m-3 soil), C(z), were calculated by multiplying the soil Cl concentration (kg Cl kg-1 dry soil) by bulk density.


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  		Table 3. Summary of background Cl concentrations.

 
Theory
Mass recovery, Mo (kg m-2), mean travel depth, E[z] (m), and variance, V[z] (m2), were calculated for all Cl depth breakthrough curves (DBTCs). Because of the limited number of samples, a lognormal probability density function (PDF) was fitted to the DBTCs from 1, 2, 3, and 4 yr after the Cl application (Jury and Roth, 1990):

[1]
where the parameters µ and {sigma} are statistical parameters and E[z] and V[z] are given by (Jury and Roth, 1990):

[2]

[3]

For the detailed transect (34 yr after tracer application) local scale transport was quantified by calculating central moments from the DBTCs measured at each of the 51 spatial locations, j, using the method of moments (Jury and Roth, 1990):

[4]

[5]

[6]
where Z is the maximum depth of sampling. Eq. [4], [5], and [6] were also used to calculate central moments for the single DBTC measured 28 yr after Cl application. Since approximately 60 depth increments (samples) per core were available, the integrals in Eq. [4], [5], and [6] were estimated numerically by rectangular summation. Relative mass recovery was calculated as Mo divided by the total amount of Cl added to the surface of the sampling area.

For the samples taken 34 yr after the initial application, resident concentrations for each depth, z, at each location, j, were averaged over the length of the transect, L, to calculate the transect scale DBTC (van Wesenbeeck and Kachanoski, 1991):

[7]
where CL(z) is the transect average resident concentration at depth, z. The transect scale mass recovery (MoL), mean travel depth (EL[z]) and variance (VL[z]) were calculated by substituting CL[z] for C(j,z) in Eq. [4], [5], and [6]. Van Wesenbeeck and Kachanoski (1991) found that the transect scale variance (VL[z]) should be related to the local scale transport parameters (Vj[z], Ej[z]) by:

[8]

That is, the transect-scale variance is equal to the average of the local scale variances (Varj[z]), plus the variance of the local scale mean travel depths (Var[Ej{z}]).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
The average dimensions and morphology of the pedogenic horizons and sedimentary layers in the first 6.0 m observed from the 51 cores sampled in the summer of 2000 and previous investigations by Meneley (1975) for depths <6.0 m are given in Table 2. The sampling area is characterized by silty loam textured A and B soil horizons, a varved layer from 1.2 to 2.5 m depth underlain by a silty layer (2.5–6.0 m depth) and sandy layer (6.0–22 m) extending to the water table (approximately 15-m depth).

Table 3 summarizes the average background Cl concentrations measured from the background core. Coefficients of variation for the background concentrations range from 6 to 31% and the concentrations are commonly at least an order of magnitude less than the Cl concentrations measured on the cores from the Cl treatment. Total Cl recovery in the background core was 0.05 kg m-2 compared with the Cl tracer application of 3.4 kg KCl m-2.

The normalized DBTCs for 1, 2, 3, 4, 28, and 34 yr after tracer application measured from the south end of the 3.4-kg KCl ha-1 plot established by Ballantyne (1980) are given in Fig. 2 . The DBTC for 34 yr after the initial application (Fig. 2f) is the average transect scale DBTC from 51 cores. The 34-yr transect scale DBTC is shown in more detail, along with the transect-average soil water content profile and the average depth of layer boundaries having the greatest differences in texture, in Fig. 3 . The error bars represent the 95% confidence interval. Mass recovery of the Cl tracer was 97% for the 34-yr DBTC, which is excellent and reflects the large sample number. Mass recovery for the other DBTCs (Fig. 2a–c) varied considerably reflecting the influence of spatial variability and the small sample numbers.



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  		Fig. 2. Summary of chloride movement over time.

 


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  		Fig. 3. Transect-scale average volumetric water content and resident soil Cl concentrations. The reference lines indicate major layer boundaries. The error bars represent the 95% confidence interval.

 
The Cl was applied in the fall after a fallow year. One year after application, the DBTC (Fig. 2a) reflects transport from spring snowmelt, evapotranspiration during the growing season, and fall precipitation. The majority of the Cl is within the first 0.45 m, but E[z] = 0.62 m because significant amounts of Cl were also transported to 1.2 m. Below average winter and growing season precipitation (Table 1) coupled with crop water demands likely explain the high concentrations at the surface.

Two years after application (at the end of a fallow period) the mean travel depth is 0.1 m greater (E[z] = 0.72 m; Fig. 2b) than 1 yr after application. Most of the Cl has been leached out of the first 0.3 m of the profile, and there are higher concentrations of Cl in the 0.9 to 1.2 m depth interval. This fallow period was marked by above average growing season precipitation (Table 1) and there would have been significantly more soil water recharge compared with the first year. This would explain the depletion of tracer from the top 0.3 m. Three years after application (after a second crop was grown), E[z] = 0.79 m and more Cl has been leached from the 0- to 0.45-m depth. There is a larger proportion of the chloride in the 0.6- to 1.2-m depth (Fig. 2c). In this case, winter precipitation was well above average (Table 1), but growing season precipitation was below average and high demand for water by the crop may also have decreased downward transport during the growing season.

Four years after the initial application (after a second fallow season), the majority of the Cl had moved to 0.9 to 1.2 m (E[z] = 1.34 m), the depth which small amounts of Cl had reached 1 yr after application (Fig. 2d). Significant amounts of the Cl have moved below 1.5 m. Again, above average over winter and growing season precipitation (Table 1) coupled with fallow conditions likely contributed to the deeper movement.

After 28 and 34 yr (both sampled during fallow seasons), E[z] is only 0.34 to 0.45 m deeper than 4 yr after the initial application, indicating very slow average movement (Fig. 2e and 2f). There are, however, significant amounts of Cl below 1.5 to 2.0 m, suggesting some deep drainage is occurring. Small amounts of the Cl tracer are as deep as 6.0 m. A significant amount of Cl is still present in the top 1.0 m suggesting transient upward transport is also occurring. The DBTC for 28 yr after application has greater amounts of Cl below 2.0 m than the DBTC for 34 yr after application. This is likely a result of local spatial variability in Cl transport. The DBTC for 34 yr after initial application is the average of 51 cores whereas the DBTC for 28 yr after application represents only one core. The E[z] values for 34 yr after application ranged from 1.53 to 1.83 m.

In summary, transport of the Cl was relatively quick in the first 4 yr of transport (E[z] = 1.34 m, after 4 yr), and slow between 4 and 34 yr after application (E[z] = 1.68 m, after 34 yr). The initially quick transport of the Cl is consistent with soil water balance models of deep drainage that include root water uptake. Precipitation inputs into the root zone are much higher than the amount of deep drainage that leaves the root zone because of evapotranspiration. Furthermore, fluxes in the root zone are depth dependant because of the infiltration process, and depth dependent root densities. For example, Tyler and Walker (1994) developed a root zone model to explain tracer movement based on the soil water balance assuming steady precipitation, and depth-dependant root extraction functions (e.g., Raats, 1974). Their model predicted that the tracer velocity in the root zone would decrease at a rate dependent on the root density profile, and reach a constant deep drainage velocity below the root zone which was dependent on the long-term soil water balance. Therefore, the average velocity of the tracer in the root zone would be greater than the velocity below the root zone. The accepted rooting depth for annual crops on the Canadian Prairies is 1.2 m (Campbell et al., 1987; Grevers et al., 1986). During the first 4 yr, Cl transport occurred dominantly through the root zone, and during the period between 4 and 34 yr after the initial application, Cl transport was dominantly below the root zone. Even so, there are still significant amounts of Cl between 0.6 and 1.2 m that may be influenced by roots. The DBTCs for 28 and 34 yr also reflect the influence of the second pulse of Cl added in 1970. The rapid transport and dispersion and mixing of the tracer within the root zone and subsequent slow transport below the root zone have merged the two-pulsed applications of Cl.

The transect-average soil water content profile measured in 2000 at the same time as the 34-yr DBTC helps to explain factors influencing the depth distribution of the Cl tracer (Fig. 3). Cumulative net infiltration by fall, winter, and spring precipitation has increased the soil water contents in the upper 0.5 m. The shallowest occurrence of the Cl tracer (above background) in the 34-yr DBTC occurs just below the infiltration front at 0.6 to 0.7 m. Below the infiltration front (0.5–1.3 m), but still within the root zone, the soil water contents are much lower; likely because the previous crop extracted most or all of the available water. Plant water extraction appears to have extended to approximately 1.3 m consistent with expectations on rooting depths in the prairies, and the start of the varved layer. The peak concentration of the Cl tracer in the 34-yr DBTC also occurs at 1.3 m (Fig. 3). The data suggest that during a cropping year, plant water extraction causes some of the Cl tracer to move upward into the rooting zone. For example, a study by Campbell et al. (1984) in southern Saskatchewan showed upward migration of subsoil nitrate into the root zone of a wheat crop. Soil water recharge in the subsequent fallow period leaches the Cl tracer downward again, toward the bottom of the rooting zone. The depth of the leaching front likely depends on the amount of annual precipitation during the fallow period, which varies considerably from year to year (Table 1).

The highest water contents occur just below the rooting zone from 1.3 to 2.5 m, and are associated with the fine-textured varved layer. Water contents decrease between 2.5 and 6.0 m and are lowest in the sand layer (z > 6.0 m). The soil water content measurements suggest the presence of a zero-flux plane (i.e., location of zero hydraulic gradient) in the 1.0- to 1.5-m zone, but this cannot be confirmed because the relationships between matric potential and soil water content have not yet been determined for the major textural layers. The influence of the varved layer, however, in limiting the depth of root water extraction and acting as a capillary- hydraulic barrier to infiltrating water is likely substantial. The water infiltration front during fallow years would have to penetrate to at least 1.2 m for the water to have a significant possibility of continuing to move down as deep drainage. For the soil water content profile given in Fig. 3, this would require an estimated 20 cm of net infiltration. The range in annual precipitation, and the fact that infiltration during fallow periods occurs over approximately an 18-mo period, suggests that deep drainage is likely occurring. The very slow average movement of the Cl between 4 and 34 yr after application, however, suggests the long-term average deep drainage is very small.

The amount of deep drainage can be estimated from the average long-term average velocity of the Cl below the root zone multiplied by the transport volume ({theta}T). Average velocity may be calculated by the change in E[z] with respect to time. The average transport volume ({theta}T) may be approximated by the average volumetric water content ({theta}) below the root zone. Factors such as anion exclusion, however, result in {theta}T being less than {theta}. Thus, an upper limit of deep drainage (Jw) was estimated from:

[9]

For the period between 4 and 34 yr after the initial application, {Delta}E[z] = 0.34 m, {Delta}t = 30 yr, making the average velocity approximately equal to 11 mm yr-1. The average volumetric water content between 1.34 m (E[z] after 4 yr) and 1.68 m (E[z] after 34 yr) is approximately 0.3 m3 m-3 making Jw = 3 mm yr-1. The deep drainage estimate of 3 mm yr-1 is comparable with the range of other estimates of deep drainage for the Canadian prairies. Joshi and Maulé (2000) used the CMB method to estimate a deep drainage of 0.2 to 2.0 mm yr-1 for a site near Saskatoon, Saskatchewan. Zebarth and de Jong (1989) used Darcy's law to estimate a deep drainage rate of 10 to 50 mm yr-1 at a site under similar management located in the black soil zone in Saskatchewan. Keller et al. (1988) found that ground water recharge for the same aquifer was spatially variable and ranged between 0.3 and 35 mm yr-1 near Warman and Dalmeny, Saskatchewan, respectively.

The horizontal coefficient of variation of local scale (single core) mean travel depth was only 4%. Thus, almost all (99%) of the transect scale (10 m) spatial variance in transport occurs at the local scale and the horizontal spatial variability of soil properties between soil cores does not appear to be a dominant factor in determining the depth distribution of the tracer. This contrasts sharply with many other field transport experiments, performed at much higher soil water flux that measured high spatial variability of local average convective fluxes. The influence of 34 yr of root zone extraction and upward movement of water during cropping years, infiltration and redistribution of water and leaching during fallow periods, and some deep net transport by drainage and diffusion has resulted in the Cl tracer being locally dispersed over a very wide depth range (0.5–6.0 m) and a large travel variance V[z] of the DBTC (Fig. 3). The transient direction of water flux, low net deep drainage, and long time periods for solute diffusion likely account for the dominance of local dispersion on the transect scale.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
The long-term (>30 yr) movement of a Cl tracer through soil in a semi-arid environment was observed. Transport was initially relatively quick with E[z] = 1.34 m 4 yr after the initial application of the Cl. Transport between 4 and 34 yr after the initial application was very slow with E[z] = 1.68 m, 34 yr after application. The relatively quick movement of most of the Cl through the root zone (0–1.2 m) and subsequent slower movement beyond the root zone is consistent with soil water balance models such as the one developed by Tyler and Walker with the root extraction function of Raats (1974). The difference in average Cl transport E[z] between 4 and 34 yr after the initial application was used to calculate a deep drainage estimate of 3 mm yr-1. Deep drainage was also suggested by the movement of some of the Cl tracer to 6.0 m. Root zone extraction and upward movement of water during cropping years, recharge of water and leaching during fallow periods, and some deep net transport by drainage and diffusion appear to be the dominant transport processes. The presence of a varved layer starting at a 1.2-m depth likely had a controlling influence on the depth of root water extraction and the amount of net deep drainage. The influence of horizontal variability of soil properties at the transect scale (10 m) on Cl transport was small. The low observed spatial variability in Cl transport, coupled with the observed homogeneity of the layers suggests that deep drainage is relatively spatially uniform within the 10-m sample transect.


   ACKNOWLEDGMENTS
 
Funding for the project was provided by the Natural Sciences and Engineering Research Council of Canada and the Saskatchewan Wheat Pool. The technical contributions of Sid Farkas, Angela Taylor, Robin Weseen, Kim Weinbender and Jaime Hogan are greatly appreciated. Special thanks go to Gordon and Kathleen Carr for the use of their land. The original work of the late Archie Ballantyne made this project possible.

Received for publication January 31, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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Right arrow Soil Hydrology

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The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Journal of
Environmental Quality