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Published in Soil Sci. Soc. Am. J. 68:1138-1144 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-1—SOIL PHYSICS

Tillage Effects on Subsurface Drainage

Toshitsugu Moroizumia,* and Haruhiko Horinob

a Faculty of Environmental Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Okayama 700-8530, Japan
b College of Agriculture, Osaka Prefecture University, 1-1 Gakuen-town, Sakai-city, Osaka 599-8531, Japan

* Corresponding author (morot{at}cc.okayama-u.ac.jp).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tillage near the soil surface may greatly influence drainage discharge and pressure head values in the subsurface zone. In this study, a controlled comparative experiment was conducted in the field under natural weather conditions, using a tilled and an untilled column, to evaluate the effects of tillage on subsurface drainage discharge and pressure head values. There were no significant differences between the two columns in subsurface drainage before tillage treatment before the study, as checked by a preliminary experiment. After tilling one column of the two columns, cumulative subsurface drainage discharge was larger and occurred earlier for the tilled column than for the untilled column. The measured drainage discharge and pressure head values were evaluated using the water movement model, HYDRUS version 6.0, which numerically solves the Richards equation. The HYDRUS model reproduced measured values well for subsurface drainage discharge and pressure head values after the tillage, as determined by root mean squared error (RMSE), mean bias error (MBE), and R2. Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns. Moreover, the effects of tillage depth on subsurface drainage discharge were simulated. The results implied that the effect of tillage on subsurface drainage were induced by the conditions of pressure head just before the first rainfall, and this effect was equal to, or greater than, the effects of changes in hydraulic properties due to tillage.

Abbreviations: MBE, mean bias error • RMSE, root mean squared error • R2, coefficient of determination


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TILLAGE OF FARMLAND plays an important role not only as a soil management tool for improving the plant root environment, but also as a factor that impacts the hydrological cycle, including infiltration, surface evaporation, subsurface drainage, and groundwater discharge. Considering the fact that farmland covers large areas, the influences of tillage on subsurface drainage and groundwater discharge could be significant.

Changes in soil hydraulic properties of the surface soil induced by tillage can alter the infiltration rate of rain or irrigation and significantly affect the subsurface drainage discharge. Bulk density, porosity, and saturated hydraulic conductivity are better indicators of the tillage effects on soil hydraulic properties (e.g., Mielke et al., 1986; Benjamin, 1993). In general, soil tillage increases porosity and reduces bulk density, leading to changes in water retention function, hydraulic conductivity, and infiltration rate. Kribaa et al. (2001) showed that the tillage increased hydraulic conductivity, especially at pressure heads close to saturation.

Bulk density and porosity, however, were not sufficient to compare tillage effects on water flow through soils, and pore continuity was an important factor for water flow in soils for comparisons between the tillage systems (Ehlers, 1975; Ball et al., 1988; Sauer et al., 1990). Benjamin (1993) reported that the no-till system had a hydraulic conductivity equal to or greater than both the moldboard plow and chisel plow systems, owing to either a greater conductivity of pores or to water flow through a few very large pores. Wu et al. (1992) indicated that a no-tillage treatment had greater numbers of continuous macropores and equal or greater saturated hydraulic conductivity than the moldboard plow tillage.

Hill et al. (1985) and Hill (1990) reported that conventionally tilled soils drained more rapidly than soils under conservational tillage. Recently tilled soil also has rapid infiltration until settled by rainfall and irrigation. However, the effects of tillage on soil hydraulic properties, infiltration rate, and subsurface drainage discharge generally depend on spatial as well as temporal variabilities (Logsdon et al., 1993; Logsdon and Jaynes, 1996). Therefore, it is necessary to grasp the characteristics (subsurface drainage and pressure head values in this study) of experimental sites under untilled conditions to more accurately examine the effects of tillage under a controlled comparative experiment.

Through a controlled comparative experimental method, the degree of difference between tillage treatments can be evaluated more precisely. Moroizumi (1998) conducted a controlled comparative experiment in tilled and untilled areas, which proved to be an effective method for evaluating the effects of tillage on soil temperature, pressure head values, and evaporation. Moroizumi et al. (2001) also investigated the effects of subsoil breaking on surface runoff, pressure head values, and soil temperature in a sloping field with both a subsoil breaking area and a controlled area. It was clear that the subsoil breaking resulted in a decrease in the surface runoff and caused variations in pressure head values to be larger than in the controlled area.

A model analysis is a useful tool for evaluating quantitatively what factors induce the effects of tillage on subsurface drainage. Several studies have examined modeled tillage effects on water movement within tilled soils. Most of them included heat transport and were aimed at finding the effects of tillage on soil moisture, soil temperature, and evaporation by using the coupled water and heat transport model (e.g., Hammel et al., 1981; Yang and Zeng, 1988; Benjamin et al., 1990; Moroizumi and Horino, 2002). However, there have been few studies on modeling the impact of tillage on soil hydraulic properties and subsurface drainage. Mapa et al. (1985) demonstrated the impact of temporal changes in hydraulic functions on soil water movement, and compared these results with water content profiles predicted with a simulation model using input functions obtained before and after irrigation. Their results indicated that appropriate input hydraulic parameters sufficiently described a simulated water movement at different times when varying the deformation of tilled soil with wetting and drying over time. Ahuja et al. (1998) proposed two simple methods for predicting the water retention curve of a tilled soil from that of an untilled soil, and obtained a good approximation with RMSE of 0.0066 and R2 of 0.99.

The objectives of this study were to estimate the influences of tillage on subsurface drainage and pressure heads using a controlled comparative experiment. Preliminary experiment was performed with two soil columns before tillage treatment to make sure that the two columns had similar soil properties. The experimental data were analyzed, using the one-dimensional water flow model, HYDRUS version 6.0. The model performance was verified by comparing the numerical results with the experimental data. Moreover, the influence of tillage depth on drainage discharge was simulated.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experiments
Field Experiment
Field experiments were performed on a 10 by 20 m bare soil area at the School of Veterinary and Animal Sciences, Kitasato University, in Aomori prefecture, Japan. This area is located in the northeastern part of the main island of Japan. The soil at this site has volcanic ash parent material, being an andosol that is called ‘Kuroboku’ soils in Japanese soil classification system. The general characteristics of ‘Kuroboku’ soil are low in bulk density (0.6–0.9 g cm–3), high aggregation, high porosity (0.6–0.8 cm3 cm–3), and high permeability (10–2–10–3 cm s–1; Soma et al., 1983; Maeda and Soma, 1979). The apparent soil texture ranged from sandy loam to sand. Two soil columns were buried in the area to estimate the effects of tillage on subsurface drainage and pressure heads.

Each soil column (49.2 cm in diameter and 110.8 cm long) was fitted with a drain pipe (3.0 cm in diameter) at the bottom. The columns were packed from the surface with sand soil material between 50- and 100-cm depths and sandy loam from 0 to 50 cm; this layering was the same as the soil profile in the field around each column. The columns were also packed with coarse sand and gravel, as a filter material, below a depth of 100 cm. Pressure head values were measured at depths of 5, 10, 25, 45, 65, 80, and 100 cm, using tensiometers with pressure transducers, and were recorded automatically at 5-min intervals with a datalogger controlled by a portable computer. The data were converted to 1-h average values, which were used for numerical analysis. The subsurface drainage discharge at the bottom was measured manually every 2 to 9 h, using a graduated cylinder. The rainfall was measured with a tipping-bucket rain gauge.

Subsurface drainage discharge and pressure heads were measured intensively from Day 247 to 255 of 1993. The experimental results reflect the transitory, short-term effects of tillage operation because the period of the measurement was limited. This period corresponds to late summer in Japan. A preliminary experiment was conducted from Day 239 to 243 to confirm whether or not the two columns showed a difference with respect to subsurface drainage discharge before tillage treatment. As a result, pressure head values for the two columns were almost the same before tillage (Moroizumi, 1998). After the pre-experiment, one of the two columns (called Column 2) was tilled crosswise by hand, using a hoe to simulate a small hand-plow. The soil around the tilled column was also tilled so that it had the similar environmental conditions to prevent from an advection effect. The depth of tillage was approximately 4.5 cm. We emphasize that this shallow tillage was conducted to measure the impacts of hydrological processes. Tillage was conducted during 1010 to 1050 h on Day 244.

Laboratory Experiments
Some of the basic physical properties of the soils were measured in the laboratory. Eight undisturbed soil cores from each layer (0 to 50 cm and 50 to 100 cm in the untilled layer, and 0 to 4.5 cm in the tilled layer) were taken immediately after finishing the experiments. The size of core sampler was 5.0 cm in diameter and 5.1 cm in height. It should be noticed that the 0 to 4.5 cm in the tilled layer was sampled using this sampler. Dry bulk density and porosity were calculated by the gravimetric method using a dry oven. Saturated hydraulic conductivity was measured with a constant-head permeameter. Particle-size distribution was measured by a mechanical analysis (hydrometer method), and the soil textures were determined by a textural triangle. A pulverizability index indicated the soil is pulverized. A sample of tilled soil was separated into six aggregate groups using the five graded sieves with opening of 0.5, 1, 2, 4, and 8 cm. Pulverizability is calculated by dividing the mass of each clod group by the total mass of the tilled soil and expressed as a percentage (JSIDRE, 1992).

Model Description
Governing Equation for Water Flow
One-dimensional vertical water movement in variably saturated soil is described by the following mixed form formulation of Richards equation:

[1]
where {theta} is the volumetric water content (L3 L–3), h is the soil water pressure head (L), t is time (T), z is the vertical space coordinate (L), and K is hydraulic conductivity (L T–1). The sink term due to plant water uptake is not included in Eq. [1] because the experimental data were collected under bare surface conditions. Equation [1] assumes that the water vapor movement due to thermal gradients can be disregarded, which is negligible during the infiltration process with rainfall and the wet condition of soils. For a relatively dry soil, however, the water vapor movement should be considered, and an analysis using the coupled heat and water flow model is required (e.g., Scanlon and Milly, 1994; Moroizumi et al., 1997).

Nonlinear partial differential Eq. [1] was solved numerically by the computer code HYDRUS version 6.0, which is free software revised and updated by Simunek et al. (1998). In this code, Eq. [1] is solved with a mass-lumped linear finite element scheme for the spatial discretization of the flow domain and a Picard iteration procedure at each time step. The time derivative is approximated by a fully implicit difference scheme. The element lengths are shorter when approaching the soil surface where pressure head values varied rapidly because of rainfall, irrigation, and evaporation (and also due to tillage).

Hydraulic Properties
The soil water retention property and hydraulic conductivity were expressed by the van Genucthen (1980) function with the Mualem pore-size distribution model:

[2]

[3]
where {theta}r is the residual soil water content (L3 L–3), {theta}s is the saturated soil water content (L3 L–3), Ks is the saturated hydraulic conductivity (L T–1), {alpha} is the fitting parameter in the soil retention curve (L–1), n is also the fitting parameter in the soil retention curve (dimensionless), and Se is the effective water content [= ({theta} – {theta}r)/({theta}s – {theta}r); dimensionless]. The pore-connectivity parameter l in the hydraulic conductivity function of Eq. [3] is 0.5; this was estimated as an average for many soils by Mualem (1976). The {alpha} and n were calculated to fit the laboratory water retention data using Powell's conjugate direction method (Powell, 1964). The water retention data were measured using the soil column method and the pressure plate method. The {theta}r was measured in the laboratory on the basis of Bresler et al.'s (1978) definition.

Hydraulic conductivity K({theta}) was estimated using the fitting parameters of the soil water retention curve.

Initial and Boundary Conditions
A surface boundary condition was specified by the Neumann condition, which was given by precipitation and the maximum potential rate of evaporation under atmospheric conditions. A seepage face at the bottom was used as a bottom boundary condition. This boundary condition often applies to a finite lysimeter, such as the one used in this study, which is allowed to drain under gravity (Simunek et al., 1998).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Physical Properties
The results of the basic physical properties of soils are presented in Table 1. The soil textures by the International Soil Science Society classification range from sandy loam to sand. They are "apparent" textures because these andosols usually do not disperse completely into separate particles by the common dispersion methods and the hydroxide particles remained aggregated (Moroizumi and Horino, 2002). The porosity and saturated hydraulic conductivity are greater in the tilled layer than in the untilled layer and the dry bulk density is reduced in the tilled, which is the general characteristic of the effects of tillage (Ahuja et al., 1998). The pulverizability of 0 to 1 cm in the tilled layer was 64.8%.


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  		Table 1. Physical properties of soils packed in the columns.

 
The relationship between measured and fitted soil water retention curves are showed in Fig. 1. The fitted parameters are also presented in Table 2. Figure 1(a) indicates that pressure heads in the tilled soil for a range in soil water content of 0.2 to 0.65 were larger (less negative) than those in the untilled soil. A decrease of bulk density by tillage decreased the soil water pressure head at the same water content (Gupta and Larson, 1979). The effects of tillage on pressure head were restricted to the wet range of soil water content; at less soil water content, the water retention was essentially unchanged (Ahuja et al., 1998). The {alpha} in the tilled soil was about ten times greater than that in the untilled soil because of the decrease in the air-entry pressure head in the tilled soil. It is inferred that 1/{alpha} is related to the air-entry pressure head because the dimension of 1/{alpha} is ‘length’. The air-entry pressure head generally increases as the bulk density decreases as seen in the tilled soils (Gupta and Larson, 1979).



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  		Fig. 1. Soil water retention curves for (a) sandy loam (tilled and untilled) and (b) sand.

 

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  		Table 2. Fitted parameters of the van Genuchten function (Eq. [2] in the text) for hydraulic properties.{dagger}

 
Figure 2 shows the hydraulic conductivity function K({theta}) estimated using the fitting parameters of the soil water retention curve.



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  		Fig. 2. Hydraulic conductivity as a function of water content for (a) sandy loam (tilled and untilled) and (b) sand.

 
Experimental Results
The results of the preliminary drainage experiment before the tillage is shown in Fig. 3. Subsurface drainage discharge started at 9 h after the first rainfall. The total subsurface drainage discharge was 22.2 mm for Column 1 and 21.4 mm for Column 2 for a total rainfall of 32.5 mm. The subsurface drainage discharge between the two columns was not significantly different at 0.05 probability level by t test. The result implies that the characteristics for drainage discharge and infiltration of rainfall were the same between the two columns. Therefore, we concluded that it was possible to evaluate precisely the effect of tillage on subsurface drainage after tillage, using the two columns.



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  		Fig. 3. Measured values for cumulative subsurface drainage discharge for Column 1 (solid circle) and Column 2 (open triangle) during the preliminary experiment from Day 239 to 243.

 
Figure 4 shows the initial condition for the pressure at the start of the experiment, which already included the effects of tillage on pressure heads in the drying process before the experiment. The dying process continued for 6 d, from Day 241 to 246.



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  		Fig. 4. Profiles of pressure heads at the start of the major experiment (0000 h on Day 247). This profile was used as the initial conditions for the drainage simulations.

 
Figure 5 presents the experimental results after one of the two columns was tilled. There were two main occurrences of precipitation on Day 247 to 248 and Day 251 to 253. The total amount of each precipitation was 45.5 mm for Day 247 to 248 and 52.0 mm for Day 251 to 253. Runoff and crust at the two columns were not observed. The onset of drainage for the tilled column was at 1700 h on Day 248, namely, 11 h earlier than that for the untilled column. The total amount of subsurface drainage discharge was 55.2 mm for the tilled column and 38.8 mm for the untilled column. The difference of 16.4 mm was significant at 0.05 probability level. We considered that the difference of drainage discharge would be mainly induced by the change of the water retention curve (Fig. 1) and hydraulic conductivity (Table 1).



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  		Fig. 5. Simulated and measured values for cumulative subsurface drainage discharge in the tilled and the untilled column, during Day 247 to 255 in 1993.

 
Model Validations
Subsurface Drainage
Figure 5 also presents simulated values for the cumulative amount of drainage water in each column. Both simulated values were slightly larger than the measured values after Day 252. The simulated value for the tilled column at the end of the experiment was 60.5 mm, which was 5.3 mm greater than that for the measured value. Similarly, the simulated value for the untilled column was 45.7 mm, which was 6.9 mm greater than the measured value.

Statistical comparisons are very useful for quantitatively verifying model performance. For the tilled column, RMSE between measured and simulated values was 2.6 mm and the MBE was –1.4 mm. The untilled column showed an RMSE of 3.4 mm and a MBE of –1.29 mm. The R2 was 0.991 for the tilled column and 0.988 for the untilled column. We judged from these statistical analyzes that the simulated and the measured values were in good agreement. These results indicate that the model satisfactorily reproduced the effect of tillage on subsurface drainage discharge.

Pressure Head
Figure 6 presents the simulated and measured pressure head values at depths of 5, 10, 25, 45, 65, and 80 cm for tilled and untilled columns. Several pressure head values (e.g., during the 1600 h on Day 249 to the 1500 h on Day 250 and the 0000 h to the 2400 h on Day 253) could not be measured because of a temporary failure of the data logging system.



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  		Fig. 6. Simulated and experimental results of the temporal changes in pressure head at depths of 5, 10, 25, 45, 65, and 80 cm, with rainfall, from Day 247 to 255 in 1993.

 
As a whole, variations in simulated head values tended to be similar to those measured, although the simulated values for the increase in pressure head values influenced by rainfall were slightly lower than the measured values. One reason might be due to the a hysteresis in the water retention curves that was not included in the numerical analysis in this study; therefore, the simulated data did not show the increase in pressure head values by rainfall, namely, a wetting process. However, the timing of the impact of rainfall on the pressure head values at depths of 5, 10, 25, and 45 cm were well reproduced. At depths of 65 and 80 cm, the pressure head values were almost unaffected by the rainfall and the simulated values were in good agreement with the measured ones.

The statistical comparisons are presented in Table 3. The RMSE and the MBE for each depth were not very large (RMSE = 2.4 to 32.0 cm and MBE = –20.4 to –0.9 cm for the tilled column, and RMSE = 2.5 to 26.4 cm and MBE = –16.6 to –0.7 cm for the untilled column). The R2 ranged from 0.76 to 0.97 for the tilled column and from 0.89 to 0.97 for the untilled column. The model is satisfactory for evaluating pressure head values as it is for the subsurface drainage discharge.


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  		Table 3. Root mean square error (RMSE), mean bias error (MBE), and coefficient of determination (R2) between measured and simulated pressure heads for the tilled and the untilled column.{dagger}

 
Simulation of Tillage Depth Influence on Subsurface Drainage Discharge
The experiment was conducted only for the tillage depth of 4.5 cm. To investigate the effects of other tillage depths on subsurface drainage, the changes in subsurface drainage discharge for varying tillage depths were simulated (Fig. 7). The initial and boundary conditions were the same as those in the tillage depth of 4.5 cm. The hydraulic properties of the tilled layer used in the field experiment were applied for each tilled layer in the simulations. The onset and the trends in time changes for the subsurface drainage discharge for tillage depths were almost the same. The discharge amounts were 59.2 mm for a tillage depth of 2 cm, 62.2 mm for a tillage depth of 10 cm, 62.5 mm for a tillage depth of 40 cm, and 63.5 mm for a tillage depth of 60 cm. Subsurface drainage discharge values increased only slightly as the tillage depth increased. The results of the simulations indicated that the change of hydraulic properties by the tillage could not be enough to explain the difference of subsurface drainage discharge between the tilled and the untilled columns.



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  		Fig. 7. Cumulative subsurface drainage discharges for the simulated tillage depths of 2, 10, 40, and 60 cm.

 
Initial Condition Influence on Subsurface Drainage
Moroizumi and Horino (2002) presented that the decrease in pressure heads beneath the tilled layer in the tilled columns was suppressed during the drying process, compared with those in the untilled column. The difference of initial pressure heads at the beginning of the experiment (Fig. 4) in this study includes the effects of tillage on pressure heads during the drying process, and therefore, it should be noted as another factor that is different from the hydraulic properties.

Soil water storage calculated from the initial profile of pressure head was 16 mm greater in the tilled column than in the untilled column. The difference of 16.0 mm was approximately the same as the difference of 16.4 mm of the final drainage discharge. The same amount of rainfall water, on the other hand, infiltrated into the two columns because runoffs were not observed at the two columns. Therefore, we can infer from these results that the tilled column stored less and drained more rainwater by this amount (16.0 mm) than the untilled column.

Essentially, the initial condition at the beginning of the drainage experiment must change in accordance with the tillage depth, because tillage develops a profile of pressure heads under the drying process as mentioned above. In the simulations, however, the same initial condition as for the tillage depth of 4.5 cm was specified for all tillage depths, and therefore, the effect of the tillage depths would be small in simulations. If the initial conditions of pressure head were suitable to all tillage depths, the drainage discharge between the tillage depths would have been more different.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
A controlled comparative experiment was conducted using a tilled and an untilled column to evaluate the effects of tillage on subsurface drainage discharge and pressure head. Before the tillage experiment, a preliminary experiment was performed to check the differences between the two columns. There were no significant differences in regards to subsurface drainage discharge values between the two columns. After the tillage, the total subsurface drainage discharge for the tilled column was larger than that for the untilled column, and the onset of drainage was earlier for the tilled column than for the untilled column due to an increase of hydraulic conductivity in the tilled layer.

Measured subsurface drainage discharge and pressure head values were analyzed by the numerical model to test performance of model. The HYDRUS model used in this study reproduced measured values for subsurface drainage discharge and pressure head values after the tillage, as determined by RMSE, MBE, and R2. Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns.

Moreover, the effects of tillage depths on subsurface drainage discharge were simulated. The simulated cumulative subsurface drainage discharge increased only slightly with increase in tillage depth, which was contrary to our expectation. It was attributed to the setting of initial condition that treated all tillage depths as the same as that of tillage depth of 4.5 cm, in spite of including the effect of the tillage depth on the profile of pressure heads. It was concluded that the initial condition was an important factor for the effects of tillage on subsurface drainage.


   ACKNOWLEDGMENTS
 
The authors thank Hiromitsu Yanagida, Mitsuhiro Sugimoto, Shuichi Kitami, Takehiro Toyosaka, Hironori Siba, and Jun Maruoka of Kitasato University for their help with this experiment. The authors also benefited from many helpful discussions with Dr. Yuichi Sato and Dr. Koichi Sato of Kitasato University. This study was funded in part by Grant-in-Aid for the Encouragement of Young Scientists in 1991.

Received for publication February 19, 2003.


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




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