A numerical simulation model (CONSERVB, van Bavel and Hillel, 1976) is evaluated as a method to help identify the soil properties that are most significant in determining the evaporative loss of soil water. Thanks to Helen Richards for typing it up. references and staff of the Library and Center for Informatics and Biometrics, Lincoln University.
LIST OF TABLES
NOTATION
Xs Ratio of the space average of the temperature gradient in the solid relative to the water phase (shape factor of solid particles).
General Introduction
Overview
The computer modeling approach is a method that enables the isolation of tillage-affected variables and therefore the determination of the soil properties that have the most significant effect on the soil water balance.
Project objectives
Thesis organisation
Literature Review
Introduction
- Wind erosion
- The effects of wind erosion on agricultural productivity
Serious wind erosion events occurred in Canterbury on 4 and 15 October 1988 when north-west Foehn wind gusts reached 93 km/h (McGuigan, 1989). Wind erosion physically removes the most fertile part of the soil, including lime, fertilizers, seeds and organic matter (Daniel and Langham, 1936) and therefore reduces productivity.
The mechanics of wind erosion
- The surface wind
- Soil particle movement
- Modes of transport
Sizes larger and smaller than 0.1 mm in diameter are less erosive by wind (Chepil and Woodruff, 1963). Relatively few particles larger than about 0.84 mm in diameter are usually moved by erosive winds (Chepil and . Woodruff, 1963).
Surface soil structure
- Soil aggregation
- Aggregate formation
- Aggregate stability
- The role of soil organic matter
- Soil aggregate breakdown
- Tillage-induced surface soil structure and crop production
- Soli temperature
- Soil mechanical impedance
- Soil nutrient factors
- The Ideal seedbed from an agronomic perspective
- Tillage and soil structure
- Primary and secondary tillage
- Tillage and soli water content
Continuous air-filled pores are required in the soil down to the optimum rooting depth of the plants (Boone, 1988). The definition of the soil condition necessary for agronomic objectives and soil development.
Surface soil water
- The effect of soil water content on soil erodibility to wind
- The surface energy balance
- Shortwave albedo
- Factors affecting shortwave albedo
- Soil heat flux
- Volumetric heat capacity
- Thermal conductivity
- Soil water evaporation
- Introduction
- Climatic Influences on evaporation
- Soli water flux
- The estimation and measurement of evaporation
The continuity equation is obtained by equating the differences between the amount of heat entering and leaving a volume of soil per unit time with the change in the heat content of that soil per unit time (i.e., the law of conservation of energy). The volumetric heat capacity of a soil depends on the relative volume fractions of the soil's constituents, i.e.
Meteorological methods of evaporation estimation
This method is not easily applied because of the difficulty in determining eo (Rosenberg et al., 1983). Each combination solution assumes that the turbulent transfer coefficients for water vapor and sensible heat are equal (Rosenberg et al., 1983).
Direct measurement of evaporation with Iysimeters
- Conclusions
The relationship between soil structure and soil water conditions is complex and involves soil porosity, soli-thermal properties, microtopography, the surface energy balance and the microclimate above the soil surface. The relationship between shortwave albedo, surface soil structure and surface water content is an important aspect, as is the net effect of tillage-induced changes in shortwave albedo on soil water conditions.
Materials and Methods
Introduction
Tillage ~nd water content effects on surface-soil properties
- Experimental site
- Experimental procedure
- Experimental measurements
- Near-saturation hydraulic conductivity
- Pore size distribution
- Shortwave albedo
Then they placed it on top of the core and monitored the rate of water draining from the infltrometer. Jackson's method is a further development of the Childs and Collis-George model (1950) and the Marshall model (1958). This is the equation that will be evaluated here to calculate the K(Vlm) ratio.
In this study, near-saturated hydraulic conductivity was used for matching due to the large macropore volume expected in the tilled soil.
Evaporation from a bare soil surface
- Experimental procedure
- Experimental measurements
- Soil water evaporation
- Soli water content
- Soil temperature
- Net radiation
- Wind velocity height profile
- Relative humidity
- Soil surface roughness
The instrument was placed 1.5 m above the ground surface in the southeast quarter of the plot. Wind speed at a height of 2.0 m was recorded at the center of the plot with a three-cup pulse-counting anemometer (Synchrotac, Melbourne, Australia). The altitudinal profile of wind speed was measured at the center of the plot using Rimco three-cup miniature anemometers (Selbys Scientific Umited) mounted on the elevations.
The air temperature was measured simultaneously with the relative humidity also at 2 m and in the center of the plot to allow for vapor pressure deficit and dew point.
The Effects of Multiple-pass Tillage on Surface Soil Physical Properties
Introduction
Aggregate size distribution
AGGREGA TE DIAMETER (mm)
Aggregate stability
Abrasion by the impact of particles transported by wind along the soil surface is an important phase of the wind erosion process on all soils. Mechanical stability of soil aggregates gives a relative measure of the resistance to disintegration by abrasion experienced by the soli when eroded by wind. Some of the clods formed under these conditions could have been reformed from fine particles showing cohesion due to relatively high water content.
The low relative stability of the aggregates produced from tillage and soil water content near the IPL is undesirable in terms of both surface crust formation and wind erosion susceptibility.
Soil surface roughness
- Introduction
- Distributional form of the data set
- A comparison of surface roughness indices·
- Tillage treatment effects on random roughness
- Soil surface area
Whether oriented roughness is removed from the data depends on the application of the resulting roughness index. LD is an estimate of the central tendency of the height difference between different points. An equation in the form of equation 4.6 was fitted to the data as suggested by Linden and van Doren (1986).
A relationship similar in form to that proposed by Linden and van Doren (1986) appears in most data sets.
Dry bulk density
Conclusions
Clods formed during tillage near the lower plastic limit are significantly less stable than clods formed during tillage at lower soil water contents. The 'minimal' tillage formed a soli structure where the dry bulk density was lower than with the other two tillages.
CHAPTERS
The Effects of Multiple-pass Tillage on Surface Soil hydraulic and
Thermal Properties, and Shortwave Albedo
Introduction
Soil porosity
- Introduction
- Total porosity
- The soil water characteristic
- Functional pore size classes
- Further discussion
Effect of soil water content before tillage and tillage operations on total porosity (Standard error of the mean = 0.86%; means labeled with the same letter are not significantly different at the 5% level as determined using the New Multiple Test of Duncan). Effect of soil water content before tillage and soil operations on macroporosity (Standard error of the mean = 1.4%; means labeled with the same letter are not significantly different at the 5% level as determined using the New Duncan's Multiple ). Effect of soil water content before tillage and soil operations on aeration porosity (Standard error of the mean = 1.7%; means labeled with the same letter are not significantly different at the 5% level as determined using the New Duncan's multiple).
Effect of pre-tillage soil water content and tillage operations on transmission porosity (Standard error of the mean = 0.77%; means marked with the same letter are not significantly different at the 5% level as determined using Duncan's New Multiple Range Test).
Near-saturation hydraulic conductivity
This would likely cause a decrease in total porosity, mainly due to a reduction in the volume of the less stable macropores (Klute, 1982). By wetting soil samples while measuring the water characteristic, the freshly tilled soil will have settled slightly. When a soil is saturated, all pores are filled with water and conductive, maximizing the conductivity of the soil and the continuity of the water films.
As more pores are emptied, the rotation of the continuous water films increases as water must flow around the air-filled pores.
Unsaturated hydraulic conductivity
- Introduction
- Sensitivity analysis
- Application of the Jackson (1972) method
Point (i) lies on the detailed water characteristic curve for initialization (i) at the position corresponding to that where the conductivity of the matching factor is determined. The values of the matching factor K were determined at the same matrix potential, but these represent different 9y values. Matching K-factor values were determined at the same 9v values, but these represent different I/Im values.
The procedure is very sensitive to a change in 9y corresponding to the matching factor K value (9y(mf)).
Soil thermal properties
The higher water content at saturation in the high porosity treatments tends to offset the corresponding lower contribution to the total thermal conductivity of the soli matrix that comes from the solid soil fraction. When saturated the same increase in total porosity resulted in an 11.4% increase in thermal conductivity. Increasing porosity led to a decrease in volumetric heat capacity of an oven-dry soil, but an increase in volumetric heat capacity of a saturated soil (Table 5.2).
This is because the volumetric heat capacity of groundwater is greater than that of the solid soil fraction.
Bare soil shortwave albedo
- Introduction
- Zenith angle effects on shortwave albedo
- Soil water content and shortwave albedo
- Surface soil structure and shortwave albedo
- Further discussion
In reflectance with water content at the soil surface shown by Idso et al. 1975) could have been an artifact of their extrapolation process. Reflection is expected to be a function of the water content of the soil surface itself. 20 mm soil depth and does not show the correlation between shortwave albedo and the water content 'at the very surface' of the soil.
However, the size and number of samples for groundwater content and the random sampling procedure should ensure that the results take into account the observed spatial variability.
Conclusions
It would be advantageous to be able to determine the extent to which the soil water regime can be manipulated by controlling surface soil structure to change shortwave albedo. The water content of surface soils has a large effect on bare soil shortwave albedo with a dry soil having a higher shortwave albedo than a wet soil. Soils with coarse surface soil texture generally had slightly reduced albedo, although the differences in reflectance across the range of surface textures produced in this study were small and not always consistent.
Bare soil reflectance is more closely related to the small scale surface roughness derived from aggregate size distribution than to the larger scale random surface roughness index (LD index).
Numerical Simulation of Soil Water Flux
Introduction
- Model development • a validation problem
- Soil-water simulation models - a review
- The CONSERVB model
- General description
- Model inputs
- Model solution sequence
- Previous evaluations of CONSERVB
- A pre-verification assessment of CONSERVB
- Conclusions
- Introduction
- Experimental method and model initialisation
- Verification method
- Model initialisation
- Results and discussion
- A preliminary investigation
- A comparison between measured and simulated evaporation
- An evaluation of the simulated soil surface energy balance
Generates the current evaporation rate from environmental weather data and from current values of soil water content and. Sensible heat flux into the air (H, W m-2) and evaporation rate (E, kg m-2 s-1) are calculated by defining. C, (J m-3) is the volumetric heat capacity of the air, and Hs (kg m-3) is the absolute humidity of the air on the ground surface.
To evaluate the sensitivity of the CONSERVB model to changes, a log-linear regression function was fitted in the K(9v) function to the data points previously calculated using Jackson's (1972) procedure.
DAY 75
- A comparison of measured and simulated soil water and temperature profiles
A similar comparison occurs in the second drying cycle where the simulated net radiation was within 40 W m-2 of the measured value. The simulated rise in morning surface soil temperature will appear to occur 20 to 30 minutes before midday. The results shown are representative of the intermediate and final stages of each drying cycle.
However, for most of the day, simulated soil temperatures are within 2-3 °c of the measured values.
DAY 78
DAY 80
DAY 97
DAY 99
- Sensitivity and error analysis
- The application of the CONSERVe model to a range of tillage-induced soil structural
The results in Table 7.7 follow the investigation of the sensitivity of the simulated soil surface temperature to errors in model input parameters. The modified value (Table 7.7) represents the calculated value when the quartz share is increased by 10% while the organic matter share remains constant (i.e. the clay share is reduced to compensate). The results indicate that precise measurements of the relative proportions of quartz, organic matter, and clay minerals are not required for estimating these soil thermal properties in soils.
The low sensitivity of the model to a change corresponding to the estimated maximum error of this parameter would indicate that a generalized emission function is sufficient in the CONSERVB model.
Model initialisation
The K(9y) function and the water characteristic (ljIm(9y)) for the soil 'B' horizon are assumed unchanged by the three tillage treatments. The functions used are the same as in previous simulations and are shown in Figures 7.2 and 7.3. The 'A' horizon water characteristic for each of the three tillage treatments used in the model.
The horizon 'A' function K(9,) for each soil treatment was estimated from water characteristics using the method of Jackson (1972).
Results and discussion
The differences in cumulative evaporation between treatments at the end of the drying cycle were large, with the 'Intermediate' treatment resulting in 8.5 mm more evaporation than the 'Excess' treatment.
