Profile of reduction in soil water storage (grass_dS) at 1000 mm soil depth and total grassland evaporation (grass3t) on a daily basis. Profile of soil water storage depletion (trees_dS) at 1000 mm soil depth and daily total transpiration of three trees (Treel_t, Tree2_t and Tree3_t).
INTRODUCTION
General problem statement
Background to the mine water problem
- Effect of different mining methods on the water in mines
- Extent of the mine water problem
- Importance of solutions to the mine water problem
- Solutions to the mine water problem
At that time, groundwater contributed to only 3% of the country's water needs compared to 13% in the 1980s (Department of Water Affairs and Forestry, 1980 cited by Hodgson et al., 2001). To reduce the intrusion of mine water, a clear understanding of the impact of mining on water quality and quantity and the environment is necessary.
Solution developed for and tested in this research project
Fast-growing trees that may use a lot of water, such as Eucalyptus, have the potential to alter the groundwater balances of sites and watersheds and address the water quality and quantity issues that mines face. The components of the groundwater balance most affected by a change in vegetation include groundwater storage and total.
Potential of solution proposed in this research project
Other international examples illustrate the impact of Eucalyptus species on water resources (Lima, 1984; Raper, 2000; Sikka et al., 2003). The effectiveness of eucalyptus species on the soil water balance under these conditions will therefore largely depend on the selected tree species, tree planting efficiency, climatic conditions, location in the landscape, planting density, topography, underground mining method, soil properties, groundwater depth and water quality .
Hypothesis of this research project
Aims of this research project
Outline of document
- Evaporation
- Transpiration
- Total evaporation
- Air temperature profile differences
- Water vapour pressure profIle differences
- A description of the heat pulse velocity equipment
- Advantages and disadvantages of the heat pulse velocity technique
- Accuracy of the heat pulse velocity technique
- Patching of heat pulse velocity data
- Gravimetric and VOlumetric soil water content
- Soil water potential
- Profile soil water content
- Soil water storage change
- Relative saturation
- A description of the technique used by the water content reflectometer
- A description of the CS615 water content reflectometer
- Accuracy of the CS615 water content reflectometer
- Advantages and disadvantages of the CS615 water content reflectometer
- A description of the heat dissipation technique
- Empirical relationships between matric potential and temperature change
- Advantages and disadvantages of the heat dissipation sensor
- A description of the thermocouple psychrometer
- Accuracy of the thermocouple psychrometer
- Advantages and disadvantages of the thermocouple psychrometer
- Actual soil evaporation
- Actual plant transpiration
- Crop growth
- Rainfall interception
- Bottom boundary condition
- General
- Climate .1 Rainfall
The electrical conductivity (EC) of soil water affects the accuracy and quality of the volumetric measurement of soil water content. The matric potential is related to the thermal conductivity of the soil water solution, equilibrated in the ceramic block.
SOUTH\AFRICA \
However, for summer a maximum daily reference evaporation of up to 6 mm d-I, and a minimum reference evaporation of 1.4 mm d-I was estimated.
Soil conditions
Vegetation
General
The Bowen ratio energy balance method requires measurements of net radiation, air temperature and water vapor pressure vertical profile differences, soil heat flux density, soil temperature and soil water content. Measurement intervals were 1 s for the air temperature and water vapor pressure profile differences, and 10 s for the net radiation, soil heat flux density, soil temperature and soil water content. The measurement of the air temperature and water vapor profile differences must be within the resolution of the sensors.
Later, a water content reflectometer (Campbell Scientific CS6l5 probe) was used to estimate volumetric soil water content at 20 minute intervals.
Methods for determining soil water content 11 and soil water potential 12
Because the accuracy of the sap flux measurements and the speed of the heat pulses depend on the distance between the probes, a drilling jig with three aligned holes was used to install the probes accurately and parallel to each other. Sensor sets (a set of sensors consists of a water content reflectometer, a heat dissipation sensor and a thermocouple psychrometer) were then installed at each site at intervals of 200 mm depth and up to 900 mm below the soil surface (Fig. 5.7). , with the first set installed at a bottom depth of 100 mm. The additional power wire was buried close to the sensors at a depth of approximately 500 mm to prevent large temperature fluctuations.
Twenty-nine wet bulb measurements were made during this period in addition to the measurement of the zero offset voltage and psychrometer block temperature (Savage et al., 1981).
Estimating climatic conditions
Five water content reflectometers per site were connected to a CR10X Campbell Scientific data logger, and five thermocouple psychrometers and five heat dissipation sensors from each site were connected to a Campbell Scientific CR7X data logger.
General
Simulation of the soil water balances with the Soil Water Atmosphere Plant (SWAP) model
The soil parameters required within SWAP for both locations were obtained from an analysis of the Rensburg soil form at the grassland location, and up to a depth of 1 m. The results of the application of the different techniques and the groundwater balance model on the grassland and E. This reduction of drainage is achieved by an increase in total evaporation and the associated decrease in groundwater storage.
Other studies have shown that groundwater storage is more depleted by Eucalyptus and other trees compared to grassland and crops (e.g.
Technique used for soil water content comparison
Patching of missing soil water content data
This malfunction of the sensors followed a large rainfall event (71.8 mm) and possible lightning damage to the sensors on 27 December 1998. There were linear relationships (data not shown) between the soil water content estimated for the 300- and 700-mm soil depths and the soil depths of 500 and 700 mm in the period prior to the malfunction (July 1 to December 26, 1998). This was the result of lightning damage to the data loggers and sensors respectively and theft of the data loggers.
Since little change in the profile of soil water storage was expected during this period, due to low transpiration rates (1 to 3 mm dol), soil water content was estimated as a linear function of time.
Seasonal changes in the total evaporation of grassland and transpiration of E. viminalis trees
Introduction
Seasonal differences in relative saturation at different soil depths
The high relative saturation at soil depths of 300 to 900 mm indicates that the grass was unable to utilize the available soil water at these soil depths. As water flows from soil depths with high soil water potential (wet soil) to those with lower soil water potential (drier soil depth), water will move downward from the 100-, 300-, and 500-mm soil depths to the 700-mm soil depth. . Although only a few soil water potential data points are available for the 900 mm soil depth, downward flow is expected to continue to propagate from the 700 mm depth to the 900 mm soil depth at the grassland site.
The water in the soil therefore moves from the wetter 500 mm soil depth to the drier 700 mm soil depth.
Cumulative effect of grassland and E. viminalis trees on the relative saturation at different soil depths under above-average rainfall
The rapid saturation of these soil depths after only 276 mm of rainfall during 1999/2000 was the result of a build-up of groundwater over time. The wettest depths at this site were the 300- and 500-mm soil depths, where little groundwater extraction occurred. The increased relative saturation for the 700- and 900-mm soil depths also indicates that tree roots could not utilize all the available soil water at these depths and that the soil wetting front reached these soil depths.
Towards the end of summer (7 February 2000), after another 360 mm of rainfall, there was little change in relative saturation in 300- to 900-mm soil depths for the grassland.
Introduction
Seasonal changes in the profile soil water contents
Cumulative profile soil water contents
Introduction
Plant and soil water relationships during spring and summer
Plant and soil water relationships during autumn and winter
Relationships between soil water storage reduction and total evapotranspiration (and transpiration) of a grass soil and E. Bottom: Reduction of soil water storage profile (grass_dS) above a depth of 1000 mm and total daily evapotranspiration of a grass land (grass3t). Bottom: Reduction of soil water storage profile (tree_dS) over a depth of 1000 mm and total daily transpiration of three trees (Treel_t, Tree2_t and Tree3_t).
During spring, the reduction in soil water storage in the pasture area exceeded that in the southeast.
Crop growth
- Grassland growth
It was hypothesized that the greatest differences in soil water balance between the two locations would occur along E. Soil parameters determined in situ and in the grassland laboratory (Tables Aland A6) were used (Table F.9). However, due to some limitations in the implementation of the SWAP model, the low saturated hydraulic conductivities for the sites (Table F.9) had to be adjusted to the lower end of the range (0.1 mm d,l) adopted in the SWAP.
The parameterization used the observed maximum soil profile depth at the test location (1200 mm).
Rainfall and total evaporation
Grass tree G-T Grass tree G-T Grass tree G-T. 1998) found that total pasture evaporation varied between 651 and 752 mm a-I, while Greenwoodet al. 1985) found that total evaporation of grazed pastures was approximately 400 mm a'l. In contrast to the lower total evapotranspiration for grasslands, Greenwood et al. 1985) it is estimated that the total evaporation from eucalyptus plantations, with a phreatophytic root system, is between 1600 and 2700 mm a,l. Therefore, there are differences in total annual evaporation between different eucalyptus species grown under different conditions.
Drainage
Eucalyptus trees, alfalfa and annual crops and pastures) was approximately equal to the difference between annual rainfall and total evaporation. After the removal of native deep-rooted eucalyptus species for agricultural and mining purposes, groundwater levels rose, causing salts to wash to the surface, increasing salinity in streams. At above-average rainfall, drainage at both sites was directly related to rainfall and the amount by which rainfall exceeded the long-decade average rainfall (Fig. 8.2).
Where the rainfall exceeded the long-term average by more than 300 mm a-I, the drainage reached maximum values above 400 mm a-I and.
Soil water storage change
Total evapotranspiration and associated profile soil water content differences calculated between grassland and E. During the experiment, the profile soil water content at the grassland site exceeded that at E. Reduction of soil water storage (soil depth 1000 mm) in the area grassland was also smaller than on E.
However, it was concluded that the differences between the reduction of groundwater storage at the grassland site and the total evaporation from the grassland will translate into the occurrence of drainage outside the root zone.
General
This work provided a comprehensive overview of the differences in the soil water relationships of grassland and E. The Highveld area of South Africa will depend on the success of vegetation establishment (trees and other) and the cover (leaf area) achieved (e.g. . Versfeldet al., 1998), as conditions in the mining area of Mpumalanga are less favorable. for forestry than other forestry areas in South Africa. As in the study by Versfeldet al. 1998) on the water use of different vegetation types in mining environments, this study does not provide "exact predictive knowledge" (Versfeldet al. 1998) in terms of water use or drainage in mining operations.
However, the current research does increase our knowledge about the possible effects of a change in vegetation on groundwater management.
Impact of trees on the regional water resources
Recommendations regarding regional water resources, the use of alternative vegetation types, and the increase of total evaporation to potential rates will be discussed below. A number of research projects have been concerned with the water consumption of grasslands and eucalyptus trees in the Highveld mining environment (Versfeldet a Jarmainet al., 2001). Local and international research into catchment hydrology has also been concerned with the effect of afforestation with eucalyptus trees on the water balance.
However, little is known in South Africa about the water use of these species, other potentially high water uses by native trees and agroforestry combinations.
Research into the increase of evaporation of trees (or high water use agricultural crops) to potential evaporation rates, through
Particle size distribution
Bulk density
Van-Genuchten parameters
Porosity
Electrical conductivity
Soil pH
Saturation extract
Sodium adsorption ratio
Colour
Structure
Consistence
Nodules
Properties of soil layers at field research sites, and that of calibration soil samples
The structure of the G horizon ranged from strong medium angular (500 mm) to weak fine blocky (700 mm). As part of the field experiment, soil water content and soil water potential were estimated at different depths. Block soil samples with dimensions of 500 mm x 300 mm x 200 mm were removed from the grassland at the end of the field experiment.
Subsoil samples were also used to determine the final soil water content and oven dry weight of the subsample.
