CHAPTER 3 FIELD SITES, MONITORING AND MODELLING
In this Chapter, a detailed description of the field sites, the monitoring undertaken and modelling and data processing, is presented. For the field site description, the location and experimental layout, the soil conditions, irrigation water qualities and cropping systems are also described. This will highlight the wide range of conditions investigated in this study. Under monitoring, the equipment used and the measurements taken on atmospheric evaporative demand, crop growth and nutritional status, soil water balance components and salt balance measurements are described. Finally, in the brief description of the modelling section, the approach taken to process the data and model the root zone soil water and salt balance, is given.
Field site locations and experimental layout
The study was carried out at four mines: Kleinkopjé Colliery near Witbank, New Vaal Colliery near Vereeniging, Syferfontein near Secunda and Waterberg CBM pilot project near Lephalale. Two irrigation systems were designed for the Waterberg CBM irrigation trial: drip and sprinkler that were set up on separate blocks. The remaining sites were centre pivot irrigated.
This Anglo Coal-mine is located in Mpumalanga Province (Latitude 26o28’S, Longitude 28o75’E, Altitude 1570 m). Pivot Major (30 ha) and Pivot Tweefontein (20 ha), abbreviated as TWF, is on rehabilitated open cast soils. These two fields have been irrigated with mine water since 1997. Pivot Four (30 ha) is a virgin site that has been irrigated since the winter season of 1999. Figure 3.1 shows the position of the pivots (Pivot Major, Pivot TWF and Pivot Four) and Figure 3.2 shows the experimental layout of Pivot Major, Pivot Four and Pivot TWF. Figure 3.2 includes the position of intensive monitoring sites and runoff weirs. During the 2000/01summer season at Kleinkopjé, two adjacent intensive monitoring stations were installed in the maize fields of all three pivots. Two adjacent intensive monitoring stations were also installed during the 2001/02 season in Pivot Four, which was planted to potatoes at the time. In all other seasons at Kleinkopjé, as well as the other sites, a single intensive monitoring station was installed in each field.
This Anglo Coal-mine is in Free State Province (Latitude 26o42’ S, Longitude 27o55’ E, Altitude 1432 m), and is located on the Southern bank of the Vaal River. The 10 ha field is placed close to the river in an area that had been mined in the past by underground mining method. Figure 3.3 shows the position of the field and Figure 3.4 shows the experimental layout of the Pivot. Monitoring at this site started in November 2001. This site was already erected before by the mines before the experiment was started. Unfortunately, the positioning of this site was inappropriate, as internal drainage problems plagued the research.
This Sasol Coal-mine is in Mpumalanga Province (Latitude 23o64’ S, Longitude 29o20’E, Altitude 1570 m). The 20.6 ha field had received some irrigation with mine water before the trial commenced, so the research did not begin with pristine conditions. Figure 3.5 shows the regional setting of the irrigation site and Figure 3.6 shows experimental layout of this field, which includes intensively monitored plots.
The Waterberg CBM pilot project is in the Limpopo Province (Latitude 23o68’N, Longitude 27o70’S and Altitude 839 m), located 30 km North West of Lephalale (Ellisras). The irrigation site selected was in the natural veld approximately 100 m from the CBM production water reservoir. The total area of the site was 1440 m2. Figures 3.7 to 3.10 show a schematic diagram and experimental layout of the drip and sprinkler irrigation systems.
The cropping systems include 18 growing seasons for TWF, 17 growing season for Major and 15 seasons of different cropping systems at Pivot Four, 7 growing seasons at New Vaal and nine harvests at Syferfontein. In the Waterberg, two irrigation trials were carried out in the winter season 2005 and summer 2005/06 seasons. Each growing period included records of leaf area index (LAI), dry matter (DM), plant chemical analysis, and volumetric water content measurements with a neutron water meter (NWM) and soil solution chemical analysis results.
The fields at Kleinkopjé were cropped to annual cash crops, and these included maize, wheat, sugarbeans and potatoes. The yields of maize and wheat are expressed as air-dry grain masses, whilst potato and sugarbeans are fresh mass. An example of maize irrigated with gypsum rich mine water is in Figure 3.11.
At first wheat and maize were the crops of choice, and then an attempt was made to produce vegetables such as peas, sweetcorn, pumpkin and soybean. An example of Sweet corn grown at New Vaal is shown in Figure 3.12.
Due to the heavy clay soil that would make cultivation extremely difficult, the mine decided to establish a perennial Fescue pasture. Five temperate and subtropical, annual and perennial pastures were then established as part of this research in small plots that were fenced off separately to prevent grazing animals from eating the fodder and damaging instruments (Figure 3.13). The pastures planted are listed in Table 3.1.
Salt tolerant crops of barley (Hordeum vulgare cv. Puma), and a mixture of an Italian ryegrass (Lolium multiforum cv. Agriton (Diploid)) and stooling rye (Secale cereale cv. Echo) were planted in the 2005 winter season (Figure 3.14), whereas cotton (Gossypium hirsutum cv. Opal) and Bermuda grass (Cynodon dactylon cv. K11) were planted in the summer 2005/06 season (Figure 3.15).
Harvests for the Waterberg CBM trial are presented for the winter 2005 and summer 2005/06 experiments. Barley and ryegrass were harvested before they reached maturity, as infiltration became problematic and ponding occurred. Bermuda grass was harvested when it reached the flowering stage and yield was determined. Cotton was harvested three times by hand from April to May 2006, and lint quality (uniformity (%), length (cm), micronaire (µg cm2), strength (grams per tex)), seed cotton mass (g) were determined using a laboratory gin by Cotton South Africa, in Pretoria. Uniformity (%) shows the degree to which the fibres in a sample are uniform based on the ratio of mean length to the upper half mean length. Length (cm) describes the average length of cotton fibres after the ginning process. Micronaire (µg cm-2) quantifies the mass of an individual cotton fibre taken in cross-section. Strength expresses the force required to break a bundle of fibres in grams per tex (a tex unit is equal to the weight in grams of 1,000 meters of fibre). Seed cotton (g) represents the mass of unginned cotton.
This section discusses the soil classification, depth, texture and initial soil salinity of the irrigated fields at the different mines, summarised in Table 3.2.
All the fields except Pivot Four and Waterberg CBM irrigation trial, experienced poor internal drainage problems, which reduces yields. Pivot TWF showed a marked reduction in hydraulic conductivity at the soil-spoil interface, and this has resulted in regions of waterlogging, especially in the summer when we had less control over the water balance. The Syferfontein pivot was on a very heavy clay soil that naturally limits drainage, and therefore did not present an ideal site for irrigation. The Waterberg soil was a coarse sand with low percentage of clay and silt in the 0-20 cm. The clay percentage increased to 11% in the 60-80 cm depths. The biggest problems, however, were found on the site with the lightest texture of all, New Vaal. This was due to clay lenses and the level of the buffer dam next to the field (Figure 3.16).
Kleinkopjé and New Vaal
The EC of New Vleishaft Dam water, which irrigates Pivot Major started off at around 250 mS m-1 in 1997, but climbed steadily to a value of 320 mS m-1 by the end of 2005 (Figure 3.17a). Sulphate levels over this period climbed from 1500 mg ℓ-1 to 3000 mg ℓ-1 (Figure 3.17d) whilst pH remained around 6.5, within the range that could favour good crop growth (Figure 3.17b). K, Na and Cl fluctuated between 5 and 30 mg ℓ-1 and Mg between 150 and mg ℓ-1 over the growing period. Ca, however, remained quite stable at 500 mg ℓ-1, during the trial period. Ca, SO4 and Mg clearly dominated this water.
At Tweefontein pan, a dam which irrigates Pivot Four and Pivot TWF, the EC of the water started off a little higher than that of New Vleishaft Dam water in 1998, which was around 300 mS m-1 and was fairly stable for several years until 2001 (Figure 3.17a). A rapid increase in EC to a level of 500 mS m-1 was observed by the end of 2005 and decreased to 450 mS m-1 in 2006. pH remained around 7.5 and was higher than that of New Vleishaft Dam (Figure 3.17b). Sulphate levels over this period increased from 2500 mg ℓ-1 to 4000 mg ℓ-1 (Figure 3.17c). Ca increased from 400 mg ℓ-1 to 600 mg ℓ-1. Mg fluctuated between 200 and 300 mg ℓ-1 over the growing period. Na, K and Cl, however, remained quite stable-with Na at 80 mg ℓ-1, K at 25 mg ℓ-1 and Cl around 50 mg ℓ-1 during the trial period. The deterioration of water quality resulted from the increase of Ca, Mg and SO4 concentrations in the water.
The dam, which irrigates pivot New Vaal, contains water with EC of around 130 mS m-1 and TDS around 1000 mg ℓ-1 (Figures 3.17a and 3.17e), and this water is predominantly rich in NaCl with some Ca and Mg. Na fluctuated between 15 and 300 mg ℓ-1 Cl between 6 and 132 mg ℓ-1, Ca between 26 and 250 mg ℓ-1, and Mg between 6 and 94 mg ℓ-1. K was only present in small quantities in the irrigation water.
At Syferfontein, water quality did not change during the experimental period (October 2001-May 2004) (Table 3.3).
The CBM deep aquifer water had highly elevated levels of salinity and sodicity, relative to water resources routinely used for irrigation. TDS is very high (5.1 g ℓ-1) and rich in sodium bicarbonate with low chloride levels and high sulphate. Concentrations of most trace elements are low (< 1 mg ℓ-1). Crops vary in their response to irrigation water salinity. According to FAO irrigation water quality guideline, the EC of the CBM water is higher than the threshold level specified for severe restriction to crop growth (300 mS m-1). The degree of restriction on use for this water is, therefore, severe for sensitive and moderately sensitive crops. For moderately tolerant and tolerant crops, the severity is related to the yield reduction.
The normal range of pH of irrigation water is 6.5-8.4. A pH value outside this range could cause a nutritional imbalance. pH of the CBM water remained around 7.5 during the trial period, which is in the range that could favour good crop growth
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS AND ACRONYMS
CHAPTER 1 General introduction
1.2 Research approach
1.3 Thesis outline
CHAPTER 2 Literature review
2.2 Soil and crop response to saline and saline-sodic water
2.2.1 Crop response to salinity
2.2.2 Soil salinity
2.2.3 Soil sodicity.
2.3 Modelling the effects of saline-sodic water irrigation on crop growth
2.3.1 Root zone modelling
2.3.3 Field scale application of the SWB model
2.4 Irrigation with mine water in southern Africa
2.4.1 Composition of mine water
2.4.2 Gypsum precipitation in a soil – the opportunity to remove salt from the water system
2.4.3 Crop production using coal-mine water
2.5 Runoff and drainage from mine water irrigated fields
CHAPTER 3 Field sites, monitoring and modelling
3.2 Field site locations and experimental layout
3.3 Cropping systems
3.5 Water qualities
3.6 Monitoring the field water and salt balance
CHAPTER 4 Crop production and plant nutrition
4.2 Crop production
4.3 Plant nutrition
CHAPTER 5 Soil properties
5.2 Kleinkopjé and New Vaal
CHAPTER 6 Field scale medium-term modelling of crop growth, soil water and salt balances
6.2 Model simulations
6.3 Soil water and salt balances
6.4 Long-term scenarios.
CHAPTER 7 Surface runoff from coal-mine water irrigated fields
7.2 Modelling surface runoff
7.3 Model calibratio
7.4 Measured runoff
7.5 Model validations.
CHAPTER 8 General conclusions and recommendations
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