Environmental Engineering Reference
In-Depth Information
Kikuyu (
Pennisetum clandestinum, Chiov
.
)
and star grass (
Cynodon dactylon
, L) under
sprinkler irrigation and furrow irrigation. About 80 ha were grown to Bana grass
(
Pennisetum typhoides
, L.
)
under furrow irrigation.
Imbwa Farm is located at about 22 km from Harare. It has grid co-ordinates 30
o
55
1
E,
18
o
05
1
N. During the period of study, it was surrounded by a large-scale commercial
farming area, which grows tobacco as the major crop. Thus soils are referred to as the
'tobacco soils' of Zimbabwe. It is in the Natural Region II and is part of the Manyame
Catchment with a mean annual rainfall of 850 mm and a mean annual temperature of 23
o
C. The study area drains into the Mtsike River to the Southwest.
The area is gently sloping with an average slope of 1:60. The natural vegetation in the
area consists of sparse “miombo” woodland with msasa (
Julbernadia globiflora
) and
mnondo (
Brachstegia sperciformis
) being the dominant tree species. Other isolated
species include muhacha (
Parinari curatelifolia
).
The geology consists of intrusive igneous rocks with granite as the dominant rock.
Soils are predominantly sands and loamy sands typical of granitic origin. Nyamapfene
(1991) classified the soils at the site as: “ Harare 6 G.3”, according to the Zimbabwe
classification, as
Gleyic Luxisol,
according to the FAO soil classification, and as
Udic
kandiustalf,
according to the USDA soil classification. The soils are 1 m deep on average
and show clay movement down the profile as observed in the field. The soils have high K
(about 164 mg/kg) and crops grown on such soils do not respond to the K fertilizer
application. Nitrogen and phosphorus are low, thus A and P fertilizers are needed in most
cases. The soils have low phosphate sorption capacity. The pH of the soils is naturally
acidic (about 4.3). The reason advanced by Nyamapfene (1991) for the low pH was that
the soils are prone to the leaching of basic cations such as potassium, calcium, sodium,
and magnesium. Additional physico-chemical characteristics of the soils including water
retention at different soil sections could be found in Purves (1976) and Nyamapfene
(1991).
A scheme of the farm is shown in Figure 10.3. The hydraulic structures on the site,
consisting of 6 ponds, which treat wastewater pumped from Zengeza Sewage Treatment
Works (ZSTW), 2 reservoirs and irrigation installations are owned by the Department of
Water Resources, which sells the wastewater to the Ministry of Lands and Agriculture.
The Ministry of Lands and Agriculture in turn leases the farm to individuals who are
compelled to operate the pasture.
Five pumps are used for irrigation but most of the times, only four are working. The
pumps discharge a total of about 30 × 10
3
m
3
per day and each pump commands about 80
ha. The pumps operate 24 hours a day throughout the year and the six laterals are moved
every 12 hours with an irrigation interval of 3 days. During the rainy season, excess
effluent is discharged through runoff channels into natural grassland. Gauging devices at
the farm include flumes at the main canal, sub-canals and at the outlet to Boronia Farm,
but during the period of study, they were not functioning properly. Also, during the study,
it was observed that part of the equipment is not functional and could not be used for
operation, which actually means that only the blocks that are irrigated have adequate
pumping and irrigation facility. Block 1 was chosen as a representative for the sprinkler
irrigated blocks, because it was the most often irrigated block.
A pipeline from ZSTW, located about 15 km to the north east of the farm, supplies the
wastewater used for irrigation. The plant treats wastewater from the satellite City of
