«BY ROICK CHIKATI (R035991L) SUPERVISOR: Dr. A Senzanje An undergraduate Research project submitted in partial fulfilment of requirements of the ...»
This therefore means the small dams storing capacity is reduced and much of the water is stored beneath the sediments.lt hence the need to extract the water stored beneath the sediments. However these small dams might be seen as a source of water borne diseases such as malaria, bilharzias and cholera
3.0 Study Area And Methods
3.1. Description of study area The study was conducted in the Mzingwane Catchments, which forms part of the Limpopo River Basin on the Zimbabwean side. One dam (Majelimane small dam ) was selected for the measurements and the investigation. The dam is in Insiza District, Godhlwayo communal area. The area is Agro –ecological region IV characterised by erratic rainfall of between 450-650mm,and frequent dry spells are experienced. Main livelihood activities are livestock production and rain feed crop production. It is against backdrop that over 856 dams (Sugunan, 1997)have been built in Insiza District to compliment the low rainfall received and improve people livelihoods.
3.2. The Limpopo River Basin The Limpopo River forms part of the northern border of South Africa, and separating South Africa, Zimbabwe and Botswana before it enters into Mozambique and drains into the Indian Ocean. The basin is therefore shared between four countries, see the fig bellow. The Limpopo River basin is demarcated by latitude 24º south, longitude 25º east and latitude 23º south, longitude 34º with other major basins, but the river is important because of its strategic value for water to the four countries (Pallet, 1997). A short and intense rainy season in the Limpopo river basin, with highly unreliable rainfall leads to frequent droughts. Crop production is not secure. On major reaches of the Limpopo and many of its tributaries, the flow of water can contain up to 30% sand and silt (Lawrence, 2000). Large areas of the land are seriously degraded, and much sediments and silt accumulates in the small dams locking water and reducing the reservoirs storing capacity.
11Figure 3.1 Map of Limpopo River Basin
3.3. Mzingwane Catchment.
The catchment is divided into four sub-catchments namely, Shashe, Upper Mzingwane, Lower Mzingwane, Lower Mzingwane, and Mwenezi.The catchment, which forms part of the Limpopo River Basin, is located in the semi-arid region of Zimbabwe. The rivers flow in the south eastern direction into the river Limpopo as shown on the map below, carrying with them sediments and some deposited along their courses. In certain parts of river courses, flow occurs only during the wet months, while during the dry months the riverbed is a sandy alluvial bed of considerable thickness and provides enormous storage of water. These alluvial formations serve as source of water for rural communities. One feature of the aridity of the catchments is the annual evapo- transpiration rates being higher than those of precipitation, so that there are long-term net fluxes of moisture from the catchment. Poverty is widespread and people are extremely vulnerable to the effects of drought or crop failure (WRMS, 2000)
12Figure 3.2: Map of Mzingwane Catchments (Source, David Love, undated)
3.4. Majelimane dam The dam was completely empty during the time of study. The dam measures on average 187m length, 70m width. The dam has maximum height of 5,65m.the dam serves almost 27 families on average. The wall of the dam poses some cracks that most of the water in the silt infiltrates out through the dam wall. The dam is believed to have been constructed around 1980s specifically for animal watering.
3.5. Materials Auger Stopwatch 10 x perforated (PVC) pipes (3m*20mm) Tape measure (50m) Digital mass balance Mussel soil color chart 20 x black polyplastic bags
3.6 Methods 3.6.1 Estimating the amount of silt in Majelimane dam.
- The dam length was measured using the tape measure
- The dam width was measured using the tape measure.
- The procedure followed was Pegs A, B, C, D, E, F were set out to lie on a straight lie such that AB = BC =CD =DE=EF
- The level was set up at point B and readings taken to staff at point A and then at C.
- The level then move to D and readings taken to staff held at A then C.
- Moved to D then reading taken to staff at A and then at C up to point E
- The Trapezoidal rule of areas was used to calculate the surface area of the silt (dam surface area).
NB. The Trapezoidal rule assumes that irregular boundary consists of parabolic arcs and the dam was divided into a series of even numbers.
A= b( b+mh) A= area b=formational width h= the silt height This is correct so long as the area of the section and way between A1 andA2 is the mean of the two this can be assumed as the truth as long there is no wide variation between successive sections.
NB: The base of the dam was assumed to be a straight line.
3.6.2. Measuring the moisture content.
Background Soil moisture may be present as gravity water in transit in the larger pore spaces or as capillary water in the small pores, as hydroscopic moisture adhering in a thin film to soil grains, and as water vapor (Max a, Kohler, 1998).
Gravity water is in a transient state. After rain, water may move downward in the larger pores, but this water must either be dispersed into capillary pores or pass through the vadose zone to the underground water or to a stream channel. Hydroscopic water on the other hand is held by molecular attraction and is not normally removed from the soil under climatic conditions (Joseph LH, Paul, 1998). The important variable element of soil moisture i.e. is capillary water.
- The dam was divided again into subsections which are approximately 30 meters apart.
- Silt samples were augured at approximately equal depth.
- Mussel Soil color chart was used to identify the silt colour.
- Silt samples were placed in the plastic bags and the initial mass of the silt was measured using digital mass balance.
3.6.3.Estimating the water that can be abstracted.
The amount of water depends with the time of the year. This means in June and November the amount which can be sustainable abstracted slightly differs because the of the changes in weather between June and November Measurements
- The dam was divided into smaller subsections
- Small observations wells were dug to a depth were shallow underground water could be abstracted.
- The diameter of the observation well was measured.
- 10 (PVC) pipes was inserted into the observation well (same diameter i.e. 20mm diameter).
- The initial height was noted.
NB: the time was started at the same time.
- For every one minute one pipe was removed from the observation well and the change in height noted.
This was done until at the point were there is no any change noted on the height i.e.
no water rise.
- The water was removed again from the observation well again the pipes were inserted into the well and the initial height noted.
- The same procedure was repeated for all different subsections throughout the dam.
- Three different reading were taken at each subsection the mid point and the two ends and an average is extrapolated as the average water present at that section..
16 Assessing how the water can be abstracted for rural livelihoods.
3.6.5 Assessing the purpose of water from silt of silted up small dams The information on how water from silted up Majelimani dam can be abstracted and how it is used was gathered through questionnaire administration.
Questions were administered orally to the users in Ndebele and answers written in English the questionnaire was designed for small dam (Majelimane) users. It contained both closed ended questions and open-ended questions.
Pre-testing of the questionnaire was done during the reconnaissance visit to asses’ potential problems in its administration such as phrasing, length of the questionnaire omission of some details and irrelevant questions. Again it was noticed that some parts needed rephrasing and that some vital information had been left out.
With the help from community members and Mr. Ncube, AREX official, sphere of influence for the dam was established. Simple random sampling was used to choose households by use of random numbers. The household that corresponded to that number was selected. This was done to ensure that every household had an equal chance of being selected.
Questionnaire Administration The questionnaire was administered orally to the users in Ndebele and answers written in English. Each questionnaire lasted for about twenty minutes. Much effort was made to get views from various age groups and both sexes. Ten respondents were interviewed for the small dam users (Majelimane). The main constraints for getting a larger sample being time for the study and resources. The questionnaire is given in appendix 1.
17 Interviews Oral interviews were conducted using pre-prepared checklist with representatives from various organizations and government departments listed below, these were to identify the uses of water from silt wells, establishing specific roles and services they offer to the community and problems they encounter in their service provision. Where possible, some secondary data was obtained which was used in the analysis Agricultural Research and Extension Services (AREX) • Zimbabwe National Water Authority (ZINWA) • Data Analysis All the information obtained from the questionnaire was recorded on a Microsoft Excel Spreadsheet. One-way analysis of variance was used to compare the mean volumes of water from different silt wells.
4.1. Results and Discussion Introduction In this section the results on the methods used to abstract water, the amount of water in the silt of silted up small dams are shown. Moreover the amount of silt in the small dam is being shown. It is again in this section that the multi-purpose nature of water from silt of silted up small dams is reviewed.
The graph also resembles the base of the dam.However sometimes the base of the dam is assumed to follow the straight line.
The area was calculated using the Simpson rule and the final volume was calculated using the trapezoidal rule as show bellow.
The line graph bellow shows how the moisture content of the silt is distributed throughout the dam length. The graph of moisture content against the dam length from the dam wall. From the graph it can concluded that the moisture content is much concentrated at the mid part and at the ends of the dam wall. It can be further be explained that in case of digging the silt wells these well must be dug in such position.
From the measurements performed it was only at the mid part of the dam and near the dam wall were there was water which can be abstracted i.e. shallow underground water.
Points P1, M1 and P2 are as described before.
Figure 4.2 The graph of water rise against time at mid point i.
e. point M1 22 0.25
The graph of water rise against time at point P2 From these results it can be deduced that the maximum recharge therefore the increase of water is calculated to be approximately equal 0,54m3/ min. This means the discharge at any abstraction level must not exceed 0,54m3/ min.
The water was only present at any distance approximately 90m from the dam • wall. All the other parts did not have water that could have been abstracted.
However the volume of water which can be abstracted was calculated by dividing • the whole area to a point of 90m distance from the dam wall to the cross sectional area of the silt well and was found to be 15 336 cubic meters.
4.4. Purposes of water from the silt of silted up small dam All respondents approximately (98%) indicated that the dam was developed mainly for livestock watering and domestic use since rivers are not perennial, the area is prone to droughts and the nearest dam was about 2km away. Of the interviewed respondents, 62% and 38% were male and female respectively. Their ages ranged from 20 to 48 years with the mean age being 30.9 years.
The water is used for various purposes as shown in figure 4.4.The current uses of water from silt wells are livestock watering, irrigation of small gardens, domestic use, brick 23 making, reeds and other smaller uses. Types of livestock that access the water on daily basis are cattle, goats, sheep and donkeys. Irrigation is confined to small gardens. In the small gardens, buckets are used to carry water to the fields. Mainly vegetables crops are grown for household consumption throughout the year and a little of the produce for selling Domestic uses of water comprises drinking, washing, bathing and cleaning.
Approximately 60% of the respondents use the dam water on daily basis. Borehole down stream provides a safe source of drinking and cooking although sometimes water from silt wells is used. Buckets are mainly used to fetch water to the homestead concrete structures built on the other side of the dam wall, which are used for washing Brick making is mainly carried out in the water when there is little work to be done in the field as it labor intensive. Winter is also favored because high temperature cause cracking of the bricks.
Reeds cypress spp, are used for roofing houses but their availability is threatened by cattle which feed on them.
Village heads and the community through appointed people from each are responsible for the management of the small dam. Most of the respondents again identified siltation as posing threat to the utilization of the dam. Competition for the use of water; from silt wells is rife particulars in the dry season and drought years.