Geophysical conditions of Zhulube

Zhulube catchment is mountainous, covering an area of 21 km² upstream of an important small dam supporting a 40 ha irrigation scheme as shown in Figure 4-2. The middle part of Zhulube catchment is dominated by rain fed farming and grazing land. The slope is fairly steep ranging between 5% and 7%. Landscape degradation characterised by gulleying is widespread in the middle part of the catchment. A large significant gulley (Gobalidanke) has formed mid-stream between the hillslopes and the downstream Zhulube Dam (Figure 4-3) and efforts to reclaim the gulley proved difficult in the past (Dondofema, 2007). The gulley was developed following an increase in human activity such as livestock grazing and digging of fishing worms (Ngwenya, 2006; Dondofema, 2007).

Figure 4-1: Location of the study area (Zhulube) in Zimbabwe

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The dam supporting the irrigation scheme is threatened with sedimentation from soil erosion emanating from up-stream and mid-stream erosion that leaves behind huge

Figure 4-2: Zhulube catchment characteristics

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gulleys. Already an old dam immediately upstream of the dam is full of sediments and was abandoned (Figure 4-4). Adoption of contour ridges in such a catchment could prove to be a very important soil and water conservation strategy as the ridges have the potential to reduce soil erosion and as a result sedimentation in the dam. However sustainability of the contour ridges depends on rain fed farmers deriving benefit from the practice. Thus the catchment was considered appropriate for research on the potential of water conservation from contour ridges.

The vegetation in Zhulube catchment is dominated by acacia species and mopane (colophospermum) trees (Dondofema, 2007). The grass is made up of both perennial and annual species whose densities vary according to rainfall received each season (Ngwenya, 2006).

Figure 4-3: Part of Gobalindanke Gulley and efforts to reclaim it

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Climate and water resources

Mzingwane catchment falls within the semi-arid region of Zimbabwe where rainfall is low and unreliable with high temporal and spatial variability (Sawunyama, 2004; Rukuni, 2006). The rainfall is convective and is characterised by dry spells and frequent droughts (Moyo, 2005). The average annual rainfall ranges from a low of about 350 mm in the south west part of the catchment to about 630mm in the northern part of the catchment as shown in Figure 4-5.

Figure 4-4: Silted Old Zhulube Dam abandoned after exessive land degradation in the catchment area

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While the average rain falls within the range in which crop production is possible there are several years where no crops are harvested at all owing to the high temporal variability (Figure 4-6 (a)). In areas where the annual rainfall averages 400mm the rainfall can be as low as below 200mm while in some few years it can reach 800mm. Zhulube catchment receives annual rainfall with an average of 540mm but has high deviation from the mean as shown by the rainfall at Filabusi a rainfall station about 20km from Zhulube (Figure 4-6 (b)).

Figure 4-5: Spatial rainfall distribution in Mzingwane Catchment

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1950/51 1955/56 1960/61 1965/66 1970/71 1975/76 1980/81 1985/86 1990/91 1995/96 2000/01

Rainfall deviation from average (mm/year)

Rainfall years (seasons) Deviation from long term mean at Filabusi

(b)

(Data available from 1952/53 to 1995/96 rainfall years)

Figure 4-6: Temporal rainfall variability at selected rainfall stations in Mzingwane Catchment (a) and deviation from the mean at Filabusi Station (b)

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There is also high intra-seasonal temporal variability experienced throughout the catchment as illustrated by the seasonal variation of rainfall of Zhulube for the year 2005/6 and 2006/7 shown in Figure 4-7. The rainfall is poorly distributed with frequent mid-season dry spells resulting in crop failures which discourage farmers from practicing rain fed farming (Chibulu, 2007). This high intra-seasonal temporal variability is largely responsible for crop failure in this catchment. As can be seen in Figure 4-7 the month of January received practically no rainfall during the year 2006/7. Such prolonged dry spell result in crop wilting and reduced yield and sometimes complete crop failure. The rainfall also occurs in sudden storms with high intensity that lead to high runoff and sediment generation from the degraded (eroded) land surface.

Surface runoff occurs for short durations following heavy rainfall storms but generally depends on rainfall intensity (Rukuni, 2006). In certain parts of the river system surface flow is largely limited to the rainfall season while during the dry period the riverbed is a thick alluvial aquifer storing some considerable amount of water (Sawunyama, 2005).

0

Oct Nov Dec Jan Feb Mar Apr

Rainfall (mm/month)

Time of year (months) 2005/6 2006/7

Figure 4-7: Intra-seasonal rainfall variation in Zhulube Catchment. (Source of Data: Ngwenya, 2006; Chibulu, 2007)

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Some large rivers maintain flow during the dry season as subsurface flow in the river bed alluvial aquifer. These alluvial aquifers are a source of water for communities and are used to support small irrigation schemes for smallholder farmers in Mzingwane catchment (Munamati, 2005).

There are very few large dams (capacity > 1Million m³) that have been constructed in the catchment (Table 4-1). These have a combined 10% risk yield of 350Mm³/annum (DWR and ZINWA, 2007). However there are more than 1000 small dams found within the catchment whose capacities range from about 10 000m³ to 100 000m³ (Sawunyama et al., 2007). Water utilisation from these small dams is constrained by the high evaporation rates experienced in the catchment as the mean annual evaporation far exceeds mean annual rainfall. For example in Insiza District where Zhulube is found mean annual evaporation is 2000mm compared to mean annual rainfall of 470mm (Mamba, 2007). The topography does not provide many sites with deep valleys thus exposing most of the stored water to evaporation. If the reservoirs were deep, they could have provided large amounts of water as the proportion of evaporation loss would be much less. Therefore in Mzingwane Catchment a significant amount of the water stored in small dams is lost to evaporation thereby yielding low dry season water volumes. Besides the low surface water potential, Mzingwane catchment also has low potential groundwater mainly due to the predominant igneous rock formations found in the catchment (Moyo, 2005).

About 70% of the surface water from Mzingwane Catchment is used for urban water supply with the bulk going to Bulawayo, the second largest city in Zimbabwe. Only about 20% of the developed water resources are used for irrigation within Mzingwane Catchment. As a result of the low water resources availability in Mzingwane Catchment the extent of irrigated agriculture is very low, leaving most families vulnerable to food insecurity as they rely on rain fed farming.

Dam Type Large Medium Small Small

Dam size (m³) >5Million 1 to 5 Million 0.1 to 1 Million < 0.1 Million

Number of dams 20 10 205 >1 000

Table 4-1: Distribution of dam sizes by capacity in Mzingwane Catchment

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