Temporal and Spatial Variability of the Hydrology of Semi-Arid Zimbabwe and its Implications on Surface Water Resources.
Mugabe, Francis Themba
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There are 170 Communal lands, totaling 163 500 km2 or 42% of Zimbabwe. About 75% of these Communal lands are in Natural regions IV and V that are semi-arid. Water development is therefore a key aspect in ensuring sustainable rural livelihoods in such areas where good yields are obtained two out of five years. Both groundwater and surface water resources provide water for domestic purposes and gardening during the dry season, and during droughts. Appropriate management of the limited water resources in the semi arid areas is hindered by inadequate knowledge of how catchments function hydrologically. A study was carried out to investigate how the hydrology of catchments found in the semi-arid areas differ, both spatially and temporarily and how it is affected by rainfall variability and land use changes and the implications on surface water resources. The temporal and spatial variability of rainfall and runoff of the Runde catchment in south-east Zimbabwe was studied using historical data. A second study compared the hydrology of Romwe and Mutangi micro-catchments that are located in the Runde catchment during two seasons representing a very wet and an average season. The effect of rainfall distribution and soil type was studied at Mutangi catchment and the implications on surface water resources were explored. A modelling approach was used to determine the effect of changing rainfall and land use changes and increased water abstraction on surface water resources. Rainfall varied from place to place within the Runde catchment. Cycles of variation of annual rainfall were observed and the long-term trends in runoff and surface water resources in dams reflect the effect of such rainfall cycles. Significant (P < 0.05) correlations between rainfall and runoff were observed for stream gauging stations E2, E4, E49, E54, E112, E117 that were either in the Mutirikwi or Tokwe sub-catchments. No significant relationships between rainfall and runoff were observed in the Upper and Lower Runde sub-catchments. The hydrology of Mutangi and Romwe catchments were different in the two seasons because of different amounts of rainfall received. Romwe received (1430 mm) about double the rainfall that was received at Mutangi (755mm) in the 1999/2000 season. The difference was only 140 mm in the following year when Romwe and Mutangi received 756 and 615 mm respectively. Romwe generated about five times the runoff that was generated at Mutangi in the 1999/2000 season, which were 520 mm and 102 mm respectively. The runoff was 82 mm and 69 mm in the 2000/2001 season at Romwe and Mutangi respectively. Rainfall distribution and soil type had an effect on runoff generation at Mutangi. When there were a sequence of daily rainfall events that completely filled the storage capacity in both the sodic and transitional soils, subsequent events produced very large runoff of more than 50% of the rainfall. The sodic soils at Mutangi appeared to generate the most runoff because of their small capacity to store water before saturation. The amount of runoff captured by a small dam at Mutangi was a small portion of runoff that was 23% and 30% in the 1999/2000 and 2000/2001 season respectively. Almost 97% of the loss from Mutangi dam was through surface evaporation with only 3% used productively, split approximately as 2% garden irrigation and 1% animal watering. Validation of the Agricultural Catchment Research Unit (ACRU) simulation model against field data revealed that ACRU adequately simulated measured streamflow, soil moisture and dam water storage changes. Removing all the remnant woodland and leaving all the cropped land fallow did not have significant (P<0.01) effects on both runoff and dam water level changes over the 27 year simulation period. However, planting trees in the whole catchment significantly (P<0.01) decreased runoff. Water abstraction from the dam could be increased up to 2, 4, 6, 8 and 10 times with the dam drying to acceptable levels for 7%, 7%, 11%, 22% and 30% of the years respectively. Construction of tied ridges significantly (P<0.05) decreased runoff from the catchment by 19% The study proved that rainfall varied both temprorarily and spatially over the Runde catchment and the hydrology of semi-arid catchments is determined by both rainfall totals and distribution during a season and soil type. Enough runoff is generated to fill up the dam in most years except those very dry years. Using the existing practices, where the use of reservoir water for gardening only begins in June, water use could be increased by a factor of five in most years thereby allowing a 2 ha garden to be irrigated. The resources available can be monitored readily using a staff gauge and a volume/depth curve. Typical monthly open water evaporation rates have been established, and these can be used (with a depth/area curve for the dam) to determine safe rates of water use. This will enable the community to make informed decisions such as the percentage of the garden that may be irrigated safely, which crops to grow, and when to stop using the dam water as it approaches the reserve level.