Temporal and Spatial Variability of the Hydrology of Semi-Arid Zimbabwe and its Implications on Surface Water Resources.

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

Estimation of Small Reservoir Sedimentation in Semi-Arid Southern Zimbabwe

Journal of water resources and protection, 2014; 6: 1017-1028Small surface reservoirs play an important role of providing ready and convenient source of water for various uses in semi-arid areas which are characterized by erratic and low rainfall. Lack of current data on reservoir capacity loss due to sedimentation is one of the challenges to the sus- tainable management of surface reservoirs. The study investigated the capacity loss due to sedi- mentation from 2000-2012, and estimated the trap efficiency of the Mutangi reservoir which is located in semi-arid Chivi, Southern of Zimbabwe. Hydrographic surveys, grab sampling and water depth-capacity methods were used to determine the capacity of the dam as of 2012. To compute capacity loss from 2000 to 2012, the 2000 and 2012 dam capacities were compared whilst the trap efficiency of the reservoir was determined using a set of empirical models that relates trap effi- ciency to the capacity-watershed area ratio and capacity-inflow ratio. The results show that Mu- tangi reservoir has a trap efficiency of 95% - 98% (av = 96.4%) and has lost 37% of its capacity due to sedimentation in 12 years (2000 and 2012). Rates of sedimentation were 8539 t·yr −1 , 9110 t·yr −1 and 8265 t·yr −1 for the hydrographic survey, grab sampling and water depth-capacity me- thod respectively, and the little difference in these figures demonstrates that any method can be used to determine sedimentation rates. The area specific sediment yield (ASY) ranged from 14 - 15.5 t·ha −1 ·yr −1 (av = 14.956 t·ha −1 ·yr −1 ). At the current rate of sedimentation the projected dead level of the reservoir will be lost to sedimentation in 8 years while the useful life of the reservoir is estimated to be 30 years. Capacity loss due to sedimentation is further complicating the already strained water scarcity situation in semi-arid areas and management decisions should be made based on the current sedimentation rates estimated by different methods. These results imply that management practices that reduce erosion, hence sedimentation in these small reservoirs should be practiced in order to prolong their lifespan

Perceived impacts of climate related parameters on smallholder farmers in Zambia and Zimbabwe

Cereal production especially maize, is central to food security in Southern Africa. However, it is highly sensitive to drought and climatic variation, and the relationship between production volatility and climate events has been established. This paper focuses on perceived impacts of climate variability on smallholder farmers in Zimbabwe and Zambia where there appears to be an increasing trend towards a late rainy season, prolonged mid-season droughts, and shorter growing seasons. 720 households in 4 districts were surveyed. 80% of farmers in both countries indicate they have noticed significant weather changes over the years