George Nyamadzawo

Hydrological Impacts of Urbanization and Urban Roof Water Harvesting in Water-limited Catchments: A Review

Roof water harvesting is a potential source of water for domestic and livelihood uses in water-scarce urban areas of the world such as sub-Saharan Africa (SSA). However, little is known about the hydrological impacts of incorporating roof water harvesting on on-site and downstream hydrology of urbanized catchments. Therefore, the current review investigates the effects of urbanization and urban roof water harvesting on hydrological processes, rainfall-runoff relationships, groundwater recharge and water contamination, and highlights future research directions. The review showed that the urban heat island effect increases the frequency and magnitude of convective storms. The high proportion and connectivity of impervious surfaces reduce infiltration, thereby increasing the runoff coefficient and Hortonian runoff. Urbanization reduces the minimum threshold rainfall for runoff generation, resulting in multi-peak hydrographs reflecting the contribution of both pervious and impervious surfaces. Urban roof water harvesting increases catchment lag time, but reduces downstream peak and total discharge, baseflow and flow velocity. Utility trenches, tunnels and buried structures form a complex network resembling a shallow urban karst system, which provides preferential flow pathways for groundwater recharge by imported water via leakages. Contrary to the widely held notion that urbanization reduces groundwater recharge by increasing impervious surfaces, empirical evidence shows significant urban-enhanced recharge in water-limited urban catchments. However, we contend that excessive groundwater abstraction for multiple uses in water-scarce regions offsets the urban-enhanced recharge, resulting in groundwater depletion. Due to the overriding collective effects of reduced soil moisture and vegetation cover on evapotranspiration in water-limited environments, we conclude that urbanization lowers evapotranspiration. Urban roof water harvesting short-cuts the urban water cycle, thereby minimizing the risk of runoff contamination that could occur during its extended flow over contaminated land surfaces. Contaminated sources of recharge, such as wastewater leakages coupled with the urban karst system, promote groundwater pollution. Overall, urban roof water harvesting imparts additional complexity to urban catchments, and has potentially adverse effects on ecohydrology. Understanding these impacts is critical for planning, designing and operation of urban roof water harvesting systems. Future research may provide a comprehensive understanding of these impacts by combining hydrological measurements and process modelling in urbanized catchments incorporating roof water harvesting.

Opportunities for optimization of in-field water harvesting to cope with changing climate in semi-arid smallholder farming areas of Zimbabwe

Climate change has resulted in increased vulnerability of smallholder farmers in marginal areas of Zimbabwe where there is limited capacity to adapt to changing climate. One approach that has been used to adapt to changing climate is in-field water harvesting for improved crop yields in the semi- arid regions of Zimbabwe. This review analyses the history of soil and water conservation in Zimbabwe, efforts of improving water harvesting in the post independence era, farmer driven innovations, water harvesting technologies from other regions, and future directions of water harvesting in semi arid marginal areas. From this review it was observed that the blanket recommendations that were made on the early conservation method were not suitable for marginal areas as they resulted in increased losses of the much needed water. In the late 1960 and 70s’, soil and water conservation efforts was a victim of the political environment and this resulted in poor uptake. Most of the water harvesting innovations which were promoted in the 1990s’ and some farmer driven innovations improved crop yields in marginal areas but were poorly taken up by farmers because they are labour intensive as the structures should be made annually. To address the challenges of labour shortages, the use of permanent in-field water harvesting technologies are an option. There is also need to identify ways for promoting water harvesting techniques that have been proven to work and to explore farmer-led knowledge sharing platforms for scaling up proven technologies.

Understanding the causes, socio-economic and environmental impacts, and management of veld fires in tropical Zimbabwe

Veld fires are a common phenomenon in the predominantly savanna ecosystems of Zimbabwe. Until now no studies have investigated the causes, and socio-economic and environmental impacts of veld fires in Zimbabwe. Yet such information is crucial for planning and implementation of fire prevention and control practices. The present study uses multiple sources of information including review of published literature, reports from regulatory agencies, informal interviews and field observations to address the following objectives; (1) to identify the key causes of veld fires in Zimbabwe, (2) to investigate the socio-economic and environmental impacts of veld fires, (3) to highlight management practices for the control of veld fires and (4) to identify research gaps on causes and management of veld fires in Zimbabwe. This review showed that the main causes of veld fires are anthropogenic. The impacts of fires are multifaceted and are a threat to the bio-physical, social and economic environment because of their trail of destruction and they directly impact all sectors of the economy. Impacts range from loss of livelihoods and income, psychosocial impacts associated with fatalities and family bereavement, loss of biodiversity and disturbance of the hydrological balance. It is very difficult, if not impossible, to prevent veld fires because, besides the negative impacts of veld fires, fire also play an important positive role in many Zimbabwean ecosystems. In view of this, fire management should move away from fire fighting to management practices such as early burning that reduces the negative impact, and enhance the positive effects of veld fires.

Limits of agricultural greenhouse gas calculators to predict soil N2O and CH4 fluxes in tropical agriculture.

Demand for tools to rapidly assess greenhouse gas impacts from policy and technological change in the agricultural sector has catalyzed the development of ‘GHG calculators’— simple accounting approaches that use a mix of emission factors and empirical models to calculate GHG emissions with minimal input data. GHG calculators, however, rely on models calibrated from measurements conducted overwhelmingly under temperate, developed country conditions. Here we show that GHG calculators may poorly estimate emissions in tropical developing countries by comparing calculator predictions against measurements from Africa, Asia, and Latin America. Estimates based on GHG calculators were greater than measurements in 70% of the cases, exceeding twice the measured flux nearly half the time. For 41% of the comparisons, calculators incorrectly predicted whether emissions would increase or decrease with a change in management. These results raise concerns about applying GHG calculators to tropical farming systems and emphasize the need to broaden the scope of the underlying data.

The effects of catena positions on greenhouse gas emissions along a seasonal wetland (dambo) transect in tropical Zimbabwe

Wetlands are major natural sources of greenhouse gases (GHGs). In central and southern Africa, one of the most extensive wetlands are dambos (seasonal wetlands) which occupy 20–25% of land area. However, there are very little data on GHG methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) emissions from dambos, and this study presents the first estimates from dambos in Zimbabwe. The objective was to evaluate the effects of catena positions; upland, dambo mid-slope and dambo bottom, on GHG emissions along an undisturbed dambo transect. Methane emissions were −0.3, 29.5 and −1.3 mg m−2 hr−1, N2O emission were 40.1, 3.9 and 5.5 µg m2 hr−1, while CO2 emissions were 2648.9, 896.2 and 590.1 mg m−2 hr−1 for upland, mid-slope and bottom catena, respectively. Our results showed that uplands were important sources of N2O and CO2, and a sink for CH4, while the dambo mid-slope position was a major source of CH4, but a weak source of CO2 and N2O. Dambo bottom catena was weak source GHGs. Overall, dambos were major sources of CH4 and weak sources of N2O and CO2.We concluded that, depending on catenal position, dambos can be major or minor sources of GHGs.

Optimizing dambo (seasonal wetland) cultivation for climate change adaptation and sustainable crop production in the smallholder farming areas of Zimbabwe

Most smallholder farming areas of Zimbabwe have low soil fertility and low rainfall, which has continually decreased over the past years. In recent years, most of the smallholder farming areas have experienced perennial droughts, poor rainfall distribution and crop failures and these have been attributed to climate change and variability. Cultivation of dambos, which are seasonal wetlands, presents a climate change and variability adaptation option for smallholder farmers. This synthesis analyses the role of dambo cultivation in climate change and variability adaptation and discusses future directions for sustainable dambo utilization. The data on current dambo farming practices were collected from literature, surveys and field observations. The results showed that farmers grow crops in dambos as an adaptive strategy to climate change and variability and have largely abandoned upland fields where yields are <1 t ha−1 in preference of dambos where yields average 2–3 t ha−1. Dambo cultivation offers a buffer against crop failures and has resulted in improved household food security. We conclude that dambo cultivation is a potentially beneficial farmer-driven climate change and variability adaptation strategy. However, if not properly designed and managed, dambo cultivation may result in their degradation hence there is need for further research to evaluate options for sustainable dambo utilization as intensification of dambo agriculture is important for food security.

The effect of hydroxide solutions on the structural stability and saturated hydraulic conductivity of four tropical soils

Industrial and domestic effluents are widely applied to soil as a way of dumping, and/or source of nutrients and water for plant growth. The potential effects of sodium hydroxide (NaOH) and potassium hydroxide (KOH), which are common hydroxides found in industrial effluents/sludge, on aggregate stability, clay dispersion and saturated hydraulic conductivity (Ks) of four tropical soils were studied. For aggregate stability and clay dispersion, the soils were pre-treated with NaOH and KOH solutions (0.003–0.3M, pH = 11.5), and for Ks the NaOH and KOH solutions were used as influent solutions. Aggregate stability and Ks decreased with increased concentration of hydroxide solutions, while clay dispersion increased as concentration of the hydroxide increased. NaOH, which is more commonly used for industrial cleaning, had more negative effects on soil structural stability compared to KOH. Long-term application of a mixture of sewage sludge and effluent to Pension farm soil reduced Ks compared to Churu farm soil (control). Ks decreased to less than 20% for Pension farm soil compared to 40% for Churu farm soil when 0.3M NaOH was used as influent solution, despite having much higher organic matter content in the former. It was concluded that disposal of sewage sludge/effluent high in hydroxides, such as those of sodium (Na) and potassium (K)1 would result in reduced structural stability and Ks. Therefore there is a need to establish critical levels of hydroxides in effluent/sludge that can be disposed onto Zimbabwean soils.

Burning, biomass removal and tillage effects on soil organic carbon and nutrients in seasonal wetlands (Dambos) of Chiota smallholder farming area, Zimbabwe

Seasonal wetland (dambo) cultivation in smallholder farming areas is important because it improves household food security. However, most farming practices, such as burning of vegetation and conventional tillage in dambo gardens, may reduce soil organic carbon (SOC) and nutrient dynamics. We evaluated the effects of simulated burning, vegetation clearing and clipping, and conventional tillage in dambo gardens on SOC, nutrient contents and biomass production over a 3-year period. The results showed that clearing and clipping of vegetation and conventional tillage reduced SOC, soil nutrient contents and biomass yields, while burning increased SOC and soil nutrient contents. For the 0–10 cm depth, conventional tillage, clearing and clipping resulted in a 37%, 34% and 18% decrease in SOC, respectively, after three seasons, burning resulted in a 25% increase in SOC, while there were no changes in the control after 3 years. For the 0–40 cm depth, the average change in SOC was 32%, 25% and 16% for conventional tillage, clearing and clipping, respectively. Locally and regionally, conventional tillage, clearing and clipping reduce SOC, nutrient contents and biomass production in dambos. Though annual burning increased SOC and nutrient contents in the short term, the long-term effects are uncertain, hence there is a need for long-term studies.

Soil organic carbon and nitrogen stocks along a seasonal wetland (dambo) transect in central Zimbabwe

Ecosystems of central and southern Africa are occupied by some of the largest seasonal wetlands commonly called dambos. Dambos are likely to store huge stocks of soil organic carbon (SOC) because of their saturated conditions. However, most available literature report average SOC concentrations while ignoring pedological and hydrological variations. The objectives of the study were to quantify effects of catena position and hydrology on SOC and nitrogen stocks along a dambo transect in Chiota, Zimbabwe. Soil organic carbon stocks varied significantly with catena and were 7.3, 9.5, 30.4, 12.9 and 7.2 Mg ha−1 for upland, margin, middle slope (midslope), lower slope and bottom, respectively, for the 0–40 cm depth. Corresponding nitrogen stocks were 0.6, 0.8, 2.1, 1.1 and 0.7 Mg ha−1 for upland, margin, midslope, lower slope and bottom, respectively. The dry-season water table was 8, 1.5, 1.0, 1.3 and 1.5 m, whereas the wet-season water table was 6, 0.5, 0, 0 and 0.9 m below the surface for the upland, margin, midslope, lower slope and bottom, respectively. Biomass stand was highest in the midslope and least in the bottom. It was concluded that SOC and nitrogen stocks varied significantly with catena and this was attributed to differences in wetness.

Rainwater Harvesting Options to Support Off-Season Small-Scale Irrigation in Arid and Semi-arid Areas of Zimbabwe

Rainwater harvesting involves the collection, storage and subsequent use of rainwater for domestic, agricultural and other livelihood activities. It consists of a wide range of technologies used to collect, store and provide water for use by humans and/or human activities such as irrigation and providing drinking water for livestock. In semi-arid areas of Zimbabwe, where rainfall is scarce and insufficient to sustain dryland crop production, rainwater harvesting can form the basis for irrigation in order to improve food security. Some rainwater harvesting and irrigation technologies that are currently in use in Zimbabwe include roof catchment systems, rock catchment systems, ground catchment systems, small dams and sand dams. Roof catchment systems collect water from roof surfaces into storage tanks or the place of use. Rock outcrops provide collecting surfaces for rainwater. Sand dams and small dams or weirs are constructed across streams and rivers to capture and store surface and subsurface flow. These various ways of harvesting rainwater can be linked to different irrigation technologies which include drip, sprinkler and flood irrigation systems. Farmers have adopted different pumping mechanisms to move water from storage to point of irrigation such as use of solar, fuel (petrol/diesel) and manual methods. Although it has been proven that these technologies are beneficial in the dry areas, most of these systems are not well developed and only a few of these technologies are operational. There is therefore need to promote RWHI and include other RWH methods such as roadside divergence ditches which are currently not being used in Zimbabwe.