The role of superficial geology in controlling groundwater recharge in the weathered crystalline basement of semi-arid Tanzania

Study region: Little Kinyasungwe River Catchment, central semi-arid Tanzania. Study focus: The structure and hydraulic properties of superficial geology can play a crucial role in controlling groundwater recharge in drylands. However, the pathways by which groundwater recharge occurs and their sensitivity to environmental change remain poorly resolved. Geophysical surveys using Electrical Resistivity Tomography (ERT) were conducted in the study region and used to delineate shallow subsurface stratigraphy in conjunction with borehole logs. Based on these results, a series of local-scale conceptual hydrogeological models was produced and collated to generate a 3D conceptual model of groundwater recharge to the wellfield. New hydrological insights for the region: We propose that configurations of superficial geology control groundwater recharge in dryland settings as follows: 1) superficial sand deposits act as collectors and stores that slowly feed recharge into zones of active faulting; 2) these fault zones provide permeable pathways enabling greater recharge to occur; 3) ‘windows’ within layers of smectitic clay that underlie ephemeral streams may provide pathways for focused recharge via transmission losses; and 4) overbank flooding during high intensity precipitation events increases the probability of activating such permeable pathways. These conceptual models provide a physical basis to improve numerical models of groundwater recharge in drylands, and a conceptual framework to evaluate strategies (e.g., Managed Aquifer Recharge) to artificially enhance the availability of groundwater resources in these regions

The El Nino event of 2015-2016: climate anomalies and their impact on groundwater resources in East and Southern Africa

The impact of climate variability on groundwater storage has received limited attention despite widespread dependence on groundwater as a resource for drinking water, agriculture and industry. Here, we assess the climate anomalies that occurred over Southern Africa (SA) and East Africa, south of the Equator (EASE), during the major El Niño event of 2015–2016, and their associated impacts on groundwater storage, across scales, through analysis of in situ groundwater piezometry and Gravity Recovery and Climate Experiment (GRACE) satellite data. At the continental scale, the El Niño of 2015–2016 was associated with a pronounced dipole of opposing rainfall anomalies over EASE and Southern Africa, north–south of ∼12∘ S, a characteristic pattern of the El Niño–Southern Oscillation (ENSO). Over Southern Africa the most intense drought event in the historical record occurred, based on an analysis of the cross-scale areal intensity of surface water balance anomalies (as represented by the standardised precipitation evapotranspiration index – SPEI), with an estimated return period of at least 200 years and a best estimate of 260 years. Climate risks are changing, and we estimate that anthropogenic warming only (ignoring changes to other climate variables, e.g. precipitation) has approximately doubled the risk of such an extreme SPEI drought event. These surface water balance deficits suppressed groundwater recharge, leading to a substantial groundwater storage decline indicated by both GRACE satellite and piezometric data in the Limpopo basin. Conversely, over EASE during the 2015–2016 El Niño event, anomalously wet conditions were observed with an estimated return period of ∼10 years, likely moderated by the absence of a strongly positive Indian Ocean zonal mode phase. The strong but not extreme rainy season increased groundwater storage, as shown by satellite GRACE data and rising groundwater levels observed at a site in central Tanzania. We note substantial uncertainties in separating groundwater from total water storage in GRACE data and show that consistency between GRACE and piezometric estimates of groundwater storage is apparent when spatial averaging scales are comparable. These results have implications for sustainable and climate-resilient groundwater resource management, including the potential for adaptive strategies, such as managed aquifer recharge during episodic recharge events

Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa

Groundwater in sub-Saharan Africa supports livelihoods and poverty alleviation1,2, maintains vital ecosystems, and strongly influences terrestrial water and energy budgets. Yet the hydrological processes that govern groundwater recharge and sustainability—and their sensitivity to climatic variability—are poorly constrained4. Given the absence of firm observational constraints, it remains to be seen whether model-based projections of decreased water resources in dry parts of the region4 are justified. Here we show, through analysis of multidecadal groundwater hydrographs across sub-Saharan Africa, that levels of aridity dictate the predominant recharge processes, whereas local hydrogeology influences the type and sensitivity of precipitation–recharge relationships. Recharge in some humid locations varies by as little as five per cent (by coefficient of variation) across a wide range of annual precipitation values. Other regions, by contrast, show roughly linear precipitation–recharge relationships, with precipitation thresholds (of roughly ten millimetres or less per day) governing the initiation of recharge. These thresholds tend to rise as aridity increases, and recharge in drylands is more episodic and increasingly dominated by focused recharge through losses from ephemeral overland flows. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by large-scale climate controls. Intense precipitation, even during years of lower overall precipitation, produces some of the largest years of recharge in some dry subtropical locations. Our results therefore challenge the ‘high certainty’ consensus regarding decreasing water resources in such regions of sub-Saharan Africa. The potential resilience of groundwater to climate variability in many areas that is revealed by these precipitation–recharge relationships is essential for informing reliable predictions of climate-change impacts and adaptation strategies

Forest cover changes, stocking and removals Under different decentralised forest management Regimes in Tanzania

By the end of the last century many countries including Tanzania moved from centralised towards decentralised forest management but little empirical evidence exists on how such changes have influenced forest conditions. The objective of this study was to provide insights on how decentralised approaches might influence forest resource conditions. Forest cover analyses from satellite images (1993, 2000 and 2009) and systematic sample plot inventories (2009) in two state forest reserves under joint forest management (JFM) and two village forest reserves under community-based forest management (CBFM) in Babati District, Tanzania were carried out. Based on the results, it was not possible to claim that the decentralised management had been successful in improving forest conditions. Proportions of closed woodland decreased significantly over time (from over 80 to 50–60% under JFM and from around 70 to almost 0% under CBFM. In all forests, numbers of regenerants were high, but proportions of larger trees were low and levels of removals (legal and illegal) were relatively high. In general the situation under JFM was better than under CBFM. Results of this study can be used by policymakers to assess the influence of decentralised forest management in Tanzania.The study was done as a part of the project 'Assessing the impact of forestland tenure changes on forest resources and rural livelihoods in Tanzania' (No. NUFUTZ-2007/10226) under the Tanzania–Norway NUFU Programme 2007–2012. We acknowledge the assistance provided by staff of the Manyara catchment forest office, Manyara natural resource office, village governments in Managhat, Haraa, Riroda and Bubu villages during data collection. We are indebted to A Masao, D Byarugaba and D Shafii who provided valuable technical assistance in the field

Impact of decentralised forest management on forest cover changes in the north eastern Tanzania

This study aimed to assess the impact of decentralised forest management on forest cover changes in the north eastern Tanzania. Six contrasting forests namely: Shagayu (JFM), Shume-Magamba (fexclusive state management) and Sagara (CBFM) in the montane, and Handeni Hill(JFM), Kiva Hill (exclusive state management) and Kwakirunga (CBFM) insemi-arid forests were studied. Forest cover changes were assessed for periods before and after decentralised forest management. Cover maps were derived from Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM) images. Forest inventory techniques were used to estimate tree harvests as major drivers of forest cover changes. The area under closed forest cover decreased substantially ranging from 0.4%.y-1 to 1.3%.y-1 in the montane forests with higher forest loss recorded under exclusive state management. In the semi-arid, the forest under JFM experienced substantial increase in forest cover (+3.5%.y-1) as compared to exclusive state management and CBFM. More tree basal area were harvested under exclusive state management in the montane study forests as compared to JFM and CBFM and the differences were significant (p<0.05). In the semi-arid study forests, higher tree harvests were recorded under CBFM followed by JFM and exclusively state and thedifferences were significant (p<0.05). It was observed in this study that, regardless of management regime, participating villagers were unable to exclude people with no formal rights to the forests under PFM. It is therefore concluded that, decentralised management can impact forest resources both positively and negatively depending on institutional arrangements. However, some empirical evidence indicates that JFM and CBFM performed better than those under exclusive state management,although uncontrolled exploitation of the forest has continued also under these regimes. Although the two regimes are promising forest decentralisation models for Tanzania, more research is needed to understand the functions of different governance structures for decentralized forest management to achieve the goal of improving forest condition.Keywords: decentralised forest management, forest cover, montane, semiarid, north eastern Tanzani

The El Niño event of 2015-2016: climate anomalies and their impact on groundwater resources in East and Southern Africa

The impact of climate variability on groundwater storage has received limited attention despite widespread dependence on groundwater as a resource for drinking water, agriculture and industry. Here, we assess the climate anomalies that occurred over Southern Africa (SA) and East Africa, south of the Equator (EASE), during the major El Niño event of 2015–2016, and their associated impacts on groundwater storage, across scales, through analysis of in situ groundwater piezometry and Gravity Recovery and Climate Experiment (GRACE) satellite data. At the continental scale, the El Niño of 2015–2016 was associated with a pronounced dipole of opposing rainfall anomalies over EASE and Southern Africa, north–south of ∼12∘ S, a characteristic pattern of the El Niño–Southern Oscillation (ENSO). Over Southern Africa the most intense drought event in the historical record occurred, based on an analysis of the cross-scale areal intensity of surface water balance anomalies (as represented by the standardised precipitation evapotranspiration index – SPEI), with an estimated return period of at least 200 years and a best estimate of 260 years. Climate risks are changing, and we estimate that anthropogenic warming only (ignoring changes to other climate variables, e.g. precipitation) has approximately doubled the risk of such an extreme SPEI drought event. These surface water balance deficits suppressed groundwater recharge, leading to a substantial groundwater storage decline indicated by both GRACE satellite and piezometric data in the Limpopo basin. Conversely, over EASE during the 2015–2016 El Niño event, anomalously wet conditions were observed with an estimated return period of ∼10 years, likely moderated by the absence of a strongly positive Indian Ocean zonal mode phase. The strong but not extreme rainy season increased groundwater storage, as shown by satellite GRACE data and rising groundwater levels observed at a site in central Tanzania. We note substantial uncertainties in separating groundwater from total water storage in GRACE data and show that consistency between GRACE and piezometric estimates of groundwater storage is apparent when spatial averaging scales are comparable. These results have implications for sustainable and climate-resilient groundwater resource management, including the potential for adaptive strategies, such as managed aquifer recharge during episodic recharge events