Local and distant impacts of water infrastructure on land use change in Africa.

The development of rural economies across the global south is often related to access to water and the development of water infrastructure. It has been argued that the drilling of wells and construction of new dams would unleash the agricultural potential of African nations that are exposed to seasonal water scarcity, strong interannual rainfall variability, and associated uncertainties in water availability. There are, however, some important environmental and social externalities that often remain overlooked, and whose impacts may be underestimated. The drilling of new wells has been reported to induce pastoralist communities to increase livestock density, often triggering overgrazing and land degradation, with important environmental impacts. For instance, overgrazing-induced loss of grass cover may allow for an increase in soil erosion and enhance the rate of dust emissions from denuded landscapes into the atmosphere with important impacts on local, regional, and global biogeochemical cycles. While this phenomenon is a known desertification pathway across subtropical Africa, its long-range impacts remain poorly understood. A region affected by overgrazing as a result the establishment of new boreholes and wells can be found in the Southern Kalahari (Southern Africa), where the loss in grass cover has been associated with the remobilization of linear dune systems. Here I use modeled pathways of air parcel trajectories starting from overgrazed regions of Southern Africa (as well as from other major dust sources in the Southern hemisphere) to investigate the potential impacts on the Southern Ocean. I found a positive relationship between ocean productivity and trajectory densities of dust emitted from Southern Africa. Water infrastructure can also have a more local impact on land use and rural livelihoods. For instance, dam construction for agriculture or other uses, is often seen as a solution to the water scarcity problem in Africa, a continent affected by high levels of economic water scarcity and a strong untapped irrigation potential. While water security is often presented as the pathway to poverty alleviation and invoked to justify large dam projects for irrigation, it is still unclear to what extent small holders will benefit from them. Are large dams built to the benefit of subsistence farmers or of large-scale commercial agriculture? Here I use remote sensing imagery in conjunction with advanced machine learning algorithms to map the irrigated areas (or ‘command areas’) that have appeared in the surroundings of 18 major dams built across the African continent between 2000 and 2015. I quantify the expansion of irrigation afforded by those dams, the associated changes in population density, forest cover, and farm size. I find that, while in the case of nine dams in the year 2000 there were no detectable farming patterns, in 2015 a substantial fraction of the command area (ranging between 8.5% and 96.7%) was taken by large-scale farms (i.e., parcels >200ha). Seven of the remaining 9 dams showed a significant increase in average farm size and number of farms between 2000 and 2015, with large-scale farming accounting for anywhere between 5.2% and 76.7% of the command area. Collectively, these results indicate that many recent dam projects in Africa are associated either with the establishment of large-scale farming or a transition from small-scale to mid-to-large scale agriculture. I analyze the value of water acquired by agribusiness investors by determining the value generated by irrigation through the enhancement of agricultural production in the command area of the same dams.

Advisor: D'Odorico, Paolo