Today maize is the most important staple food crop in Africa. Much of this maize is grown on small farms or individual smallholdings, where farmers are dependent on the seasonal rains to irrigate their crops. A run of droughts can be catastrophic for these farmers, and understanding how global change is impacting water availability is vital if they are to preserve their livelihoods.

With this in mind Lyndon Estes from Princeton University, US, and colleagues studied changes in water availability during the maize growing season between 1979 and 2010. They used a bias-corrected meteorological dataset (using real meteorological station data to remove bias from weather simulations and satellite data) to look at precipitation, evapotranspiration and overall water availability for 20 different maize-growing African countries.

The results show a varied picture across the continent. In Southern Africa, particularly South Africa, maize-growing regions benefited from increased water availability over the period studied. Reduced evapotranspiration appears to be the driving force in this case, primarily resulting from declining net radiation, and to a lesser extent from increasing vapour pressure and declining temperatures. The reasons for the reduction in net radiation are unclear but may be linked to increased aerosols in the atmosphere or increased cloud cover (or perhaps data error).

Estes and his colleagues speculate that an increase in the use of irrigation may also help explain the trend, with respect to the roles of temperature and vapour pressure. “When you irrigate, you increase the water available for evaporation from crop transpiration and the soil surface,” said Estes. “Evaporation has a cooling effect by transferring heat from the state in which we feel it into water vapour. So if you irrigate a large enough area, then it makes sense that this might have a cooling effect.”

Sahelian zone countries in West Africa, along with Ethiopia in East Africa, meanwhile, appear to have benefited from a rebound from the long-term Sahelian drought. How long that recovery will last is not clear, however. “Evidence suggests that the region may alternate between wet and dry states, each of which may persist for decades,” said Estes.

A small number of East African countries, including Tanzania and Malawi, experienced declines in water availability, mostly due to decreased rainfall but also because of an increased demand for water.

Although the results cannot completely explain the changes in maize yields across Africa – factors not included in this study, such as increasing use of technology, political instability and subsidy policies all play a big role – Estes and his colleagues believe that their findings could still help maize farmers plan ahead. “These results can help guide more detailed studies that will give us more understanding of why yields have changed,” he said. “If the changes in water supply prove to be important, then that could guide policy on irrigation and crop variety, for example.” Exactly the kind of information that Tanzanian maize farmers are looking for.

The team reported their study in Environmental Research Letters (ERL) as part of the ERL Focus on African Environmental Processes and Water-Cycle Dynamics.

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