Fire/water link expanded
NASA scientists have added new details to the relationship between bushfires and drought.
Droughts are caused by a number of factors (usually in combination) including temperature shifts, overgrazing (which reduces vegetative cover, and therefore the ability of the soil to retain moisture) and deforestation.
Replacing the moist soil of vegetative cover with bare, shiny desert soil reflects sunlight directly back into space, diminishing the capacity for rainfall.
Similarly, fires dry out the soil and stymie the convection that brings rainfall.
Small particles called aerosols that are released into the air by smoke may also reduce the likelihood of rainfall. Water vapour in the atmosphere condenses on certain types and sizes of aerosols called cloud condensation nuclei to form clouds; when enough water vapour accumulates, rain droplets are formed, but too many aerosols make the water vapour spread out more diffusely to the point where rain droplets do not materialise.
A new study published in the journal Environmental Research Letters by Charles Ichoku, a senior scientist at NASA's Goddard Space Flight Center, has taken a more comprehensive look at the relationship between fire and the water in northern sub-Saharan Africa.
Ichoku and colleagues used satellite records from 2001 to 2014 to analyse the impact of fires on various water cycle indicators, namely soil moisture, precipitation, evapotranspiration and vegetation greenness.
They also focused on examining the interactions between clouds and smoke and the effects of fires on surface brightness.
When Ichoku used satellite data to match fire activity to hydrological indicators, a pattern emerged.
“There is a tendency for the net influence of fire to suppress precipitation in northern sub-Saharan Africa,” he said.
For example, in years that had more than average burning during the dry season, measurements of soil moisture, evaporation and vegetation greenness - all of which help to trigger rain -decreased in the following wet season.
Even within dry seasons, the amount of water decreased in areas with more humid climates as the burning became more severe.
The findings will be used to improve modelling, which may then be able to explain some of the study's seemingly paradoxical findings, including the fact that even as fires decreased by 2 to 7 percent each year from 2006 to 2013, precipitation during those years did not increase proportionately.
Ichoku thinks one possible reason a decrease in fires did not result in more precipitation has to do with the change in the types of lands that are being burned.
Across the study period, more forests and wetlands were converted to cropland than in previous years.
Ichoku notes that recent droughts have drawn people to farm areas that have more water. The drawback is that such land types provide a significant amount of moisture to the atmosphere that eventually becomes rain, so their conversion to farmland poses a threat to future water availability.
“The removal of vegetal cover through burning would likely increase water runoff when it rains, potentially reducing their water retention capacity and invariably the soil moisture,” Ichoku said.
“The resulting farming would likely deplete rather than conserve the residual moisture, and in some cases, may even require irrigation. Therefore, such land cover conversions can potentially exacerbate the drought.”