Establishing the Linkages Between Forest Cover and Precipitation in Ghana

The conversation about ecosystem services in many ways revolves around forests.  Forests provide carbon sequestration benefits, support biodiversity, reduce erosion, filter water, to name just a few.  One ecosystem service that often gets overlooked is forests’ ability to create or maintain microclimates.  Models of the Amazon forest predict that a significant decrease in precipitation as forest cover decreases and runoff increases, with that freshwater lost to the ocean, rather than be returning to the atmosphere above the forest through evapotranspiration, later to return as precipitation.  As the hydrological cycle in the region is disrupted, overall precipitation levels can decrease, dramatically changing the climate.

Rainfall and Forests in Ghana

Understanding whether this relationship holds true in other regions can be an important tool in demonstrating the values of forest.  The Nature Conservation Research Centre, the Water Initiative’s partner in Ghana, commissioned a study to examine this relationship. Rebecca Asare, West Africa Coordinator of Forest Trends Incubator for Ecosystem Services, explained, “The reason we did the study was because our strategy in Ghana, from a policy and institutional perspective, is to underscore the importance of forests, to have another iron in the fire for our arguments.  Ghanaian decision makers are getting better at understanding the carbon value of the forest, few people understand how it’s important for climate.  In Ghana, agriculture is cocoa, oil palm, and food crops. If you lose your forest, then theoretically you’re losing rainfall as well.”

As we continue to develop new models of investments in watershed services, the ability to empirically connect precipitation events with forest or other land cover will open up a whole new avenue for investments.  Redefining watershed services in this way would be a radical contribution to the approach, since water availability as a result of precipitation is often thought of as beyond our control, changing only due to larger weather variation. “This kind of study is strategic in Ghana, because water is relatively plentiful, but there are concerns for the longer term availability of water”, said Phil Covell. “We want to draw out the relationships between the agricultural sector, rainfall, and hydrological services. The agricultural sector mainly relies on rainfall, so the question is whether we have enough evidence to justify a form of compensation for forest restoration and protection based on this relationship.”

The Approach

Because accurate precipitation data was not available, the researcher, Dr. Alexandra Morel of Oxford University, used cloud cover as a proxy for rain events. Using satellite data from 2004-2012, cloud frequency (how many clouds there were) and cloud initiations (where and when the clouds started).  By overlaying the cloud data on a land cover map, a correlation between cloud presence and land cover could be examined.  Because cloud cover also depends on a number of other variables – including time of day and year – results were averaged over the study period for each month.

Final_report.pdf   Google Drive

And the results were…

On average, the research showed that cloud frequencies are consistently higher over forested areas throughout the year.   However, a detailed look also showed some important variations.  First, the transition from coast to land plays an overwhelming role in cloud initiation.  Many more clouds formed in the forested coastal regions than in non-forested coastal regions; but this difference disappeared in the inland regions.  Intuitively, this result makes sense – the coastal soils and sands are warmer than the intact forest, so cloud cover forms as the prevailing winds push air from the coast towards the land.

Looking at cloud frequency – how much of the time cloud cover is there – rather than cloud initiation, inland forested areas did have slightly more clouds than the non-forested inland areas, despite no different in cloud initiation rates.  The clouds seem to be sticking around longer, which, by extension, could mean that they are producing more precipitation.

Where’s the tipping point?

Although the study has its limitations, the work clearly shows that the remaining patchwork of forest present in coastal Ghana plays an important role in local cloud patterns and freshwater availability.  “The forest reserves across the country were carved out by during the colonial era for watershed protection – they’re relics of the British government,” said Rebecca. “Our partner, Professor Yadvinder Malhi, pointed out that the British almost couldn’t have done a better spatial distribution of forest reserves. It’s almost like the way they fall across the landscape is the perfect distance before you start losing rainfall generation power. From one forest to the next is near perfect spacing in order to maintain rainfall generation across the landscape.”

Despite the patchiness of the forest cover, this study suggests that they are doing their job of maintaining the rainfall of Ghana. However, if this optimal spacing is lost and the forests are degraded, precipitation could decrease, threatening Ghana’s economy.  The question that should be on everyone’s tongue is, “What’s the tipping point?” As projects like the Water Initiative work to stress the economic value of forests, studies like this one provide more firepower for the argument for using financing mechanisms to pay for conservation of our forests – whether protecting colonial-era forest reserves or investing in best management practices on farms.

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