How surface water in the Amazon is affected by land cover change 2-fold

(source: click here)

The way surface waters in the Amazon are interlinked with its land cover is summarised well in Coe et al (2009)'s paper. Through land cover changes, humans affect the quantity of surface waters by "changing how incoming precipitation and radiation are partitioned among sensible and latent heat fluxes, runoff, and river discharge and altering regional and continental scale precipitation patterns". 

While it is generally observed elsewhere on the planet that deforestation induced increased runoff, micro-scale comparisons of forest and deforested (pasture) land-cover responses to precipitation confirm this. Moraes et al (2006) looked at two areas in eastern Amazonia, and found that runoff increased from 3.2% to 17% of annual through fall when the land-cover was altered from forest to pasture.

On a much larger scale (a whole river catchment), the discharge of the Tocantins River (south-eastern Amazonia) was analysed together with precipitation records over two time periods that different in both their proportion of catchment used for agriculture vs natural forest, and the rate of deforestation (time-period 1 and 2). Here, we can draw a similar conclusion to that found on a micro scale - the river discharge was greater with greater pasture land-cover. However what I found interesting to evaluate upon is that this increase in discharge was not at all caused by an alteration to the precipitation pattern (the precipitation of the two time-periods were not significantly different!). The discharge in time-period 2 was 24% greater than in time-period 1, due to land-cover change altering the aggregate hydrological response of the catchment. 

(What is good to note here, and what I will touch on in the next blogs, is that this higher proportion of rainfall ending up in rivers and streams over pasture land is also due to less water being lost to the atmosphere via evapotraspiration compared to natural forest cover).

I want to give a quick preview to my next blog topics, in order to evaluate the points above within a larger climate-change context. It is predicted through many modelling studies that large-scale deforestation in the Amazon will reduce precipitation and moisture convergence over the land and also increase temperatures (e.g. Costa and Foley, 2000).

Thus, we have two opposing forcings on surface waters in the Amazon, both induced by deforestation (land-cover change): 1) the alterations made to the hydrological response to precipitation results in higher runoff proportions and thus increases river discharge. 2) the greater climatic response to a large-scale shift to pasture land-cover will reduce moisture convergence, reduce annual precipitation over the area and thus decrease river discharge.

This evaluation is obviously just plausible when looking at the aggregation of surface waters in the region. Local and regional changes to river discharge will more likely be determined by one effect over another other. Also, the large-scale changes in precipitation patterns over the Amazon have not yet been uniformly observed over the basin and are often also just regional responses to intense land-cover change rates.