Lost and found in the Amazon: hydro signals emerge years later

Posted on September 21, 2011

It’s well known that land clearing can have profound impacts on hydrology.   In the tropics, deforestation typically increases soil compaction and runoff and  has complex effects on evaporation.  In areas of pasture, cropland and recent  regrowth, evaporation levels are modest, but they are significantly higher at the edges of forest fragments and in areas of older regrowth.  Variations in land cover, soil moisture and leaf characteristics affect heat transfer, humidity, atmospheric circulation and rainfall patterns.  Analyses of time-series data on rainfall and streamflow, together with comparisons of basins with different patterns of land use, have revealed clear hydrological responses to land clearing at the microcatchment level.  However, responses at larger scales are complex and poorly understood.  By relating hydrological parameters to landscape indices in the Ji-Parana Basin, Amazonia, Brazilian scientists explored how the strength of the hydrological response varied with spatial scale.  Gauging stations provided streamflow data and satellite imagery was used to generate indices of forest cover, patch density and the amount of forest edge.  The study covered the period from 1978 to 2006.  Although deforestation was carried out aggressively in many parts of the basin through the 1980s, there was no detectable response in the contemporary streamflow data.  However, significant responses at the sub-basin scale appeared in later data from the 1990s and showed that run-off and peak flows had increased.  These changes weren’t due to rainfall, which showed no trend over the same period.  Responses of hydrology to deforestation were weaker at larger spatial scales, presumably because of interactions between climate feedback, variable land use and other factors.   Hydrological responses were also time-dependent, being strongest after a lag of several years.  These lagged correlations might have been due to changes in topsoil properties or to higher evaporation rates in cleared areas with vigorous growth of invasive vegetation.  Lagged responses represent a challenge for mathematical modellers, most of whom have assumed that the effects of land-clearing are immediate.

 Reference:  Rodriguez, D.A., Tomasella, J. & Linhares, C.  2010.  Is the forest conversion to pasture affecting the hydrological response of Amazonian catchments?  Signals in the Ji-Paran´a Basin.  Hydrological  Processes 24, 1254–1269.