Study impacts of land use change in southern South America.

N. L. PESSACG; S. A. SOLMAN

Florianópolis

Workshop; Meeting CLARIS-LPB M26; 2010

CLARIS-LPB PROJECT

Two of the main specific objectives of WP5 are: • To study the land-atmosphere feedbacks to identify the regions of larger interaction and to analyze the role of soil moisture conditions and land use on the development of precipitation. • To examine the regional effects of climate change and variability on the La Plata basin hydrologic cycle, with emphasis in land surface-atmosphere feedbacks and their impact on extreme events. In this context we are working to study the impact of changes in land-use in the climate over LPB. With this objective, sensitivity experiments to land use/land cover change were performed with the Fifth-generation Pennsylvania-State University-NCAR nonhydrostatic Mesoscale Model (MM5). Land use changes over South America have been estimated from transformed maps based on satellite observational corresponding to 2008 and 2000 respectively. Take in our account these evidences of land use changes over South America, three idealized land use scenarios were defined, where the natural land cover was replaced over most of the LPB region by dry land crop pasture (CROP simulation), by evergreen broadleaf (FOREST simulation), and by bare soil (BARE simulation) respectively. One-year length experiments for the land use change scenarios and control simulation were compared. The period simulated corresponds to 1996-1997 and the focus is on SON and DJF, the austral spring and summer season These highly idealized scenarios have the purpose of evaluating what extent large changes in land use may impact on the climatic characteristics over the target region. Three factors largely control the impact of deforestation: albedo, evaporation and surface roughness length (Sellers 1992). Modifications in these variables linked with land cover changes, induced several positive and negative feedbacks. We can detect that the decrease of roughness and roots between the forest scenario and the bare soil experiment drives to a reduction of latent heat flux associated with a reduction of the aerodynamic roughness of surfaces and also with a decrease of the capacity of root for extracting soil moisture. Other remarkable feedback is associated with the albedo increase between these scenarios, which leads to a decrease in the net short wave radiation. The solar and long wave changes together imply in the most of regions analyzed a decrease in the net radiation at surface. This decrease in the net radiation in combination with the important reduction of latent heat flux lead to a large increase in the surface energy budget, that drives to a significance increase of temperature in BARE simulation respects FOREST run. The reduced availability of moisture near the surface together with a relatively more stable planetary boundary layer resulted in a decrease of low and medium-level cloud cover and a decrease in precipitation.