The impact of deforestation of the Gran Chaco in the precipitation of Gran Chaco and surrounding region

PATRICIA BLATTER, JUAN RUIZ Y MANUEL PULIDO

Foz do Iguazu

Congreso; American Geophysical Union, Meeting of the Americas 2010; 2010

American Geophysical Union

The atmospheric response to land use changes in the Gran Chaco region (a region that covers the Northeast of Argentina, East of Bolivia, most of Paraguay and a small part of Brazil) is examined using the WRF mesoscale model which includes NOAH module as land model. We focus on the modifications to the precipitation introduced by land use changes and on explaining the mechanisms of land-atmosphere interaction. Two periods were selected one where the observed precipitation is lower than expected and the other where a strong precipitation is found in the region associated with a low-level jet event. A series of 15 days numerical simulations were performed for both case studies using NCEP-GDAS as initial and boundary conditions with a 10 km horizontal resolution in a 3000x3000 km domain. The land use includes experiments with: evergreen broad leaf (from the USGS classification), actual land use classification (control case) and grassland in a rectangle representing the Gran Chaco region. Numerical experiments for each land use also include changes to the initial soil moisture. In particular, the actual soil moisture and 50% of the actual value are used for grassland cover and 100% and 150% of the actual soil moisture are used for the evergreen broad leaf cover and equivalent changes to the vegetation fraction are imposed. A strong sensitivity of the precipitation to land use changes is found. An increase of the mean precipitation is found for the evergreen broad leaf experiment with respect to the control case, particularly over the forcing region. This response is stronger if the initial soil moistureis 150% of the observed value. On the other hand the total precipitation decreases for grassland cover with respect to the control case. The precipitation changes are mainly associated with evaporation changes. In both case studies the precipitation differences between the evergreen broad leaf and the control case show a dipolar pattern with a negative response, i.e. a decrease of the precipitation for the evergreen broad leaf case, located to the southeast of the forcing region. Two possible explanations of this dipolar pattern are raised. A weaker LLJ is found in the evergreen broad leaf experiment and therefore the humidity transport to the southeast region is expected to diminish. Furthermore, the increase of the convection in the forcing region produces an increase of the subsidence which explains the dipolar pattern in the precipitation differences at fixed times and could also explain the pattern found in the mean precipitation differences. However the evidences are not conclusive with the two case studies that are examined in this work.