Replacement of Dry Forest by Crops in a semiarid region of Argentine: its effects in the energy budget

HOUSPANOSSIAN, J; NOSETTO, M D; JOBBÁGY, E G

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

American Geophysical Union

Dry forests (DF) of Argentina, extending over 900000km2, have been increasingly replaced during the last 10 years for crop and forage production. This land cover change may strongly affect the surface energy budget and the hydrologic cycle, with potential effects on the local, regional and global climate. In this study, we estimated the climatic radiative forcing associated with the albedo change of this land cover transformation. Additionally, we characterized the surface temperature and NDVI patterns as surrogates of latent heat and carbon fluxes, respectively. We focused on the Espinal region in central Argentina (San Luis, Lat:-35; Lon:-65.25) on an area originally dominated by a dense cover of Prosopis caldenia which is being replaced by annual crops (mainly soybean and corn). We used observations of albedo, surface temperature (Ts) and NDVI for the 2001-2002 growing season from MODIS sensor on board of TERRA satellite to characterize 40 sites covered with native dry forests and annual summer crops. Actual albedo was estimated from MCD43A3 (albedo shortwave, diffuse and direct) and MYD08 (aerosol optical depth) MODIS products. Remote sensing information combined with atmospheric profiles (re-analysis ERA-40 database) and ground meteorological data fed a columnar radiative transference (CMR) model to estimate upwelling shortwave fluxes at the tropopause level. The mean annual actual albedo was ~60% higher for annual crops compared to dry forests (0.16 and 0.10 for crops and dry forests, respectively). This implies that replacing dry forests by crops increased by ~30% the reflected shortwave radiation, with a cooling radiative forcing of -13.2 W m-2. In both dry forests and crops, maximum albedo values were observed in summer (0.19 and 0.12 for crops and dry forests, respectively) at the time of the peak of the growing season, as suggested by NDVI patterns. Interestingly, in spite of their higher radiative energy inputs, dry forests had lower canopy temperatures than crops (26.4 and 20.9 °C), suggesting a higher partition of absorbed energy into latent heat flux for the former. This study suggests that the replacement of dry forest by annual crops has a strong impact on the surface energy budget and on the partition between sensible and latent heat flux. The net effect of these vegetation change on climate is still uncertain, yet, given its growing extent, it should be considered in regional and global climatic models. Other biophysical (e.g. cloudiness) and biogeochemical (e.g. carbon balance) processes need to be considered for a full accounting of its net radiative forcing effect.