A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Seasonal variations of soil moisture in an open savanna (campo sujo)in central Brazil.Quesada, C.A.; Santos, A.J.B.; Breyer, L.M.; Miranda, A.C.; Miranda, H.S. & Viana, S.Departamento de Ecologia, Universidade de Brasília, Brasília-DF. quesada@unb.brSavanna formations with scattered shrubs, few trees and a continuous herbaceous layer,characterise the Campo Sujo, an open form of the Brazilian savannas. High seasonality isan important characteristic what may suggest strong water stress during the dry seasonwhat is about five to six months long. The present study was carried out from August 1999to September of 2001 at the Reserva Ecólogica do IBGE, 35 km south of Brasília, Brazil(15 o 56′41′′ S e 47 o 51′02′′ W). The soil is a well-drained yellowish red oxisol with claytexture (60% of clay) and less than 2º of slope. The measurements of soil water contentwere taken with a neutron probe in three access tubes, measuring 3.60 m in length. Soilmoisture was marked seasonal, the variation between the wettest and driest day was403.3 ± 7.7 mm 65% of which occurred below 1m. The profile storage at the last days ofthe 1999, 2000 and 2001 dry seasons was very similar despite a difference in dry seasonduration and large differences in rainfall in the preceding wet seasons, indicating that thevegetation is conservative in its water use. A water balance was done to determinateevapotranspiration rates (E) and others components of the annual water balance. E from wetseason was determinate as 2.4 mm/d and 1.6 mm/d in the dry season. During the end of allthe measured dry seasons the top 0.6 m water content did not change for about two monthsuntil the onset of the subsequent wet season, suggesting that the vegetation has already usedall the available water content from this soil layer.