Deep Soil Water Dynamics and Depletion by Secondary Vegetation in the Eastern Amazon

Secondary/fallow vegetation is found extensively in the Eastern Amazon. The role of this deep-rooting vegetation in the hydrological cycle is unknown. We studied the water dynamics of this vegetation with emphasis on the deeper soil by means of a soil water model. The soil hydraulic properties were optimized in an inverse modeling procedure using the soil water model Hydrus-1D providing rainfall, actual evapotranspiration (Ea) determined with the Bowen ratio energy balance method (BREB), rooting depth, and distribution, as well as the in situ control measurements of soil water pressure head and soil moisture. In 1997, Ea according to BREB measurements amounted to 1174 mm, in 1998 this was 1475 mm. Modeled drainage at a 10-m depth in the 2 yr was 951 and 1016 mm, respectively. The model indicated that around 27% of the soil water was taken up below the 0.9-m depth in the 2-yr study period. During the severe 1997 dry season, according to the soil water model Ea was reduced drastically, as the soil water storage was depleted. According to micrometeorological measurements, however, Ea was not reduced as extremely. This difference might be due to general uncertainties of the soil water model as well as BREB measurements. On the other hand, formation and subsequent evaporation of early morning dew apparently contributed to Ea, which was not considered in the soil water model. In general, besides slightly lower interception, the water budget of a young secondary vegetation did not differ from that reported for Amazonian primary forest.