title: Soil moisture depletion under simulated drought in the Amazon: impacts on deep root uptake.
abstract: *Deep root water uptake in tropical Amazonian forests has been a major discovery during the last 15 yr. However, the effects of extended droughts, which may increase with climate change, on deep soil moisture utilization remain uncertain. *The current study utilized a 1999-2005 record of volumetric water content (VWC) under a throughfall exclusion experiment to calibrate a one-dimensional model of the hydrologic system to estimate VWC, and to quantify the rate of root uptake through 11.5 m of soil. *Simulations with root uptake compensation had a relative root mean square error (RRMSE) of 11% at 0-40 cm and < 5% at 350-1150 cm. The simulated contribution of deep root uptake under the control was c. 20% of water demand from 250 to 550 cm and c. 10% from 550 to 1150 cm. Furthermore, in years 2 (2001) and 3 (2002) of throughfall exclusion, deep root uptake increased as soil moisture was available but then declined to near zero in deep layers in 2003 and 2004. *Deep root uptake was limited despite high VWC (i.e. > 0.30 cm(3) cm(-3)). This limitation may partly be attributable to high residual water contents (theta(r)) in these high-clay (70-90%) soils or due to high soil-to-root resistance. The ability of deep roots and soils to contribute increasing amounts of water with extended drought will be limited.
authors.affiliation: Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, GA 30602, USA. dmarke@warnell.uga.edu
authors.forename: Daniel
authors.initials: D
authors.lastname: Markewitz
authors.forename: Scott
authors.initials: S
authors.lastname: Devine
authors.forename: Eric A
authors.initials: EA
authors.lastname: Davidson
authors.forename: Paulo
authors.initials: P
authors.lastname: Brando
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Nepstad
mesh_terms: D001938:Brazil; D002138:Calibration; D003198:Computer Simulation; D055864:Droughts; D008954:Models, Biological; D018517:Plant Roots / Q000378:metabolism*; D018526:Plant Transpiration / Q000502:physiology; D011891:Rain; D012987:Soil; D014197:Trees / Q000502:physiology*; D014867:Water / Q000378:metabolism*
chemical_list: D012987:Soil; D014867:Water