13. Title: Effect of woody cover removal on soil water use in the Arid Chaco analyzed at three
spatial scales
PI: Roberto Fernandez
Budget: USD $195,000
Period: 2007-2009
Agency: ANPCyT – Ministerio de Ciencia, Tecnología, e Innovación Productiva – Argentina
Summary:
Agricultural expansion and intensification has noticeable effects
on ecosystem structure. An example of this process is occurring in the
Arid Chaco of north-central Argentina, where trees and shrubs are
removed in order to increase forage production and accessibility to
cattle. The available evidence suggests that this change in physiognomy
is associated with equally important changes in ecosystem function but,
as occurs for other parts of the world, actual data are scarce. This is
particularly true regarding our understanding of the water cycle,
largely because of the difficulties in assessing its changes at the
appropriate spatial scale, which is that of the management unit
(paddock). Our central hypothesis is that woody plant removal, by
eliminating deep-rooting tress and shrubs, reduce the ecosystem ability
to evapotranspire rain water, and thus modifies its spatial and
temporal dynamics. We will compare deforested vs. control situations in
the San Luis province combining methods at three levels of detail. At
the most detailed scale (plot), we will monitor soil water dynamics,
shrub and tree sap flow, direct evaporation from the soil surface; deep
seepage will be assessed by using natural tracers. We will establish
permanent stations to monitor water seasonal and spatial (vertical and
horizontal) changes by non-destructive samples (repeated measurements)
and destructive ones (in order to increase spatial replication for both
water and tracers). At the paddock level, we will use
micrometeorological data, mainly moisture and temperature gradients, to
evaluate the effect of deforestation upon total evapotranspiration; we
will apply energy balance theory (Bowen ratio approach) from both
low-height towers and ultralight aircraft-based sensors. The main
advantage of this approach from the point of view of our objectives is
that, taking the necessary precautions, they are able to evaluate
changes at the paddock spatial scale. At the landscape scale, we will
use information provided by satellite-based sensors (visible and
thermal radiation) in order to spatially extrapolate the observations
from our permanent and airborne sensors, and to understand the
consequences of deforestation on the sink-source relations for vapor
and sensible heat. Finally, we will use simulation models applicable to
the larger scale to assess to which degree these changes can interact
with, and eventually modify, atmospheric dynamics. The project
activities will allow a group of young doctoral and postdoctoral
students to complete their education. Project results will contribute
to the rational design of land management practices and will allow
knowledge-based decisions affecting the use of hard-to-replace natural
resources (soil, water).