Geoscience Reference
In-Depth Information
carbon, and other elements in terrestrial systems by runoff, land-atmosphere
exchange, and other surface processes connects the biogeochemical cycles operating
in soil-based, freshwater, and marine systems. Quantifying changes in the water cycle
associated with climate change is therefore a critical element of building an
understanding of future changes in biogeochemical cycles.
Reconstruction of the monthly discharge of the largest rivers by Labat et al.
(2004) indicates that global continental runoff increased during the 20th century.
Changes in runoff have been linked to changes in precipitation, evapotranspiration,
and land use. Modeling of the relative contributions of precipitation, temperature,
carbon dioxide concentration, land cover, and land use to increases in river discharge
in the 20th century indicates that increases in precipitation are the dominant driver of
global increases in discharge (Gerten et al., 2008). Precipitation is expected to
increase with increasing temperature, though the rate of increase may be moderated
by the influence of tropospheric greenhouse gas forcing and black carbon aerosols on
precipitation (e.g., Frieler et al., 2011). Land use practices also contribute to increases
in discharge, particularly in watersheds characterized by extensive agriculture or
deforestation. For example, there is a strong correlation between agricultural land
cover in the Mississippi River basin and increased discharge under average
precipitation conditions, with agricultural land use accounting for more of the
increase in Mississippi River discharge in the past 50 years than do increases in
precipitation (Raymond et al., 2008). This agriculturally enhanced runoff can carry
high concentrations of nitrogen, phosphorus, and carbon (in the form of bicarbonate)
that impact the biogeochemistry of the receiving rivers and downstream marine
systems.
The role of climate-related changes in evapotranspiration in the intensification
of the water cycle is more challenging to sort out, in part because of feedbacks
between evapotranspiration and soil moisture. Elevated atmospheric carbon dioxide
has been tied to decreases in stomatal conductance (e.g., Leakey et al., 2009), which
could lead to decreased evapotranspiration and increased soil moisture (e.g., Gedney
et al., 2006). However, several lines of hydrological evidence (water balance
estimates, lysimeter and pan evaporation measurements, length of growing season)
point to an increase in evapotranspiration in temperate regions over the past 50 years
(Huntington, 2008). These results suggest that, at present, the effects of higher
temperatures are generally able to offset the effects of increased carbon dioxide on
evapotranspiration, though their relative effects are likely to vary geographically and
may change with future changes in climate and land cover.
While there are relatively long and spatially distributed records of runoff and
precipitation, fundamental hydrological parameters like soil moisture and
evapotranspiration are difficult to measure and, for the most part, existing data are
temporally and spatially sparse. To advance the science, measurements at points on
the landscape (e.g., from networks of flux towers) will have to be integrated smoothly
with areally distributed estimates derived from remote sensing (e.g., satellite
measurements of soil moisture). All these measurements will have to be coordinated
through new data assimilation methods with new theory appropriate for landscape
and regional scales. These and other new approaches to quantifying essential
hydrological parameters are necessary to resolve spatial and temporal trends in the
Search WWH ::
Custom Search