|Working Group I: The Scientific Basis|
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14.2.5 Precipitation, Soil Moisture, and River Flow: Elements of the Hydrological
Changes in precipitation could have significant impacts on society. Precipitation
is an essential element in determining the availability of drinking water and
the level of soil moisture. Improved treatment of precipitation (see Chapter
7, Section 7.2.3) is an essential step.
River systems are linked to regional and continental-scale hydrology through
interactions among precipitation, evapotranspiration, soil water, and runoff
in terrestrial ecosystems. River systems, and more generally the entire global
water cycle, control the movement of constituents over vast distances, from
the continental land-masses to the world’s oceans and to the atmosphere.
Rivers are also central features of human settlement and development.
It appears, however, that a significant level of variance exists among land models, associated with unresolved differences among parametrization details (particularly difficulties in the modelling of soil hydrology) and parameter sets. In fact, many of the changes in land-surface models since the SAR fall within this range of model diversity. It is not known to what extent these differences in land-surface response translate into differences in global climate sensitivity (see Chapter 8, Section 220.127.116.11) although the uncertainty associated with the land-surface response must be smaller than the uncertainty associated with clouds (Lofgren, 1995). There is model-based evidence indicating that these differences in the land-surface response may be significant for the simulation of the local land-surface climate and regional atmospheric climate changes (see Chapter 7, Section 7.4).
Much attention in the land-surface modelling community has been directed toward
the diversity of parametrizations of water and energy fluxes (see Chapter
7, Sections 7.4, 7.5, and Chapter
8, Section 8.5). Intercomparison experiments (see Chapter
8, Section 8.5.4) have quantified the inter-model differences in response
to prescribed atmospheric forcing, and have demonstrated that the most significant
outliers can be understood in terms of unrealistic physical approximations in
their formulation, particularly the neglect of stomatal resistance. Some coupled
models now employ some form of stomatal resistance to evaporation.
Climate-induced changes in vegetation have potentially large climatic implications,
but are still generally neglected in the coupled-model experiments used to estimate
future changes in climate (see Chapter 8).
There is, obviously, a direct coupling between predicted soil moisture and
predicted river flows and the availability of water for human use. Complex patterns
of locally generated runoff are transformed into horizontal transport as rivers
through the drainage basin. Moreover, any global perspective on surface hydrology
must explicitly recognise the impact of human intervention in the water cycle,
not only through climate and land-use change, but also through the operation
of impoundments, inter-basin transfers, and consumptive use.
Recognition of the importance of land hydrology for the salinity distribution
of the oceans is one reason for seeking improvements in models for routing runoff
to the oceans (see more precise cites here and in Chapter
7). Most coupled models now return land runoff to the ocean as fresh water
(see Chapter 8). Runoff is collected over geographically
realistic river basins and mixed into the ocean at the appropriate river mouths.
Although this routing is performed instantaneously in some models, the trend
is toward model representation of the significant time-lag (order of a month)
in runoff production to river-ocean discharge. What is needed for a variety
of reasons, however, is for river flow itself to be treated in models of the
climate system. (See Chapter 7, Section 7.4.3.)
On land, surface processes have until very recently been treated summarily
in Atmospheric General Circulation Models (AGCMs). The focus of evaluating AGCMs
has been on large-scale dynamics and certain meteorological variables; far less
so on the partitioning of sensible and latent heat flux, or the moisture content
of the planetary boundary layer. When the goals of climate modelling are expanded
to include terrestrial biosphere function, such aspects become of central importance
as regulators of the interaction between the carbon and water cycles. Terrestrial
flux and boundary-layer measurements represent a new, expanding and potentially
hugely important resource for improving our understanding of these processes
and their representation in models of the climate system. (See Chapter
7, Section 7.4.1.)
The spatial resolution of current global climate models, roughly 200 km, is
too coarse to simulate the impact of global change on most individual river
basins. To verify the transport models will require budgets of water and other
biogeochemical constituents for large basins of the world. This requires ground-based
meteorology in tandem with remotely sensed data for a series of variables, including
information on precipitation, soils, land cover, surface radiation, status of
the vegetative canopy, topography, floodplain extent, and inundation. Model
results can be constrained by using a database of observed discharge and constituent
fluxes at key locations within the drainage basins analysed. Climate time-series
and monthly discharge data for the past several decades at selected locations
provide the opportunity for important tests of models, including appraisal of
the impact of episodic events, such as El Niño, on surface water balance
and river discharge. It will be necessary to inventory, document, and make available
such data sets to identify gaps in our knowledge, and where it is necessary
to collect additional data. Even in the best-represented regions of the globe
coherent time-series are available for only the last 30 years or less. This
lack of data constrains our ability to construct and test riverine flux models.
Standardised protocols, in terms of sampling frequency, spatial distribution
of sampling networks, and chemical analyses are needed to ensure the production
of comparable data sets in disparate parts of the globe. Upgrades of the basic
monitoring system for discharge and riverborne constituents at the large scale
are therefore required.
In sum, hydrological processes and energy exchange, especially those involving clouds, surface exchanges, and interactions of these with radiation are crucial for further progress in modelling the atmosphere. Feedbacks with land require careful attention to the treatments of evapotranspiration, soil moisture storage, and runoff. All of these occur on spatial scales which are fine compared with the model meshes, so the question of scaling must be addressed. These improvements must be paralleled by the acquisition of global data sets for validation of these treatments. Validation of models against global and regional requirements for conservation of energy is especially important in this regard. As noted in Chapter 8 (Section 18.104.22.168), “Uncertainty in land surface processes, coupled with uncertainty in parameter data combines, at this time, to limit the confidence we have in the simulated regional impacts of increasing CO2.”
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