Working Group I: The Scientific Basis

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14.2.5 Precipitation, Soil Moisture, and River Flow: Elements of the Hydrological Cycle

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.

Soil moisture is a key component in the land surface schemes in climate models, since it is closely related to evapotranspiration and thus to the apportioning of sensible and latent heat fluxes. It is primary in the formation of runoff and hence river-flow. Further, soil moisture is an important determinant of ecosystem structure and therein a primary means by which climate regulates (and is partially regulated by) ecosystem distribution. Soil moisture is an important regulator of plant productivity and sustainability of natural ecosystems. In turn terrestrial ecosystems recycle water vapour at the land-surface/atmosphere boundary, exchange numerous important trace gases with the atmosphere, and transfer water and biogeochemical compounds to river systems (see also the discussion in Chapter 7, Section 7.4.3 and Chapter 8, Section 8.5.4). New efforts are needed in the development of models, which successfully represent the space-time dynamics interaction between soil, climate and vegetation. If water is a central controlling aspect, then the interaction necessarily passes all the way through the space-time dynamics of soil moisture. Finally, adequate soil moisture is an essential resource for human activity. Consequently, accurate prediction of soil moisture is crucial for simulation of the hydrological cycle, of soil and vegetation biochemistry, including the cycling of carbon and nutrients, and of ecosystem structure and distribution as well as climate.

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 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, “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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