Annex 5: Hydrological Data in Global Climate and Terrestrial Observing Systems

(prepared by Mr Kaczmarek)

Background

1. This document presents a summary of discussion held during the second session of CHy Working Group on Operational Hydrology, Climate and the Environment (Silver Spring, 2 to 7 October 1995). In addition to WG members, a number of invited experts joined the meeting under the agenda item dealing with collection and processing of hydrological data for global climate and terrestrial observing systems.

2. The following discussion papers were distributed among the participants:

(a) Hydrological data in global climate and terrestrial observing systems (submitted by Mr. M. Beran, chairman of the Working Group);

(b) Hydrological data for global observing systems (submitted by the WMO Secretariat). Material contained in these papers was also employed in writing this summary.

3. Participants informed the meeting on developments regarding the following international activities relevant to global observing systems:

• Global Climate Observing System (GCOS),

• Global Terrestrial Observing System (GTOS),

• World Hydrological Cycle Observing System (WHYCOS),

• Flow Regimes for International Experimental and Network Data (FRIEND) project,

• Global Runoff Data Center in Koblenz,

• Intergovernmental Panel on Climate Change (IPCC).

4. The following strategy problems concerning the role of hydrologic data in global observing systems were investigated:

(a) detecting and documenting climate change and variability;

(b) detecting and documenting change in hydrological systems and its impacts to water resources;

(c) providing hydrological input to global climate model development and applications;

(d) enhancing the role of hydrology in studying land/soil degradation, natural ecosystems, biodiversity loss, etc. Main conclusions of the discussion on the above mentioned subjects are presented below. They should be considered as preliminary opinions and in most cases require further investigations.

Hydrological data for detecting climate change

5. The hydrological cycle affects and is affected by the earth's climate system in a fundamental sense. It contributes in a major way to the energy exchange between the earth surface and the atmosphere. The inherent difficulties, however, in using hydrologic observations in detecting and documenting climate variability and change are:

(a) low values of signal-to-noise ratio; the maybe climate change component is weak as compared to the strong natural variability of hydrologic variables;

(b) a possible compensatory effect: a rise of temperature (and consequently of potential evapotranspiration and increase in precipitation may lead to a small net effect on runoff;

(c) strong non-climatic anthropogenic impacts on water resources in many regions of the World, difficult to distinguish from the climate change signal.

6. To detect climate change and variability, the following standard hydrologic variables are to be considered:

6a. Discharge data: monthly data for seasonal and inter-annual changes, and instantaneous or event-based data for changes of extremes. Discharge measurements are routinely made by hydrological services (or other agencies), and easy to obtain. For the climate change assessment observations from catchments with minimal anthropogenic impacts (benchmark, pristine), or with accountable impacts should be used. A study is needed to ascribe a number of stations in various parts of the globe to document climate change and variability: density will not be uniform and depend on several factors, such as topography.

6b. Lake levels: data collected by national hydrological services and other institutions, for a set of large lakes with negligible/accountable anthropogenic impacts may be used. The required temporal resolution: monthly to annual.

6c. Ice phenomena in lakes and rivers: time series of dates of freeze-up and ice cover break-up may serve as an indicator of climate change. Such observations are usually made by national services or research institutions, but in some cases data may be available also from other sources (e.g. ice cover break-up reported in the press). It is required that for each typical GCM grid (e.g. 2.5° by 2.5°) a representative group of water bodies (one river, one shallow lake, and one deep lake) be selected for analysis.

6d. Snow cover: time series of the extent, duration and water equivalent of snow cover may serve as climate change indicator. Temporal resolution and spatial densities of snow measurements are described in WMO guidelines.

7. There is a number of other hydrologic variables of importance for detecting climate change, but presently observed mostly in a research context, as e.g. evapotranspiration, groundwater table in pristine aquifers, and lake stratification patterns. If available, such data should also be collected and analysed.

Detecting change in hydrologic and water resource systems

8. Most of the above mentioned hydrological observations, used for detecting change in climate may also serve to identify non-stationarities in hydrologic processes, and consequently assess changes in regional water supply and demand, and to evaluate possible enhancement of water-borne hazards (e.g. floods).

9. In particular, such data should be systematically compiled and analyzed for:

• globally significant freshwater bodies,

• pristine and/or “stable” ecosystems,

• research basins used for studying universal-type problems.

The variables under investigation should be appropriate to element/problem concerned, and dependent on element exhibiting impact (droughts and floods, ice break data, ecology of aquatic systems).

10. The globally significant freshwater bodies should include:

• a set of largest rivers of the World, accounting for at least 50% of total runoff to oceans;

• selected large lakes;

• largest World's aquifers;

• water bodies representative of major anthropogenic influences, selected on a basis of population, economic development, possible water induced international conflicts, etc.

11. The pristine and/or “stable” ecosystems should be distributed throughout the World reflecting various climatic and physiographic conditions. The stability concept should be rigorously defined.

12. Research basins should be selected in order to address universal problems, e.g. effects of urbanization, deforestation, role of mining, etc. Irrigated agriculture may be particularly sensitive to changes in precipitation, potential evapotranspiration and soil moisture. The selected experimental catchments should allow to study most critical problems identified in Agenda 21, be representative geographically and of different scales.

Hydrological data for environmental changes

13. The various 'problem areas' dealt with under this heading can be broadly divided into those where there is direct anthropogenic change leading to a defined problem and others whose changes are indirectly influenced by man's activity. Table 1 shows that broad division and the associated role of hydrological sciences.

14. It would be difficult to consider hydrologic data needs at the level of individual variables, because of the great diversity that exists in the underlining processes. Moreover, the hydrologist would in most cases be in support role to the specialists most directly involved in a given problem area.

15. This is less so in the case of water quality in rivers and lakes, and in wetlands. For water quality issues, a tier system was found to be a useful concept (about the GCOS tier system see e.g.: “GCOS/GTOS Plan for Terrestrial Climate-related Observations, version 1.0”: WMO/TD - No. 721).

16. There are processes of pollution and its interaction with the aquatic environment for which a tier 1 and tier 2 approach is required. GEWEX and IGBP activities need to consider this issue, especially focused on environments where processes are understood. Also tier 3 gives a reasonable fit, because it is likely that hydrometric stations in WHYCOS would also serve as the location for water quality measurements.

17. The meeting agreed that it was only possible to give generalized guidance as to the role of hydrologic data in other areas. Hydrologists would play an important though supportive role in the design and implementation of the networks required to serve the needs at the various tiers.

Input to climate model development and applications

18. A new focus for hydrology is what has been termed “macro-modelling”. In essence this comprises modelling procedures for the elements of the water balance for very large basins, that are not too demanding of data and are consistent across main subcatchments or down the main river stem. Macro-modelling is justified both for water resources assessment and management in major basins, and because it is capable of adding surface and subsurface transports to soil-vegetation-atmosphere transport schemes (SVAT) that provide the land-surface description of general circulation models (GCM's).

19. Physically based models of land surface processes used as parameterization of sub-grid land surface processes in atmospheric GCM's require hydrologic and land surface data shown in Table 2.

Table 2

20. The following aspects of model development and application require hydrological data gathered according to different data collection strategies:

(a) Model development phase:

• model formulation,

• estimation of model parameters,

• testing before application.

(b) Application phase:

• data to produce model inputs,

• data assimilation to produce model initial conditions,

• model testing, evaluation and verification.

21. Most components of surface water and energy budgets cannot be measured sufficiently well for budget to close, except possibly for limited experimental situations. For example, soil moisture at model scales cannot be measured directly and has not been measured historically at the scales of its influence on the atmosphere. Data sets of water and energy budget variables organized for model development and application must be derived or assimilated from a wide range of observations. Some of these data sets are needed historically as well as in the future for model development, testing/evaluation, and for simulation of atmospheric processes.

22. Variables in water and energy budgets (both fluxes and state variables) include components of variability over a wide range of space and time scales (i.e. they are heterogeneous in space and time). In addition, most of the processes that govern their dynamics also have heterogeneous elements (e.g. soil characteristics, vegetation and topography).

23. Typical modelling scales for General Circulation Models and Numerical Weather Prediction Models (NWP's) range from about 10 km to more than 100 km. These models resolve processes at scales above grid scale, but process dynamics below grid scales must be parameterized at grid scale. The data needed to develop, test and operate these models must be derived for these scales from observations at other scales, routinely performed by national hydrologic and meteorological services.

24. An example of hydrologic data requirements for a continental-scale hydrologic project (GEWEX/GCIP - Mississippi river) is shown in Table 3.

Table 3

General issues

25. Other matters that have been discussed include:

• data quality control,

• archiving, accessibility, availability and processing of the data,

• the requirement and means of compressing large volumes of data and products into a usable form,

• the relative merits and possibilities of using distributed versus centralized data bases,

• establishing links to other data sources,

• the need for a long-term commitment from data suppliers, and

• the possible effects of commercialization of national services.

There was a general agreement on the importance of collecting metadata on hydrologic stations: its location, instrumentation, collection procedures etc.