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| Water discharge (Surface)
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| Definition |
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Surface discharge is a measure of the volume of water flowing through a river channel cross-section per unit of time.
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| Rationale |
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As parameter used in water budget equations, discharge is a very important quantity for GCM validation. Independently measured discharge can thus provide a check on other parameters computed by the numerical climate model (e.g. the surface evapotranspiration rate on the seasonal time scale). Freshwater discharge from river mouths also strongly influences oceanic circulation at interannual to decadal time scales. Discharge is a high priority observation because it is essential for climate impact modellers (e.g. for models of climate change impacts on water resource availability and extreme events). Discharge from pristine river basins may be a fundamental climate change detection element, since it naturally integrates an area and is very sensitive to climate change. Also, measurement errors are likely to be random at this scale.
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| Users |
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Discharge is a critically important variable for GCM modellers, climate impact modellers, modelling of extreme events, hydrological modellers, and national and regional analysts/planners.
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| Assessment method |
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Tiers 1 to 3: direct measurement.
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| Units of Measure |
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m3 /s.
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| Frequency of measurement |
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Depends on the specific question being asked (see Spatial resolution). With regards to data archiving, for tier 1 experiments it is generally expected that 15 minute to 1 hour data will be needed. For tier 2, hourly to mean daily data is needed and, while tier 3, mean daily to monthly data is required. The data should be collected as specified by WMO.
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| Spatial resolution |
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The spatial resolution of the data very much depends on the specific question being answered and on sampling tier:
Tier 1 (detailed experimental catchments): process-based studies generally conducted in catchments of up to 100 km2;
Tier 2 (small pristine catchments up to 1000 km2): climate change detection. These catchments should have minimum interference from anthropogenic influences;
Tier 3 (catchments greater than 100 km2): used for national and regional runoff studies along the lines managed by the GRDC. Such catchments provide an overlap with tiers 1 and 2.
Note that there should be some flexibility in the area specified for each tier depending on the specific question that is being answered and the environmental conditions of the site (e.g. mountains vis-à-vis plains).
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| Accuracy/precision required |
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The goal for all discharge data is within ± 5% of true value;
Due to interfering factors, ± 15-20% accuracy could be acceptable.
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| Associated measurements |
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River level, stage-discharge relationships, global digital map of river basin boundaries, time-dependent adjustment factors to river discharge data, and accounting for withdrawal of water for human use (irrigation, municipal water supplies, etc.).
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| Present status |
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Discharge measurements are routinely made by hydrological services (or other agencies), and are easy to obtain. For 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 (the density will not be uniform and will depend on several factors, such as topography).
The quality of river runoff data collected by national, state, local services is very variable. Measurement accuracy is poor due to a lack of reliable stage-discharge relationships (as logistic problems and lack of funding prevents a regular updating, especially at high stages), unknown changes in channel cross-section of different flood stages, and lack of current metre rating. Geographical coverage is deficient in many areas, especially Africa and Latin America (coincidentally where the major rivers of the globe are located). Overall, the data in the developing world are insufficient to meet national needs and to allow the global hydrological cycle to be closed. Moreover, in recent years the network has declined significantly in quality and quantity. WHYCOS could be the solution, but needs to achieve global coverage and solve the logistical problems of national hydrometric agencies in developing countries in a sustainable way.
Data are not often archived nationally or internationally in ways that facilitate their long-term use. An effort to be commended and encouraged is that of GRDC in Koblenz (Germany) which is concerned with the collection, archival and distribution of global river discharges (but the data sets have serious deficiencies for geographical coverage and duration of the data records.
It is immediately possible to specify 150 to 200 stations at tier 3 that are required by the GRDC to obtain regional and continental discharge to the oceans. These stations can become operational in the next two years. However, to define a very specific IOS, particularly for tiers 1 and 2, the climatologists and GTOS community need to define clearly the specific questions to be answered.
The network design needs to be carried out in conjunction with national agency needs. It is unrealistic to expect that individual nations will establish and operate stations exclusively for GTOS and GCOS. Therefore it is critical that sites be selected that serve multiple purposes. The selection process of stations will be different for developing and developed countries. In developed countries the most appropriate catchments from the existing set should be selected. In contrast, in most developing countries, it will be necessary to provide for upgrading and/or rehabilitation of stations to obtain a sufficient density.
There are a number of networks already in existence that will meet many of the climate needs of GTOS and GCOS. Many of the existing national networks are sufficient for climate purposes. Governments need to be encouraged to make their data readily available. It is suggested that GCOS and GTOS work through the FRIEND programme to encourage governments to make at least a subset of their data available to the GCOS/GTOS community. 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.
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| R and D needed |
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Spatial and temporal aggregation and disaggregation for application of measured data for modellers' needs. Improve measurement technology appropriate for large rivers and rivers subject to ice jamming.
For climate change detection, in addition to the 150 to 200 stations mentioned above, there is a need for additional stations at tier 3, located in homogeneous climates and with a minimum of anthropogenic influences (such as hydrologic structures). In the longer term, it is desirable to have at least one station for every 2˚ by 2˚ grid. The network design needs to be carried out in conjunction with national agency needs, since it is unlikely that individual nations will establish and operate stations exclusively for GTOS/GCOS. Therefore it is critical that sites be selected that serve multiple purposes.
Generally, it has to be assumed that the data provided are of sufficient quality, as there is so much variability in hydrological data that checking data quality almost impossible.
The following actions are most urgently needed:
- Filling gaps in spatial and temporal resolution;
- Upgrade existing stations to improve the collection, quality and transmission of data;
- Rehabilitate stations where the infrastructure has to be replaced;
- Establish the macro-scale network proposed by GRDC and enhance it to obtain a homogeneous time series. These stations should be asked to update data and fill time gaps;
- Improve transmission of data utilising WHYCOS where possible;
- The priority (in decreasing order) should be given to filling temporal gaps in existing records; upgrading of stations; rehabilitation of existing stations; and installation of new stations to fill spatial gaps.
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