GCOS Essential Climate Variables - Terrestrial

Introduction: River discharge plays an important role in driving the climate system, as the freshwater inflow to the oceans may influence thermohaline circulation. The statistical properties of river discharge are an indicator for climatic change and variability, as they reflect changes in precipitation and evapotranspiration and are influenced in the longer term by land cover. River discharge data are also required for the calibration and validation of climate and impact models, trend analysis, and socio-economic investigations. Monthly observations of river discharge are generally sufficient to estimate continental runoff into the ocean, although daily data are needed to calculate the statistical parameters of river discharge, e.g., for impact analyses of extreme discharge.

Most countries monitor river discharge, yet many are reluctant to release their data. Additional difficulties arise because data are organized in scattered and fragmented ways, i.e., data are managed at sub-national levels, in different sectors, and using different archival systems. Even for those data providers that do release their data, delays of a number of years can occur before data are delivered to International Data Centres such as the Global Runoff Data Centre (GRDC). In addition to the need for better access to existing data, the trend toward shrinking observing networks in some countries (especially the closing of stations with long records) needs to be reversed.

Research concerning interferometric and altimetry-based approaches to river water level and discharge monitoring from satellites are being developed by the space agencies, encouraged by GCOS, TOPC and IGWCO.

With current technology in situ systems offer the most complete basis for river discharge monitoring. The GRDC has a mandate, through resolution 21 (WMO Congress XII, 1995), to collect river discharge data on behalf of all Members in a free and unrestricted manner, in accordance with resolution 25 (WMO Congress XIII, 1999). However, there are major gaps in the data received by the GRDC, both in terms of the number of rivers monitored and the time it takes for the GRDC to receive the data. Based on past demands for data, the GRDC has proposed a baseline network of river discharge stations near the downstream end of the largest rivers of the world – as ranked by their long-term average annual volume These stations, a subset of existing gauging stations around the world, collectively form a new GCOS/GTOS baseline network, the Global Terrestrial Network – River Discharge (GTN-R). Data from these stations will capture about 70% of the global freshwater flux to the oceans (see figure below). All these stations have reported at some time in the past, and most are operating today. This network is now being adjusted in consultation with national hydrological services, and a total of 185 stations have been confirmed. The status of another 265 stations has not yet been clarified.

Baseline River Discharge Network (GTN-R) based on the GRDC priority stations (Source: GRDC).

The WMO, through CHy, will request that the National Hydrological Services responsible for the stations marked in Figure 12 (a) evaluate the identified gauging stations, determine their operational status and provide the GRDC with this information, i.e., all existing data and metadata, including the measurement and data transmission technology used; and (b) ensure that daily discharge data be submitted to the GRDC within one year of its observation (definition of “near-real time”).

Whilst the emphasis will be initially on the priority stations and a reporting delay of no more than one year, it is a limited but important step towards the ultimate goal of near real-time receipt from as many stations as possible on all significant rivers. Some stations are currently able to transmit near real-time data; others need to be upgraded. The GTN-R, in cooperation with WMO CHy, will develop standards for the near real-time transmission to the GRDC of river discharge observations taken by the National Hydrological Services. When these standards have been developed, they will be presented to CHy for its approval, and then promulgated by WMO. Implementation will be assessed by the number of priority stations reporting annually with a maximum one-year delay, by the number of near real-time stations established, by the amount of data transferred or made accessible, and by the number of countries submitting timely data to the GRDC. Long-term, regular measurements of upstream river discharge on a more detailed spatial scale than GTN-R within countries and catchment areas are necessary to assess potential impacts of climate change on river discharge in terms of river management, water supply, transport and ecosystems. Parties should take steps to assess their individual needs for such measurements, and propose steps to fill gaps in their river gauge network, as identified.

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Measurement: For many rivers, discharge measurements are either non-existent or not available or not available quickly.

Data gaps: In-situ discharge measurement and observations are well established and standardized. Changing measurement and observation techniques, new data transmission and processing techniques are constantly developed, tested and implemented. The technical aspects of monitoring have been well addressed. However, access to the discharge data and the provision of the discharge data to international data centers needs urgent attention. Compliance with WMO resolution for the free and unrestricted exchange of hydrological data and information and a timely provision of the discharge data to international data centers are needed. Additional research in the the development of altimetery and other sensing techniques and algorithms that many make it possible to derive more information about river dicharge from existing and planned satellite systems is needed. (Source: ECV T1 River Discharge - Assessment of the Status of the Development of Standards for the Terrestrial Essential Climate Variables - Draft version 8 - November 2008 (GTOS-56))

References: