WMO / OMM Volta-HYCOS Project Training module on Hydrological Expertise and IWRM Design and optimisation of hydrometric networks IRD - Unite OBHI (Observatoires Hydrologiques and Ingenierie) Design and optimisation of hydrometric networks 1. Objectives of hydrometric networks 2. Design of hydrometric networks 3. Diagnostic of hydrometric networks 4. Rationalisation and optimisation of hydrometric networks 5. Application : – Diagnostic of the hydrometric network on the Volta Basin – Definition and Design of the Volta-HYCOS hydrological network 1. Objectives of hydrometric networks • A hydrometric network is aimed at giving the hydrological information to be used for the following needs : – Assessment of the regional or national surface water resources and of their trends (climatic and anthropogenic impacts) – Water resources planning for management and utilisation – Estimation of environnemental, economic and social impacts of current or planned management practices on WR – Analysis and forecasting of extreme events (warning) : drought, exceptional floods 2. Design of hydrometric networks • The design of a network should answer the following questions : – What hydrological variables are to be recorded ? – Where will they be recorded ? – What is the frequency of recording? – What is the duration of the recording programme? – What is the level of quality of the recorded parameters? 2. Design of hydrometric networks • Definitions : – Minimum density of network: • = 1st step in the establishment of a hydrological network • It enables us to determine the hydrological characteristics at any point in the region • But cannot answer to specific needs of WR management and utilisation • The stations in such networks should be monitored continuously and their data should be of quality – Optimal network 2. Design of hydrometric networks • Definitions : – Optimal network: Optimal networks enable a good interpolation between the stations at any point in the area covered by the network, with enough accuracy for WRs management and utilisation purposes 2. Design of hydrometric networks • Types of networks : – National / regional networks – Representative or experimental basins • From a few km2 to several km2 • Measurements are carried out during one year up to several years • All the hydro-climatic variables are measured (Water levels, discharge, evaporation, evapotranspiration, infiltration…) in order to model the relation between rainfall and discharge 2. Design of hydrometric networks • Types of stations making up a network : – Main gauges Permanent stations and continously and correctly monitored. These are reference stations for statistical analysis – Secondary gauges Maintained for a limitted number of years but sufficient in order to establish good correlation with data at main stations – Special gauges Based on specific needs : irrigation, navigation, flood forecasting, dams management, … 2. Design of hydrometric networks Minimal Density WMO proposes a norm of per station hydrometric stations Physiographic Units area in km2 per density for a minimal station network : Costal zones 2750 (Guide of Hydrological Montaneaous zones 1000 Practices, 1994) Interior plains 1875 Hilly Regions 1875 Small islands 300 Polar and arid zones 20 000 2. Design of hydrometric networks Some rules and criteria • For transboundary water balance : it is indispensable to have for each international river a gauge at the entrancy and/or the outlet of the country • Confluence between a major and a minor tributary : it is useful to have a gauge in order to appreciate the discharge variation for the main river, downstream of the confluence • Along a river, installation of a gauge should consider the other stations available on the river : if the difference between the flows at 2 stations is inferior to the margin of error of flow measurement, it is useless to intercalate a supplementary station 2. Design of hydrometric networks • Case of dams : – Station upstream of a dam enable to monitor the flows which enter the dam – Station downstream of a dam enable us to estimate with precision the spilled flows (and/or turbined flow hydro-electric production) Dam of Toesse-Kanazoe (White Volta, Burkina) 2. Design ofhydrometric networks • Case of dams : – Valoriser of the information provided by stations situated on dams : reconstitution of natural flows from the water level in the dams with hydrological balance : ∆V= Qentering * ∆ Tflow + (P * Srandenue) – (Ev * Srandenue) – Abstractions •∆V:Variation of volume of dam (m3) •∆ Tcrue : Duration of flood (s) •P : Rainfall on the dam (mm) •Srandenue : Surface of dam (m2) •Ev : Evaporation on the dam (mm) •Abstractions : Abstractions of water (AEP, irrigation, …) in the dam 2. Design of hydrometric networks Some rules and criteria : • Rivers passing through or near a city where there’s important abstraction from the river : a station upstream and a station downstream of the city are necessary • Region with large irrigable area with appreciable abstraction from the river : a station upstream and a station downstream of the irrigable area are necessary Water Supply pomping station at Nawuni (Ghana) Water Supply pomping station at Nawuni (Ghana) 3. Rationalisation and optimisation of hydrometric networks • To optimise a network, is to find the best compromise between the richness and the interest of hydrological information on the one hand, and the cost of acquisition of data on the other hand • Context in West Africa: important degradation of networks since the 80’s -90’s (reduction of budget of hydrological services / pullout of IRD / ex-ORSTOM in francophone countries) • Necessity of rationalising existing networks by selecting the stations which must be monitored as priority 3. Rationalisation and optimisation of hydrometric networks • Technics of optimisation of networks (complementary) – Survey among users of data on the usefulness of each station of the network ; – Multi-criteria analysis based on indicators which enable us to appreciate the usefulness of each station in the network and to characterize it 3. Rationalisation and optimisation of hydrometric networks 1. Survey technics – Objective :Highlight the level of utility of each station on the basis of the use of data by users – Method : questionnaires – The survey enables us to highlight : A) The type of use of the station : – stations which are used for management and other decision; – station which are used for regional and long-term analysis of water resources – station which are used for design and planning purposes 3. Rationalisation and optimisation of hydrometric networks 1. Survey technics – The survey enables to highlight : B) The use of data at the station: – Flood forecast and warning; – Current Management (navigation, level control, dam management, monitoring of floods and drought,..) – Legal obligation (minimum discharge to be maintained) – Long term statistics (flood frequency analysis, trend analysis, quartiles etc) – Regional hydrological Analysis (Regional regression equation for quartiles, regional parameters of hydrological models,..) 3. Rationalisation and optimisation of hydrometric networks 1. Survey technics – The survey enables to highlight : B) The use of data at the station : – Hydraulic and hydrological design (design of reservoirs, hydro-power infrastructure, hydraulic infrastructure,..) – Planification of water resources (planification for water distribution, ..) – Water quality analysis (quality monitoring, quality modelling, sea water intrution,..) – … 3. Rationalisation and optimisation of hydrometric networks 1. Survey technics C) Identify the users of data at the station : – Governmental institutions (NHSs, Agriculture, Fishery, …) – Basin organisations; – Dams managers; – Consulting firms; – environmental monitoring institutes – Research institutes, Universities, – … 3. Rationalisation and optimisation of hydrometric networks 2. Multi-criteria analysis of network – Objective : diagnose the stations of a network on the basis of different criteria / indicators – Mean : Points can be attributed to each criteria in order to determine a global score for the station, which will be monitored in priority – Criteria of evaluation of stations 3. Rationalisation and optimisation of hydrometric networks 2. Multi-criteria analysis of network Criteria of evaluation of stations : - Regional representation of the station : - Is the station representative of a basin and/or of a climatic zone ? – Drainage area at the station – Lengh of time of the series at the station : - Longevity of the station (reference stations with series of data > 30 years) - Regularity of observations of water levels (series with few gaps) – Level of correlation between the station and other reference stations 3. Rationalisation and optimisation of hydrometric networks 2. Multi-criteria analysis of network Criteria of evaluation of stations : - Stability of the station in time - Constructions upstream or downstream of the station can modify conditions of flow - Beware of loss of staff gages or reference pillars (bench marks) (eg : following to construction of bridges, …) - Quality of data at the station : - Quality of water level data (recordings and readings) - Quality of rating curve : number flow measurements, and stability of rating curve 2. Multi-criteria analysis of network Criteria of evaluation of stations : - Usefulness of the station (on the basis of survey) : • Forecasting of High and low flows • Evaluation of flows within riparian countries ; • Evaluation of