Use of a conceptual rainfall runoff model in water resources planning

Transcription

1 The influence of man on the hydrological regime with special reference to representative and experimental basins L'influence de l'homme sur le régime hydrologique avec référence particulière aux études sur les bassins représentatifs et expérimentaux (Proceedings of the Helsinki Symposium, June 1980; Actes du Colloque d'helsinki, Juin 1980): IAHS-AISH Publ.no Use of a conceptual rainfall runoff model in water resources planning DAN LUNDQUIST Norwegian National Committee for Hydrology, Oslo, Norway Abstract. A Norwegian conceptual rainfall-runoff model, the SNSF model, has been used for simulation of runoff data for two rivers in southern Norway. In this part of the country there are conflicting interests which make water resources planning essential so the model is also applied to the analysis of the effects of planned activities in the river basins. Although not suitable for detailed analysis of water resources systems in its present form the model described seems to be a useful tool for evaluation of general effects in a river basin. Utilisation d'un modèle conceptuelpluie-débits pour la planification des ressources en eau Résumé. Un modèle conceptuel pluie-débits norvégien, le modèle SNSF, a été employé pour la simulation des données de l'écoulement pour deux rivières du sud de la Norvège. Dans cette partie du pays, il y a des intérêts opposés qui rendent essentielle la planification des ressources en eau; par suite le modèle est également appliqué à l'analyse de l'effet des activités projetées dans les bassins versants. Bien que dans sa forme actuelle, il ne convienne pas à l'analyse détaillée des systèmes de ressources en eau, le modèle présenté dans cette communication paraît constituer un outil utile pour l'évaluation des effets généraux sur un bassin versant. INTRODUCTION When planning activities that affect the flow regime of rivers, both the planner and the decision-maker are often faced with the problem of unrepresentative runoff data. This applies especially to smaller river basins where the economical output of the river has not been regarded as essential and therefore no data have been collected. With various conflicting interests arising, as for example irrigation, water supply, hydropower, fishing, recreation, waste water disposal, etc., an effect analysis often has to be made for smaller sub-basins where the planned activities have critical implications, usually with insufficient knowledge of the river regime in such basins. To deal with this problem, the use of rainfall runoff models for simulating runoff for ungauged basins was suggested and applied to two rivers in southern Norway, the Rakkestad River and the Lena River. THE SNSF MODEL The SNSF model was first formulated for simulation of hydro chemical interactions in small basins (Lundquist, 1977). Since then, the model has been used primarily for quantitative hydrological applications (Lundquist, 1978; Saelthun, 1978). The main philosophy of the model is that different types of land have different hydrological properties. Therefore a basin should be divided into land use categories of importance in the actual applications, e.g. bare rock, forest, bog, cultivated land, lake, etc. The areal cover of these sub-basins is readily measured from maps where the most significant drainage patterns are also estimated. This means that the definition of the macrostructure of the model varies from basin to basin (Fig. 1). In the model, observed precipitation is first converted from measured point precipitation to mean areal precipitation and thereafter distributed to the different sub4>asins. Within each sub-basin the precipitation is drained through a vertical profile, where some water is lost due to évapotranspiration, some is retained within the profile and the rest leaves as runoff. The profiles are described as linear reservoirs 445

2 446 Dan Lundquist r. PRECIPITATION CORRECTION AND DISTRIBUTOR OF P SNOW-ROUTINE i BOG OR CULT. LAND SUMMATION r AND ROUTING OF Q FIGURE 1. The SNSF model: schematic representation. with buffer volumes from which only évapotranspiration is allowed and with saturation volumes which result in overflow when exceeded (Fig. 1). The runoff from a sub-basin can be routed to another sub-basin or directly to the stream. If the basin is large relative to the time step used, the streamfiow also has to be routed through the channel system. An important part of the model is the snow routine which decides whether precipitation falls as rain or snow and whether melting or refreezing is taking place in the snowpack. The amount of accumulated snow in the basin and meltwater volumes are also calculated. This routine models the local climate in different altitude zones of the basin and was originally formulated by Bergstrôm (1975). THE RAKKESTAD RIVER As a result of a general trend in Norway towards utilizing the hydropower potential in smaler rivers, four power plants of approximately 1 MW each are planned on the Rakkestad River (465 km 2 ). No runoff measurements for the river exist and therefore the SNSF model was used for simulation of synthetic runoff data. These data are needed for the dimensioning and production calculations necessary when deciding on optimal instaëations. Special attention has also been focused on water use by irrigation, water supply for domestic and industrial use and waste water disposal. Since no runoff data are available for the Rakkestad River the model is calibrated on the nearby Hob^l River (306 km 2 ) where runoff data exist (Fig. 2). The two river basins show great similarity in their physiography and there are no special objections to transferring parameter values between these two basins. To take account of the variations in meteorological conditions between the two basins, meteorological data (precipitation and temperature) from As and Eidsberg are used for the Hob^l River and data from Eidsberg and Str^msfoss for the Rakkestad River (Fig. 2). Examples of the simulations are also shown in Fig. 2. A runoff series of 20 years was then simulated for the Rakkestad River. From these data flow duration curves for each month of the year have been constructed and used when deciding on the power plant installations. The flow duration curves have also been compared with the prognosis of water demand for irrigation, domestic

3 Use of a conceptual rainfall runoff model 447 FIGURE 2. The Rakkestad River application: location of basins and meteorological stations and examples of simulations. use and industrial use. The result indicates that there is a risk that the river might dry up completely during the summer months. This will impose restrictions on the water consumption and it will also make waste water disposal impossible for part of the year. Therefore, restoration of some old reservoirs in the basin is needed to help support a minimum discharge during these critical months. A detailed description of this model application is given elsewhere (Lundquist, 1979). THE LENA RIVER During the last 10 years the increasing demand for various water uses has accentuated the need for water management in Norway.To gain knowhow about analysing methods, regulation schemes and administrative cooperation, the Lena River (Fig. 3) has been chosen for a pilot study. This river basin has all the usual management problems connected with water consumption and demand. The consumptive use of water in the basin is estimated to be 02 m 3 /s for water supply and 0.1 m 3 /s for irrigation, but the last figure is expected to be 3 4 times greater before the end of this century (Thaulow et al., 1979). On the other hand, the water demand is estimated at OS m 3 /s for fishing and 1.0 m 3 /s for hydropower; figures for industrial demand and for waste water disposal are not yet available. Since mean annual discharhe is estimated at 45 m 3 /s and the discharge in low flow periods can be as low as 0.1 m 3 /s, there is obviously a need for coordination between the different interests concerned. For comparative analysis in the river basin, the river itself is to be simulated as a hydraulic system which is receiving input from or giving output to the different water consumers in the basin. The hydraulic system is a hierarchical structure with defined points where water quantity and/or quality is changed. Within this structure, the SNSF model will be used to give the natural inflow of water to the river from different land types and altitude zones.

4 448 Dan Lundquist FIGURE 3. The Lena River: land use and measurements. Runoff data for the whole basin are available for the period from 1973 to the present and it is with these that the model is calibrated. When the pilot study was decided upon in 1978, runoff measurements in six other locations within the basin were initiated and the model has been tested on the data available so far. The test revealed a commonly experienced problem with this kind of model application, namely the varying representativity of point measurements of precipitation and temperature. Furthermore, this problem was complicated by the large variations in local climatic conditions within the basin. Not even the use of three different precipitations stations (only one of which includes temperature observations) and varying temperature and precipitations gradients in the sub-basin managed to overcome these problems. Two examples of snowmelt simulation are shown in Fig. 4, FIGURE 4. Simulated spring floods for two sub-basins to the Lena River (sub-basin no. refers to discharge station no. in Fig. 3).

5 Use of a conceptual rainfall runoff model 449 and these clearly show the differences in the development of the spring floods and the discrepancies of those simulated for these two sub-basins. A meteorological station is now planned within the basin and hopefully this will give input data of acceptable representativeness for the model simulations. REFERENCES Bergstr^m, S. (1975) The development of a snow routine for the HBV-2 model. Nordic Hydrol, 6, (2). Lundquist, D. (1977) Modellering av hydrokjemi i nedb^rfelter (Modelling hydrochemistry in rivet basins). Report IR 31/77, SNSF-project, PO Box 61, 1432 As-NLH, Norway. Lundquist, D. (1978) Practical use of a simple conceptual model. In Contributions to Nordic Hydrological Conference Helsinki 1978: Norwegian Water Resources and Electricity Board, Hydrological Department, Oslo, Norway. Lundquist, D. (1979) Simulering av en avl(6psserie for Rakkestadelv (Simulation of a runoff series for the Rakkestad River). Report no. 2, Norwegian National Committee of Hydrology, Oslo, Norway. Saelthun, N. R. (1978) Comparative tests of hydrological models for operational use on Norwegian catchments. In Contributions to Nordic Hydrological Conference Helsinki 1978: Norwegian Water Resources and Electricity Board, Hydrological Department, Oslo, Norway. Thaulow, H., Moland, T. and Râhe'im, J. M. (1979) Fremdriftsrapport om arbeidet med Lenalev som pr^vevassdrag for metodeutvikling av vannbruksplanlegging (Progress report on the work with the river Lena as a pilot study for developing methods for water management). Norwegian Institute for Water Research, Oslo, Norway.

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