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Quantitative Criteria for Hydrological Drought in Bulgaria

This paper proposes quantitative criteria for hydrological drought in Bulgaria, in order to determine when flow shortages can be considered as "exceptional drought circumstances". The criteria align with international terminology and aim to harmonize with UN/CCD, ISDR resolutions. The paper discusses conceptual and practical definitions of hydrological drought and explores drought identification methods.

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Quantitative Criteria for Hydrological Drought in Bulgaria

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  1. LOW FLOW AND DROUGHT SPATIAL ANALYSISSnejina DakovaNational Institute of Hydrology & Meteorology, BASc, 66 Tzarigradsko Shosse,1784 Sofia, BulgariaE- mail Snejana.Dakova@meteo.bg BALWOIS CONFERENCE OHRID, 25-29 MAY 2004

  2. This paper is one attempt to propose a quantitative criteria of hydrological drought suitable to Bulgarian conditions i.e. to estimate when the scares of flow can be adopted as “exceptional drought circumstances”, when these conditions are beyond those that could be considered part of normal risk management..

  3. Also, this is an opportune question as well in relation to activities and documents of United Nations Convention to Combat Desertification (UN/CCD), to resolution of International Strategy for Disaster Reduction (ISDR) and to harmonizing with international terminology.

  4. General idea of drought: source Maine State Climate Office-Cape Canaveral

  5. DEFINITIONS OF HYDROLOGICAL DROUGHT • I. Conceptual definitions Drought is presented into two meaning depend of the used drought’s characteristics: • First one definite hydrological drought asa period of time of abnormal dry weather sufficiently prolonged to give rise to a shortage of water as evidenced by below normal streamflow and lake levels and/or the depletion of soil and moisture and lowering of groundwater levels.(International glossary of hydrology UNASCO&WMO, 1992; Linslay 1986, Van Chow 1988, Laaha ,2002 etc.). • The second one suggests drought as alow flow below some kind of threshold. (Kudzevich 1997, Bonachi 1993, FRIEND 1993, Kovach 1994, Sevinc 2003, Carria etc. ).

  6. DEFINITIONS OF HYDROLOGICAL DROUGHT • II. PRECTICAL DEFINITIONS OF DROUGHT • The practical definition takes into account the efficient use of the water resources. The drought definition often included the terms as “supply and demand” phenomenon. Drought can be considered asa situation where water demand for any particularly system use significantly exceeds water supply from traditional water sources for the system. (WMO/TD 1037, UNESCO 1994.)

  7. DEFINITIONS OF HYDROLOGICAL DROUGHT • ANOTHER DEFINITIONS based on the type of used water (running or regulated) are familiar preliminary in United States experience. “Stored water droughts” occurs when large storages of water in manmade reservoirs, natural lakes and ground water aquifers are depleted by very long unusually low period of precipitation. “Natural water droughts” happen quickly and frequently after just a few weeks or months of below-normal rainfall.

  8. DEFINITIONS OF HYDROLOGICAL DROUGHT ANOTHER DEFINITIONS • Concerning within-year events or annual variability the drought is presented as “short drought” and “long-term drought” depending of the used time unites.

  9. Drought identification methods • Taking into consideration the conceptual definitions, the approaches can be summarized into two groups: Direct indication methods. Drought is indicated as a consecutive number of times unite during which the flow is either equal to or below of single value, named truncation level or threshold, which determine both onset and termination of the drought by checking whether a single value of hydrologic time series lies above or below a given threshold.A negative run, under threshold, is calling Drought. Commonly it is characterized by mean of the three basic indexes: length of drought duration, run sum or total deficit of the run and mean intensity deficit as the ratio between run sum and length.

  10. Choice of the threshold level. The threshold level affects the analysis considerable and varies according to location and purposes. The mean value, • a percentage of mean, • a linear combination of the mean and the standard deviation are used in many works. Dracup (1980 ), Ben-Zevi (1987), Ozga-Zielinska (1989) etc. adopt the mean minus the standard deviation σ as a truncation level in order to compare different hydrological regimes. • The portion of daily flow duration curve with exceeded probabilities greater than 30 % is defined as low flow by Demuth (1997), Pau-Shan (2002). Also, the mean daily flow below a specific threshold, water discharge with probability 95% (Q95), is assumed as low flow by Laaha (2002).

  11. Drought identification methods • 2. Indirect methods. Drought is indicating through different mathematical approaches included different elements as rainfall, previous rain, previous discharge etc.

  12. Bulgarian experience • Low flow is namedthis periodical repeated extended status/state of the river when the flow is steady low, the surface supply is leave of and the river is predominantly fed by ground water.The highest limit is mean annual flow and the lowest limit is a zero value of minimum flow i.e. the river is dry. • Low flow could be express by its different faces: summer and/or winter flow, minimum flow with different time duration ( 1,2 ..N days, month, season, dryness). (Marinov, 1958, Dakova 1985)

  13. Different regime of Bulgarian river’s flow generated in correspondent climate region’s conditions

  14. Effect of the water intakes on Mresta river flow at gauging station Jakorouda within-year distribution of Mesta river

  15. Effect of the influence of multi-purposes reservoir Ïskar”( constructed 1956) on the streamflow of Iskar river at gauging station Kurilo situated under Sofia

  16. ESTIMATION OF LOW FLOW CHARACTERISTICS • The choice of estimating methods of low flow depends first of all on the type of the used data: data of natural flow or data of flow influenced by human activities. This respectively is to be specified by the purpose of the relevant studies. • The real state of the river is important for the purposes of irrigation with running waters, for a small water power plant function with use of running waters, for estimating the rate of preliminary purification of the waste water before the inflow into the river, for obtaining the self-purification ability of the streamflow, for normal live of the river’s ecosystems and so on, Hence, the values of the data observed at the hydrological gauging stations (HGS) have to be used. • The values of natural floware compulsory when low flow is considered as a part of water resources, an element of water balance equation, for identification of climate change impact on the river flow etc.

  17. HYDROLOGICAL DROUGHT’S INTRODUCTION • In the frame of within-year aspect, drought is indicated as such river state when the flow is either equal to or below of mean summer base flow with duration longer than that of the normal dry season (3-4 months). • The value of Hydrological drought index (HDI) >1 is a criterion for introduction of drought of multi-year analysis

  18. Monthly time series of registered flow and base flow during a very dry year like 2000

  19. Monthly values of BFI in mean year and in dry 1994 and 2000 at stations along the Mesta river. G.st.52650is situated in mountain reaches, G.st 52700 –in the middle and G.st 52880 –in the lower reaches.

  20. . Values of correlation coefficient between Base flow and the elements of low flow ** M- Mesta river S – Struma river

  21. Base flow as an enveloping curve of annual duration curves with probability 50%P, 75%, 90%P and 95% P. Mesta river

  22. Hydrological drought index (HDI) is a criterion for multi-year drought HDI = (Qi – Q av)/ Cv

  23. Hydrological drought of Mesta river

  24. Hydrological drought of Struma river

  25. CONCLUSIONS • Drought is accepted as a particular case of low flow when the values of low flow are either equal or below mean summer base flow with duration longer then the normal dry season (3-4 months). • Different criteria for indication the drought are introduced: Within-year drought Q ≤ BFL • Multi-year hydrological drought: the method of moving average of standardized discharge is used to introduce three categories of drought. (HDI) >1

  26. CONCLUSIONS • The priority of the proposed criteria and approaches of identification of drought is, that in spite of large complexity of the drought process, hydrological data are required only and they are easily applicable. • This is a contribution to further investigation on the assessment of drought risk and drought consequences and failures

  27. BALWOIS Conference,Ohrid, 25-29 April 2004 • Thank you for your attention

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