LOW FLOW AND DROUGHT SPATIAL ANALYSIS Snejina Dakova National Institute of Hydrology

1
LOW FLOW AND DROUGHT SPATIAL ANALYSISSnejina
DakovaNational Institute of Hydrology
Meteorology, BASc, 66 Tzarigradsko Shosse,1784
Sofia, BulgariaE- mail Snejana.Dakova_at_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 droughts characteristics
• First one definite hydrological drought as a
  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 UNASCOWMO,
  1992 Linslay 1986, Van Chow 1988, Laaha ,2002
  etc.).
• The second one suggests drought as a low 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
  as a 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 s 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 named this 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
(No Transcript)
14
(No Transcript)
15
Different regime of Bulgarian rivers flow
generated in correspondent climate regions
conditions
16
Effect of the water intakes on Mresta river flow
at gauging station Jakorouda
within-year distribution of Mesta river
17
Effect of the influence of multi-purposes
reservoir Ïskar( constructed 1956) on the
streamflow of Iskar river at gauging station
Kurilo situated under Sofia
18
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 rivers 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 flow are 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.

19
HYDROLOGICAL DROUGHTS 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) gt1
  is a criterion for introduction of drought of
  multi-year analysis

20
Monthly time series of registered flow and base
flow during a very dry year like 2000
21
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.
22
. Values of correlation coefficient between Base
flow and the elements of low flow
M- Mesta river S Struma river
23
Base flow as an enveloping curve of annual
duration curves with probability 50P, 75, 90P
and 95 P.
Mesta river
24
Hydrological drought index (HDI) is a criterion
for multi-year drought HDI (Qi Q av)/ Cv
25
Hydrological drought of Mesta river
26
Hydrological drought of Struma river
27
(No Transcript)
28
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)
  gt1

29
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

30
BALWOIS Conference,Ohrid, 25-29 April 2004

• Thank you for your attention