The relationship between meteorological and hydrological drought

of Barlad River catchment area

Silvia Chelcea and Mary-Jeanne Adler

National Institute of Hydrology and Water Management, Bucharest, Romania

AbstractMost of the drought indices are derived from meteorological observations (primarily precipitation andsometimes, temperature). For low flows analysis and drought identification we apply the theoryunderlying the calculation of the Standardized Precipitation Index (SPI) to define a Standardized FlowIndex (SFI), emphasizing in this way the dry hydrological periods. The pilot basin was the Barlad catchmentarea located in the eastern part of Romania, which was identified as being vulnerable to water scarcity.Another index applied in this paper is the Standardized Precipitation and Evapotranspiration Index (SPEI).These three indeces (SPI, SPEI and SFI) were calculated using more than 50 years of data (precipitation, airtemperature and montly discharge) at Vaslui, Barlad and Tecuci stations situated on the Barlad Rivercatchment area. Dry periods, from a meteorological point of view, were identified based on a droughtindex that takes into account only precipitation (SPI) and another one that takes into account bothprecipitation and air temperature (SPEI).Barlad station was found to be the most affected by the drought phenomena, given the downward trendof the SPI index, as well as SPEI, at seasonally and annually resolution. By analyzing the trends in timeseries of the standardized anomalies of precipitation and air temperature, it is observed that the mostaffected station is also Barlad. Here the precipitation shows a significant decrease; while the airtemperature has a positive and significant trend.To highlight the dry periods from a hydrological point of view we applied the procedure for calculating theSPI to the monthly discharge time series for three hydrometric stations (Vaslui, Barlad and Tecuci) situatedon the Barlad catchment area. The results show that the highest variability between SFI and SPI isidentified for the summer season, where the response time between hydrological and meteorologicaldrought at different time scales is up to 3 months. On the annual time scale, the response time is at least 6months on the Barlad River catchment area.

Data and methodsOur study is based on three data sets:Monthly mean air temperature time series (1961 - 2009) for Vaslui,Barlad and Tecuci stations; Precipitation time series (1961 - 2009) for Vaslui, Barlad and Tecucistations;Monthly mean discharge time series (1963 - 2010), in natural flowregime, measured at the hydrometric stations: Vaslui, Barlad and Tecuci.Used methods consisted in: Computation of the Standardized Precipitation Index (SPI) based on themethodology developed by McKee in 1993; Computation of the Standardized Precipitation and EvapotranspirationIndex, using the water balance method of Thornthwaite (1948). Computation of the Standardized Flow Index (SFI), using themethodology of the SPI computation.The monthly precipitation data set was used to calculate the value ofstandardized precipitation index SPI for the three stations.Also, to calculate the values of standardized precipitation andevapotranspiration index SPEI at the three stations, this data set, as well as,the mean air temperature set were used.Monthly mean discharge data set was used for the value calculation ofstandardized flow index SFI, by the same method as in the case ofstandardized precipitation index.

Barlad River catchment area

The catchment area of Barlad River issituated in the eastern part of Romania.

Barlad River is a tributary of the Siret River.Its total length is 207 km and the

average altitude is 374 m.

Classification of SPI, SPEI and SFI indices

CONCLUSIONS

Acknowledgements. Part of this work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI; project number PNII-ID-PCCE-2011-2-0073 CLIMHYDEX

Annual evolution and the trend of the standardized anomalies of precipitation and mean air temperature

at the following stations: Vaslui, Barlad and Tecuci

Annual evolution of the SPI, SPEI and SFI indices for 1, 6 and 12 months at Barlad station

The trend of the standardized of precipitation and mean air temperature at annual and seasonal level

The trend of the SPI and SPEI indices for 1, 3, 6, 9, 12 and 24 months at annual level

The trend of the SPI and SPEI indices for 3, 6, 12 and 24 months at seasonal level

Annual values of the correlation coefficient between SFI and SPI, SPEI

Seasonal values of the correlation coefficient between SFI and SPI, SPEI

Frequency based on drought classes (moderate, severe and extreme)for SFI1, SFI6 and SPI6, SPI12 at Vaslui, Barlad and Tecuci stations

Annual time evolution of SFI6, SPI12 and SPEI12 at Barlad station

Frequency based on drought classes (moderate, severe and extreme) for SFI6 and SPI12 at Barlad station, for the period 1964 – 1985

compared to the period 1986 – 2009

The annual trend of precipitation at Vaslui and Tecuci stations is not significant but, in the case of Barlad station, theannual precipitation shows a negative trend. The mean air temperature shows a positive trend at all the analyzed stations. Especially after 1989 the mean airtemperature is characterized by really high values. By analyzing the trend of precipitation and mean air temperature it was found that the most affected station, interms of decreasing precipitation amount and increasing temperature, is Barlad. Also, the Barlad station is the most affected stations in terms of meteorological drought. Here the SPI as well as theSPEI index present a negative trend both for the annual values as well as on seasonal time scale. At annual time scale, for Barlad station, the negative trend for SPI and SPEI is mostly significant starting with SPI6and SPEI6. Based on the temporal evolution of SFI6 it was found that the periods characterized by hydrological drought werethe following: 1963 - 1964, 1968 - 1969, 1986 - 1987, 1990, 1994 - 1995, 2001, 2007 and 2009.Starting with class 6, SFI presents a negative trend on annual time scale as well as for summer and autumn.

The response of the hydrological drought to the influence of the meteorological drought differs from oneseason to another. For winter season the highest correlations are found between SFI and SPI12. The responsetime suggests that during winter the hydrological drought appears as an affect of reduce precipitation duringthe whole precedent year. The most variable relationship between SFI and SPI was identified for the summer season, where theresponse time is maximum 3 months. Based on the analysis of drought frequency, over the Barlad river catchment area, it was found that for SFIthe moderate drought frequency increases together with the analyzed classes. For the severe and extremedrought, the frequency of SPI12 is much higher than in the case of SFI6. The Pettitt statistic test has helped to identify, for SFI6, SPI12 and SPEI12, the years with a positive leap:1985, in the series of SFI6 index and 1981 in series of SPI12 and SPEI12 indices. A period of increase in thedrought seasons and their variability has followed after these years mentioned above. By comparing the drought frequency between two periods: 1964 – 1985 and 1986 – 2009, for SFI6 and SPI12,it has been found that the moderate drought has increased in the second period.

The diagram of dry spells for SFI6 at Barlad station Number of cases of dry spells for SFI6 at Vaslui, Barlad and Tecuci stations, for the

period 1964 – 1985 compared to the period 1986 – 2010

e-mail: silvia_chelcea@yahoo.com