Multivariable Frequency Analysis For Regional Hydrological Drought

Regional drought now has become a typical extreme natural phenomenon on thebackground of global climate change. In recent years, the frequency as well as theloss of drought is increasing with the effect coming from global climate change andecological environment destruction, as well as the growing water consumptionowing to rising population, developing agriculture and industry, and rapidurbanization process. Drought frequency analysis and research will help intensifythe precaution and emergency management of drought hazard, and promote thereasonable distribution of water resources as well as the capacity of droughtresistance and disaster mitigation. On the basis, the variables of water cycle areused as the main indicators on drought extent, and the theoretical research methodof multivariable frequency analysis of regional hydrological drought is establishedand applied to Huaibei Plain. The main results obtained are as follows:(1) The relational expression between the two hydrological variables of HuaibeiPlain, which are groundwater depth variation and rainfall, is derived. And thenegative correlation of groundwater depth variation and rainfall is verified by thereal data from1975-2006of15cities and counties in Huaibei Plain. Empiricalmode decomposition (EMD) method is also used for the analysis of regionalhydrological characteristics and the results further prove that the negativecorrelation between the short term fluctuations of groundwater depth variation andrainfall. The EMD results of the representative county of Guoyang show that theeffect of human activity to groundwater is enhancing.(2) Drought comprehensive Z index (DCZI) containing hydrology andmeteorology factors is constructed based on the negative correlation betweengroundwater depth variation Z index and rainfall Z index of the cities and countiesin Huaibei Plain. The relationship between DCZI and standardized precipitationindex (SPI), and the identified drought processes with indices DCZI, groundwaterdepth variation and rainfall show that DCZI is objective to reflect the regionaldrought extent and is reliable to be used for hydrological drought frequencyanalysis. (3) The distribution function of each drought characteristic variable isdetermined by curve fitting method instead of maximum likelihood estimation.Archimedean copulas are applied to construct the joint distribution of droughtcharacteristic variables. Further, the drought return periods are calculated bycopulas. DCZI and groundwater depth variation are respectively regarded as theindices for drought frequency analysis of cities and counties in Huaibei Plain. Theempirical results show that: the DCZI can integrally reflect the degree of droughtexactly to avoid the fault of single index; copula theory provides an available pathfor multivariable analysis of drought characteristic, and Gumbel-Hougaard,Clayton and Frank copulas can reflect the relationship between droughtcharacteristic variables well; curve fitting method avoids the irrationality fromestimation of parameters based on data so as to make the results of copula-basedfrequency analysis more objective and reliable.