# Hydrological Drought Frequency Analysis in Water Management Using Univariate Distributions
## 1 Introduction  
Drought is a complex phenomenon and can have various approaches. Based on [PERSON] and [PERSON] [1], [PERSON] [2], usually different types of droughs are distinguished, such as meteorological drought, soil moisture drought, hydrological drought and socio-economical drought. In general, drought analysis is based on drought indices (Standardized Precipitation Index, Effective Drought Index, Streamflow Drought Index, Surface Water Supply Index, etc.) or drought frequency analysis. [PERSON] defines drought as an extended period of dryness, and drought analysis should be performed according to magnitude and duration [3]. [PERSON] defines drought \"as a natural disaster that occurs when precipitation is significantly less than its normal time\" [4]. The objective of this article is the frequency analysis of hydrological drought, important in the management of water resources, in the context of economic development, but also of climate change. According to [PERSON] [5], hydrological drought is defined as \"a lack of water in the hydrological system, manifesting itself in abnormally low streamflow in rivers and abnormally low levels in lakes, reservoirs, and groundwater\". In this article we analyze hydrological drought represented by water deficit which is characterized by minimum flows, duration of drought and volume of water deficit. They are defined according to various thresholds generally chosen as fractions of monthly average flows or values from the duration curve of daily average flows [6; 7; 8; 9]. The Romanian methodology regarding the analysis of minimum flows is usually carried out by extracting some data from the duration curve of average daily flows or minimum monthly flows. Another methodology is statistical processing to obtain insurance curves for the minimum annual flows, volumes and durations of the annual deficit [3; 10], but it is not yet implemented in Romania, and there is no standard regarding their statistical determination. International methodology recommends performing statistical analyses using the minimum data of the 1-day, 7-day and 30-day moving-average chronological series of minimum annual flows [3; 7; 9]. For proper analysis, droughs are also defined by other variables, namely durations and deficit volumes. Thus, a statistical analysis of these variables is required. In water management, durations and annual cumulative volumes are important. The time series of these two variables, resulting from the imposition of thresholds, may contain many zero values. This aspect leads to the need to use the conditional probability model [7; 11; 12]. In water management, the statistical analysis of the water deficit is important for the management of water resources, including the improvement of the ecological state of rivers. In Romania, until the year 2020, flow requirements of aquatic ecosystems represented the flow with a duration of 95% of the duration curve of average daily flows. The new legislation, H.G. 148/2020 [13], chooses to define periods of drought with the maximum value between the minimum monthly mean discharge with a 95% probability of occurrence and the lowest value among the monthly ecological flows; this leads in many cases to flows much lower than those before 2020. For example, for a multitude of rivers in protected areas (Porumbacu, Sambata, Visitsoara, Uz,... ), the values based on the new methodology affect the hydomorphological indicators [14] that characterize the good ecological condition of rivers for most of the year, constituting a real threat to the environment. The Faculty of Hydrotechnics, following hydrological research, started a procedure of statistical analysis of the minimum flows using a multitude of statistical distributions but also mathematical methods for determining periods of hydrological drought, being part of more extensive research regarding the correction of the ecological flow legislation.