An evaluation method for drought level changes based on probability matrix and its application

Drought levels are multi-level and transitional. A method quantitatively evaluating changes in drought levels and overall changes in drought are needed. By constructing a probability matrix of drought level change and introducing the multipliers of rate and degree of change, this study derived their formulas and developed a quantitative method for evaluation of drought level change. Standardized precipitation-evapotranspiration index and standardized streamflow index were employed to analyze drought levels in the Dongjiang River basin as an example. Changes in drought levels from meteorological drought to hydrological drought and those in hydrological drought levels under the influence of reservoirs regulation were evaluated at monthly, seasonal and annual scales. Results showed that: ① The degree of overall change in drought was small, but the change in drought level was distinct. The rates of overall change in drought levels at monthly, seasonal and annual scales were 45.9%, 46.8%, and 34.9%, respectively. ② The change in drought level decreased by approximately 1.4 level in October, but increased by 1.3 level in Spring. ③ The impact of reservoir regulation on the hydrological drought was significant at monthly and seasonal scales. The rate of overall change in drought level was 33% and the drought level decreased approximately by one level in spring. The proposed method can be applied not only for drought evaluation, but also for the assessment of other disasters.