The Frequency Analysis of the Hydrological Drought in Doroodzan Dam Basin Based on Stream Flow Drought Index

According to some researchers, predeting and preventing water shortages is not possible. However, by fitting the distribution of the probability to the data of a river, it is possible to determine the incident corresponding to the probability of the occurrence or a certain period of return, and with the management and implementation of the drought management programs to reduce its effects and consequences. Hydrological droughts affect vast areas and are detected by reducing lake water storage, lowering groundwater levels, and decreasing river flow flux. This decline in river flow fluctuates from two environmental and management perspectives on water resources. On the other hand, hydrological drought affects the agricultural sector and causes irreparable damages to this sector. Therefore, the importance of studying hydrological droughts is shown in relation to other droughts. Droodzan dam basin is one of the mountainous basins of central plateau of Iran located in the northwest of Fars Province. It provides approximately 760 million cubic meters of water per year, including drinking water, and agricultural and industrial facilities in and around the area. Recently, due to drought, it has faced severe water losses. Therefore, the preservation and maintenance of the reservoir of this dam and the continuity in exploitation of water resources requires attention to its watershed and the potential of water production in the    river basin. Therefore, studying the hydrological status of surface waters of the basin can have a special place in the management of drought in the country.

In this study, 5 stations that had appropriate data during a long statistical period were selected. Reconfiguration of the statistical defects was done using the correlation between stations and using the SPSS software. After reconstructing the statistical defects and completing the data, their homogeneity was analyzed using run test and SPSS software. Then, the river flow index was calculated at 3, 6, 9, and 12-month intervals during the statistical period of 28 for each station. In this study, for the frequency analysis, the data from series of limit values ​​were used and fitted with the probabilistic theoretical fit. Chi-Square and Kolmogorov-Smirnov tests were used to determine the appropriate distributions for each time scale using Chi-square fit test. Then, the return period for each of the drought conditions was obtained on the basis of the river flow index at each time period and was extracted from the frequency analysis curve of each station.

The results of the data homogeneity were studied. The data of all stations were at a level of confidence of at least 99%. In general, the results of the river flow index were very similar at different time scales and indicated the severity of the drought in these years (64-65, 80-78, and 92-87). This increase was observed at 9 and 12-month intervals. Also, the number of years in which mild drought occurred at a time scale of 9 and 12 months, was compared to the 3 and 6-month time scales. In the abundant analysis, the normal distribution of the 3-month river flow index, the distribution of generalized limit values ​​for the river flow index of 9 and 6 months, and the exponential distribution of the 12-month river flow indices were recognized as the best distributions.

 The results obtained from the calculation of the studied indices showed an increase in the severity of the hydrological drought, especially in the recent decades. River flow index is one of the indicators of the hydrological drought assessment, which can be a good measure for assessing the drought phenomenon in the region. This indicator is also very efficient and has a high sensitivity to other drought indicators. In addition, the zoning maps showed that in the northern and eastern regions of the mild drought, less than the other regions, except for the 6-month period, they were less likely to return to the rest of the period. In the case of the moderate and severe droughts, this was true in the northern parts of the region.  In general, with the increase in the return period, the extent and severity of the drought in the area increased. Particularly the northern and eastern areas of the studied watershed were more prone to these natural disasters. Therefore, this area was exposed to economic and social damages and hydrological droughts, so water resources studies require more attention. Thus, the drought risk poses these areas to severe economic, social, environmental, agricultural, and ecological degradation. These areas can serve as short-term and medium-term goals of integrated strategic management and operational plans.