Hydrological drought refers to 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. It is part of the bigger drought phenomenon that denotes a recurrent natural hazard. hydrological drought indices refers to Precipitation deficits over a prolonged period that affect surface or subsurface water supply, thus reducing streamflow, groundwater, reservoir, and lake levels, will result in a hydrological drought, which will persist long after a meteorological drought has ended.
Surface Water Supply Index (SWSI)
The SWSI evaluates the overall amount of water in a basin or sub-basin to the amount of water available in the same month in previous years. The SWSI scale ranges from -4 to +4, with -4 denoting extreme drought and +4 denoting ample water supplies. Because runoff projections are only produced in the winter and spring, the data used in the computation changes throughout the year:
For the spring and summer water usage seasons, the Surface Water Supply Index (SWSI) is a predicted measure of total surface water availability within a watershed. Pre-runoff reservoir storage (carryover) is combined with projections of spring and summer streamflow based on current snowfall and other hydrologic factors to produce the index. When compared to historical data, SWSI values range from +4.1 (abundant supply) to -4.1 (very dry), with a value of zero (0) suggesting media water supply. SWSIs are calculated this way to match other hydroclimatic indicators like the Palmer Drought Index and the Precipitation Index.
It’s best to think of it as a simple scale of 1 to 99, with 1 indicating the driest conceivable conditions and 99 being the wettest possible conditions, and 50 representing average conditions. This rating scale is also a percentile rating; for example, a SWSI of 75 percent indicates that this year’s water supply is greater than 75% of all historical events, and that it has only been exceeded 25% of the time. A SWSI of 10%, on the other hand, suggests that 90% of historical events have been higher than this one, with only 10% having had less total water supply.
It is separated into three sections: low, average, and high. From 1 to 33, the water supply is below normal, 34 to 66, medium, and 67 to 99, the water supply is above average. Another advantage of the SWSI is the calculation table, which shows the true ranking of each year. The present SWSI can be directly compared to the years that are most similar to it. To put it another way, is this year similar to the floods of 1983 or the drought of 1977? Managers can then look back at records from earlier years to figure out how to deal with the current year’s water supply. It’s also clear whether the reservoir component or the streamflow component is the primary driving force at any particular time.
Stream flow Drought Index (SDI)
SDI was established by Nalbantis and Tsakiris (2009) to define hydrological drought by evaluating monthly stream flow value.
where Vk is the volume of cumulative stream flow for the ith year. The value of k = 1 for 3 months SDI and k = 2, k = 3, and k = 4 for 6 months, 9 months, and 12 months SDI, respectively. SDI is calculated for the ith hydrological year using cumulative flow values for each k: