What are the Hydrological Drought Indices?

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. This means drought might be reflected in a number of hydrological phenomena, which, in many cases, are related to one another. The prominent ones are streamflow, springflow, snowmelt, lake level, and groundwater level, these symptoms are referred to as "Hydrological drought". For quantifying the hydrological drought events, we need the proper indices.

Hydrological Drought Indices

This group of indices aims at providing a comprehensive characterization of delayed hydrologic impacts of drought. Each of these indices requires different variables as input data in their formulas. The most important and well-known hydrological drought indices are:

• The Surface Water Supply Index (SWSI): Developed by Shafer and Dezman (1982)* in Colorado, the SWSI is a better indicator of water availability in the western United States than the Palmer Drought Index. It is calculated based on monthly non-exceedance probability from available historical records of reservoir storage, streamflow, snowpack, and precipitation. The ranges of SWSI's value are between -4 to 4.
• The Reclamation Drought Index (RDI): Developed by Weghorst (1996), the RDI improved SWSI by incorporating temperature and hence calculated a variable water demand as input.
• The Streamflow Drought Index (SDI): Based on the SPI developing concepts, the SDI was developed by Nalbantis and Tsakiris (2009). The SDI is a very simple and effective index for hydrological droughts. Positive SDI values reflect wet conditions while negative values indicate a hydrological drought. Based on the SDI, five states of hydrological drought are defined which are denoted by an integer number ranging from 0 (non-drought) to 4 (extreme drought).
• The Palmer Hydrologic Drought Index (PHDI): By modifying the PDSI calculation procedures to produce the PHDI, an index is developed that better assesses moisture anomalies that impact river flow, groundwater availability, and lake or reservoir levels. The PHDI uses a modification of the PDSI to assess moisture anomalies that affect streamflow, groundwater, and water storage, being more sensitive to hydrological components.

Detailed Explanation of Hydrological Drought Indices

The Surface Water Supply Index (SWSI) was introduced in Colorado in the early 1980s as a better indicator of water availability in the western United States than the Palmer drought index. The SWSI is calculated based on monthly non-exceedance probability from available historical records of reservoir storage, streamflow, snowpack, and precipitation. Its values range from -4 to +4, with positive values indicating above-average water supply conditions and negative values indicating below-average conditions.

The Reclamation Drought Index (RDI) includes temperature as a variable, which allows it to calculate water demand more accurately. This index is particularly useful in regions where temperature variations significantly affect water supply and demand.

The Streamflow Drought Index (SDI) was developed recently and is based on the same concepts as the Standardized Precipitation Index (SPI). The SDI is simple to use and provides a clear indication of hydrological drought conditions. Positive SDI values indicate wet conditions, while negative values signify drought conditions. The SDI defines five states of drought severity, ranging from non-drought to extreme drought.

The Palmer Hydrologic Drought Index (PHDI) is an adaptation of the Palmer Drought Severity Index (PDSI). The PHDI is designed to better capture hydrological drought conditions by focusing on streamflow, groundwater, and reservoir levels. This index is more sensitive to hydrological components, making it a valuable tool for assessing the impacts of drought on water resources.

Analyze Hydrological Drought using Surface Water Supply Index - SWSI

Importance of Hydrological Drought Indices

Hydrological drought indices are crucial for water resource management and planning. They help in:

• Monitoring and predicting drought conditions.
• Developing drought mitigation strategies.
• Informing policy decisions related to water allocation.
• Assessing the impact of drought on agriculture, industry, and urban areas.
• Enhancing our understanding of drought patterns and trends.

By using these indices, stakeholders can make informed decisions to manage water resources more effectively and mitigate the adverse impacts of drought.