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. It 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 showed "Hydrological drought". For quantifing 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. Every of these indices requires different variables as input data in their formulas. The most important and well-known hydrological drought indices are:

1. The SWSI (Surface Water Supply Iudex) developed by Shafer and Dezman (1982)* in Colorado.
2. The RDI (Reclamation Drought Index) developed by Weghorst (1996).
3. The SDI (Streamflow Drought Index) developed by Nalbantis and Tsakiris (2009).
4. The PHDI (Palmer Hydrologic Drought Index) developed by Palmer (1965).

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 is the Palmer drought index.The surface water supply index (SWSI) (Shafer and Dezman, 1982) was primarily developed as a hydrological drought index and it is calculated based on monthly non-exceedance probability from available historical records of reservoir storage, streamflow, snow pack, and precipitation. The ranges of SWSI's value are between -4 to 4.

RDI (Weghorst 1996) improved SWSI by incorporating temperature and hence calculated a variable water demand as input.

Based on the SPI developing concepts, the SDI was developed. The streamflow drought index (SDI), developed recently by Nalbantis and Tsakiris (2009), 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 should better assess moisture anomalies that impact river flow, ground-water availability, and lake or reservoir levels. The Palmer Hydrological Drought Severity Index (PHDI) is very similar to the PDSI, using the identical water balance assessment on a two-layer soil model. The Palmer Hydrologic Drought Index (PHDI) uses a modification of the PDSI to assess moisture anomalies that affect streamflow, ground water, and water storage (Steinemann,2003), being more sensitive to hydrological components.

* Shafer and Dezman (1982)