What are the hydrological drought definition?

The droughts are generally classified into four categories (American Meteorological Society, 2004),which include:1- Meteorological drought, 2- Agricultural drought, 3-Hydrological drought, and 4- Socio-economic drought.

Hydrological droughts are more related to water demands and they appear whenever a marked reduction becomes appreciable in natural streamflow or groundwater levels, plus the depletion of water storage in dams and lakes for water supply. Hence, hydrological droughts are very important and significant for urban areas or industrialized regions as well as for agricultural activities. 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.

The main impact of hydrological droughts is on water resources systems. Hydrological drought is not attached to the precipitation decrease only, but additionally decreases in the surface flow and drops in the groundwater levels provide joint impacts (Sirdas and Sen, 2003). Hydrological droughts are related to agricultural droughts by its effects on irrigation systems dependent on surface water and on river water quality. Hydrological drought is even further removed from the deficiency of precipitation since it is normally defined in terms of the departure of surface and subsurface water supplies from some average condition at various points in time.

Analyze Hydrological Drought using Surface Water Supply Index - SWSI

Hydrological drought refers to the period when water levels in natural systems such as rivers, lakes, reservoirs, and aquifers fall below the statistical average for a given time frame. Unlike meteorological drought, which is defined by the lack of precipitation, hydrological drought focuses on the impact of dry conditions on water supply.

Key Aspects and Definitions

Hydrological Definition

• Streamflow Deficit: Occurs when river and stream flows are significantly lower than the long-term average.
• Reservoir Storage Deficit: Happens when the water storage in reservoirs falls below normal operational levels.
• Groundwater Level Deficit: Defined by significantly lower groundwater levels compared to historical averages.

Impact on Water Resources

• Reduced Water Availability: Lower water levels in rivers, lakes, and reservoirs lead to decreased water availability for municipal, agricultural, industrial, and ecological uses.
• Decreased Hydropower Production: Reduced water flow can limit hydropower generation, affecting energy supply.
• Water Quality Issues: Lower water levels can concentrate pollutants, leading to degraded water quality.

Causes of Hydrological Drought

• Prolonged Meteorological Drought: Extended periods of low precipitation and high temperatures reduce water input into hydrological systems.
• Changes in Land Use and Land Cover: Urbanization, deforestation, and other land-use changes can affect runoff and infiltration, altering hydrological regimes.
• Overexploitation of Water Resources: Excessive withdrawal of surface and groundwater for various uses can exacerbate drought conditions.

Measurement and Monitoring

• Streamflow Records: Monitoring flow rates in rivers and streams provides critical data for assessing hydrological drought.
• Reservoir Storage Data: Tracking water levels in reservoirs helps in evaluating water supply status.
• Groundwater Monitoring: Using wells to measure groundwater levels over time aids in understanding the impact on aquifers.
• Drought Indices: Indices such as the Surface Water Supply Index (SWSI), Palmer Hydrological Drought Index (PHDI), and Standardized Streamflow Index (SSI) are used to quantify and monitor hydrological drought.

Hydrological Drought Indices

• Surface Water Supply Index (SWSI): Combines data on streamflow, reservoir storage, snowpack, and precipitation to provide a comprehensive measure of water supply conditions.
• Palmer Hydrological Drought Index (PHDI): Focuses on the hydrological impacts of prolonged periods of dryness, particularly on surface water and groundwater levels.
• Standardized Streamflow Index (SSI): Standardizes streamflow data to provide a comparative measure of drought severity across different regions and time periods.

Impacts on Ecosystems

• Aquatic Habitats: Low water levels can stress aquatic ecosystems, affecting fish and other aquatic species.
• Riparian Vegetation: Drought conditions can harm plants that rely on consistent water supply from rivers and streams.

Management and Mitigation

• Water Conservation Measures: Implementing policies and practices to reduce water usage during drought periods.
• Drought Preparedness Plans: Developing comprehensive plans that include monitoring, response, and mitigation strategies to manage drought impacts.
• Sustainable Water Management: Promoting practices that ensure long-term sustainability of water resources, such as protecting recharge areas and reducing overextraction.

Hydrological drought is a complex phenomenon influenced by both natural variability and human activities. Effective monitoring and management strategies are crucial to mitigating its impacts on water resources and dependent ecosystems.