Drought Assessment under the Manual for Drought Management

Drought severity describes how seriously a prolonged shortage of water affects crops, vegetation and available water supplies. Under India’s Manual for Drought Management, 2016, assessment combines rainfall conditions with impact indicators and field verification, rather than treating rainfall deficit alone as proof of severe drought.

Rainfall indicators initiate the assessment

Rainfall indicators establish whether meteorological conditions warrant further drought assessment. They capture both the amount of rain received and its distribution over time.

• Rainfall deviation measures the departure of actual rainfall from the normal rainfall for the relevant period.
• The Standardised Precipitation Index (SPI) expresses precipitation deficiency relative to the long-term rainfall record.
• Dry spells identify prolonged interruptions in rainfall, which can damage crops even when cumulative seasonal rainfall does not show an exceptionally large deficit.

Four categories of impact indicators

After the initial rainfall assessment, the Manual uses indicators from four categories to determine whether rainfall deficiency has translated into agricultural and water stress. These indicators are evaluated against the Manual’s prescribed severity thresholds.

• Agricultural indicators assess the area under sowing relative to the normal area, helping identify whether inadequate moisture has prevented or delayed cultivation.
• Remote-sensing vegetation indicators include the Normalised Difference Vegetation Index, Normalised Difference Water Index and Vegetation Condition Index, which help assess vegetation condition and moisture stress.
• Soil-moisture indicators include the Moisture Adequacy Index and Percentage Available Soil Moisture, which indicate whether moisture is sufficient to support crop growth.
• Hydrological indicators assess conditions in streamflow, reservoir storage and groundwater, showing whether water shortages extend beyond rainfall and crop conditions.

Determining and verifying severity

Severity emerges from the combined assessment of the relevant indicators, not from a single observation such as low rainfall or declining reservoir storage. The Manual provides a structured procedure for identifying moderate or severe drought and checking the assessment on the ground.

• Ground-truth verification examines actual crop conditions and damage in affected areas to corroborate the indicator-based assessment.
• Meteorological, agricultural and hydrological evidence can reveal different aspects of the same drought; their combined interpretation distinguishes rainfall shortage from its wider consequences.