The Monsoon in Rajasthan

6.1 SW Monsoon Onset and Progress

The South-West Monsoon is the dominant rainfall mechanism, delivering 75–80% of Rajasthan's annual precipitation.

Chronology of monsoon advance in Rajasthan:

• Late June (20–25 June): Arabian Sea branch crosses Kerala coast (June 1 national average), reaches Gujarat by mid-June, enters southwestern Rajasthan (Banswara, Dungarpur, Pratapgarh) by June 20–25
• Early July (1–10 July): Bay of Bengal branch (deflected NW by India's upper body) enters southeastern Rajasthan — Jhalawar, Baran, Kota
• Mid-July (10–20 July): Monsoon advances to cover Jaipur, Ajmer, Bikaner (eastern parts)
• Late July (after July 20): Northwestern Rajasthan — Jodhpur, Barmer, Nagaur, Ganganagar receive monsoon rainfall
• August: Normal rainfall period; break monsoon phases can cause temporary dry spells
• Mid-September: Monsoon withdrawal begins from northwestern Rajasthan — one of the earliest withdrawal points in India

6.2 Monsoon Rainfall Distribution

The isohyet pattern in Rajasthan shows a clear SE–NW decreasing trend:

• 100 cm isohyet passes through the Kota-Udaipur-Sirohi belt in southeastern/southern Rajasthan
• 75 cm isohyet runs through Jaipur-Ajmer-Bhilwara-Chittorgarh
• 50 cm isohyet (the critical drought threshold) runs roughly through Sikar-Ajmer-Pali-Jalore
• 25 cm isohyet runs through Bikaner-Jodhpur-Barmer
• 10 cm isohyet encircles Jaisalmer — the hyperarid core

6.3 Monsoon Variability

Rajasthan's monsoon is characterised by:

1. High inter-annual variability: Coefficient of variation (CV) of 40–60% in western zones vs. 20–25% in southeastern zones. A "normal monsoon year" for Jaisalmer may deliver 8 cm or 18 cm — both within statistical range.
2. Break monsoon: Periodic dry spells during the monsoon season (7–10 day gaps), more frequent in Rajasthan than other states. Coincides with northward shift of the Inter-Tropical Convergence Zone (ITCZ).
3. Flood years vs. drought years in the same state: While western Rajasthan faces drought, eastern Rajasthan occasionally floods (Chambal, Mahi rivers). 2019 saw both simultaneously.

6.4 Western Disturbances

Western Disturbances are extratropical low-pressure systems originating in the Mediterranean Sea that travel eastward along the mid-latitude westerlies, bringing winter precipitation to northern India including Rajasthan.

• Local names: Mawat or Mahawat — the winter rainfall event that follows a Western Disturbance
• Timing: November to March; peak frequency December–February
• Frequency: 4–8 Western Disturbance events per winter season; each lasts 2–5 days
• Rainfall: 1–5 cm per event in northern Rajasthan; occasionally 5–10 cm in Sikar, Jhunjhunu, Alwar
• Agricultural significance: Critical for rabi crops — wheat, mustard, gram. A season with 3–4 good Mawat events produces bumper rabi yields; zero-Mawat winters cause rabi failure
• Districts most affected: Ganganagar, Hanumangarh, Bikaner, Sikar, Jhunjhunu, Alwar, Jaipur, Bharatpur
• Snowfall: Western Disturbances occasionally cause snowfall on Mt. Abu and upper Aravalli peaks; frost in plains districts

Why Western Disturbances matter for Rajasthan specifically: Unlike peninsular India (which depends entirely on SW monsoon and NE monsoon), Rajasthan has a dual rainfall regime — summer SW monsoon + winter Western Disturbances. The western arid districts may get 60–70% of their meagre rainfall from the SW monsoon and 30–40% from Mawat. The loss of either source in a given year is sufficient to trigger drought.

---