Rainfall in large parts of the State is not only inadequate but also varies sharply from year to year and place to place. The average rainfall of Rajasthan is 564.89 mm (1960 to 2009) compared to the all-India average of 1,100mm and a significant variation is seen across different regions.
The South-west monsoon brings the maximum rainfall in the state. In certain areas south-east and intermediate rainfalls also contribute towards the total rainfall. Pre-monsoon showers start towards the end of June and post-monsoon showers may continue till the first week of October.
At many of the places highest rainfall is received in July and August. The period of monsoon is shortest, ranging around 2 to 2.5 months. Its onset is late and withdrawal early as compared to other States and one or two dry spells is a common phenomenon. 90 percent of the total rainfall is received during monsoon season (July-September).
In the western Rajasthan, the average annual rainfall during 1980 to 2009 ranges from less than 171.12 mm in north-western part of Jaisalmer (lowest in the state) to 400 mm in Sikar, Jhunjhunu region and along the western periphery of the Aravalli range. In the eastern region, the rainfall ranges from around 400 mm in Ajmer to 762.19 mm in Jhalawar. In plains, Banswara (795.25 mm) and Jhalawar (762.19 mm) districts receive the maximum annual rain.
The highest rainfall is received in the southwest region of the State. The annual spatially averaged rainfall is highly variable and it is most erratic in the western region with frequent dry spells, punctuated occasionally by heavy downpour in some years associated with the passing low pressure systems over the region .
The number of rainy days during the south west monsoon period from June end to mid-September over Rajasthan varies from 10 in Jaisalmer to 40 in Jhalawar and to 48 in Mount Abu. The quantum of rain and number of rainfall days during the rest of the year in different parts of Rajasthan range from 2.1 cm at Jaisalmer to 7.2 cm at Jaipur, distributed over 2.5 to 6 rainy days .
During the rainfall deficit year of 2002, the state received just 220.4 mm rainfall up to September, against the normal of 518.6 mm in the overall monsoon .The maximum average rainfall of 726 mm was recorded in 1996 and minimum 291.6 mm was recorded in 1987, prior to 2002.
Rajasthan is heavily dependent on rainfall as it is the major source of water resource in the arid / semi-arid state. All the rivers of Rajasthan are rain-fed, the only exception being river Chambal.
The major land use is rain-fed cropping, the Eastern Rajasthan that falls in the semi-arid 500–1000mm annual rainfall zone and is intensively cultivated. Thus, any fluctuation of rainfall in the climate sensitive state of Rajasthan can pose a multitude of socio-economic problems
Category of Intensity of Rainfall
- Abnormal 60% or more
- Excess 20% to 59%
- Normal 19% to (-)19%
- Deficit (-)20% to (-)59%
- Scanty (-)60% or less