Rainfall  Pattern in Uttarakhand :

Understanding of rainfall is an important issue for Uttarakhand, India which having varied topography and due to that extreme rainfall causes quick runoff which warns structural and functional safety of large structures and other natural resources.

Rainfall is considered as the main source of domestic water for living as well as for agriculture in the Uttarakhand State. The state has varied topography, leading to great variation in rainfall spatially and temporally. An analysis of the rainfall series of the state would enhance the management of water resources as well as its optimum use. One of the challenging tasks with rainfall data is to deal with interpreting past records of rainfall events in terms of future probabilities of occurrence. Therefore, the understanding of the rainfall distribution that causes flood might play an important role for the sustainable development and conservation of natural resources of the state.

Analysis of rainfall data :

Analysis of rainfall data plays an important role for any water resource planning as well as for hydrological modeling. The average annual rainfall recorded is 1069 mm for the whole duration in an average. During this period, the highest amount of rainfall was about 1982.15 mm in 1936, whereas the lowest amount of rainfall recorded was about 559.98 mm during 1987. The dark line in the figure represents the annual average rainfall. If the annual rainfall in a year departs from the average annual rainfall by greater than or equal to 25%, then it is declared as a drought (meteorological drought) year. On the basis of 25% departure from the average annual rainfall, 30.4% times there were dry years. There was sufficient rainfall from July to September to meet evapo transpiration demand and vice versa from the October to June.

It can be inferred that the annual rainfall of the Uttarakhand State is spatial and there is a wide variation in annual rainfall amount. The average maximum annual rainfall of 2426.77 mm occurred in Champawat, whereas the lowest average annual rainfall 406.70 mm occurred in Haridwar.

To identify the seasonal rainfall distribution, the whole year divided into three periods, namely monsoon (June–September), post-monsoon (October–February) and pre-monsoon (March–May). This reveals that the area receives about 82% of the total annual rainfall during the monsoon season, 10% during the post-monsoon season, and 8% during the pre-monsoon season. It indicates that more than 82% of rainfall occurs in the monsoon season, and in the remaining 8 months the crop suffers from moisture stress. Therefore, it is necessary to predict the expected rainfall to design a water conservation system. Based on the drought criteria, the years 1903, 1918, 1941, 1944, 1965, 1974, 1979, 1987, 1991 and 2001 can be characterized as drought years for Uttarakhand State.

               The Uttarakhand State is facing the problem of quick translation of rainfall to surface runoff because of slope and faces the problems of landslides. Thus to cope up with these issues, an organized calculation of probability distribution to understand and selection of the best-fit probability distribution on an annual series required.

The choice of best probability distribution could also be used to influence decisions relating to local economics and hydrologic safety systems.

Further to this seasonal rainfall distribution, it reveals that the area in an average receives about 82% of the total annual rainfall during the monsoon season, 10% during the post-monsoon season, and 8% during the pre-monsoon season. This preliminary result will help the water resource planner in hydrological modeling and the policy maker to frame general guidelines for the best use of rainfall for Uttarakhand.