Study of impact of wet and dry spells in rainfall records on the water availability estimates – A research report by National Institute of Hydrology

The report examines the effect of periodicity in the rainfall records on the water availability of Damanganga basin in Nasik and Thane districts of Maharashtra. Based on the rainfall-runoff record of a shorter period, runoff series for a longer period is developed using a water balance model. The per cent deviation in water availability estimations computed using short term wet and dry records from the complete records is examined. The effect of length of time-series on the water availability estimates is also studied.

The analysis is performed in two steps. In the first step, the effect of wet and dry spells on the water availability is examined. It is observed that on a yearly basis, its effect is more pronounced as compared to monthly basis. Also, for wet spells, an increase of about 23 per cent and 34 per cent respectively, on monthly and yearly basis is observed. For dry spells, this decrease is about 11 per cent for monthly and yearly water availability computations.

In the second step, the effect of length of data on the water availability is examined. As different sets of consecutive months are considered, dry as well as wet spells in the data are automatically included and the best fit line obtained as a result contains the effect of these periods.

Based on these analyses, it is observed that for water availability analysis on monthly basis for Damanganga basin, a minimum of 90 months of record length is needed and for computation of water availability at higher dependabilities, flow record must be of sufficient length so as to represent both dry and wet periods effectively to avoid the over or under estimation of flow values.

For shorter length of records, a relationship between per cent dependable flow value and mean rainfall can be established and then using this relationship and long term average rainfall value, dependable flow values can be modified to represent the long term trend and behavior of rainfall.

Results indicate that a minimum of seven and half years of record is needed for estimation of water availability on a monthly scale for Damanganga basin and that wet and dry spells plays a crucial role in the determination of water availability.

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