Purna River is an important west flowing river with its catchment lying in Ahwaa,Valsad and Navsari districts of Gujarat and in Nasik district of Maharashtra. The Purna basin can be divided into three prominent physiographic regions, i.e. i) eastern parts, (ii) the middle reaches and (iii) the coastal zones. The eastern parts of the basin cover a chain of rugged mountain ranges of the Western Ghats running at an elevation of above 1300 m and descending to an elevation of about 100 m at the edges of uplands of the Surat district. The middle reaches of the basin area are marked by high relief zone with ridges and valleys. The hilly zone then merges into the plains through an undulating piedmont coastal zone running parallel to the sea.

The river Purna rises in the Saputara hills of the Western Ghats near the village Chinchi in Maharashtra. The length of the river from its source to outflow in the Arabian Sea is about 180 km. The important tributaries of the Purna River are Dhodar nala, Bardanala, Nagihpar nala, Girna River, Zankari River and Dumas khadi. The catchment area of the Purna basin is 2431 Sq. km. The basin lies between 72° 45’ to 74° 00’ East longitude and 20° 41’ to 21° 05’ North latitude. There is only one H.O station of CWC near the mouth of the river at Mahuwa in Surat district of Gujarat state. State wise distribution of drainage area is shown in Table:

Accordingly to Koeppan’s Scheme, the climate of the basin is classified as AWTropical Savannah as most of the peninsular plateau, south of Tropic of Cancer is classified. In the initial reaches, the climate is influenced by the Western Ghats which becomes continued as the river reaches the coastal plains. The climatic variations are experienced in the patterns of temperature, rainfall & winds, rhythm of seasons and degree of wetness or dryness. These are described as follows:

The Temperature is maximum in the month of May and Minimum in the month of December to January. The maximum, minimum temperatures observed at site Mahuwa varies from 27° C to 46° C and 30° C to 10° C respectively.

The basin receives most of the rainfall from the South West monsoon from June to September. Average annual rainfall in the basin is 1596.8 mm.

The average monthly wind speed varies from 2.9 km/h to 0.2 km/h .In the pre and post-monsoon period, the wind speed is generally higher. The pre dominant wind direction is NE followed by SE and W.

The relative Humidity in Purna basin at site Mahuwa varies between 97.7% and 66.4% depending upon the season. It is naturally maximum in the monsoon period and is around 84.2 to 97.7%. In the winter months of November and December, the relative humidity decreases.

The whole basin can be divided in to three prominent physiographic zones viz. i) the Eastern zone ii) the middle zone and ii) the coastal zone. The Eastern zone of the basin covers a chain of rugged mountain ranges of the Western Ghats. The middle zone of the basin is marked by high relief zone with ridges and valleys. The hilly region then merges in to the plains through a coastal piedmont coastal zone running parallel to the sea. Deccan traps occupy the most parts of the basin. In the East there are high ridges and deep valleys and towards the west, they merge into the lower reach composed of recent and sub recent alluvium and blown sand. The straight graphical sequences of the rocks found in the basin are Neogene’s, Palaeogene and early Palaeogene.

Soils of Purna basin can be classified into three groups viz lateritic soils, deep black soils and coastal alluvial soils.

Water Year Book 2013-2014, West Flowing Rivers, CWC, Narmada and Tapi basin Organization, Hydrological Observation Circle, Gandhinagar, March 2015.