The State of Goa is located on the West Coast of India and receives abundant rainfall from the South-West monsoonal winds. The Konkan belt locked between the Sahyadri range on the Eastern side and the Arabian Sea on the West is very conducive for relieving the heavy moisture laden winds during monsoons. As a corollary, rainfall pattern is heaviest along the mountain ranges and on its western slopes, which gradually reduces towards the coastal plains. The majority of entire precipitation is limited to a period of just four months (i.e. June to September). Annual rainfall pattern in Goa over the last 14 years, is very erratic and rainfall in recent years has been below an annual average of 3000mm. Though the State has a huge potential of assured water, a high percentage of the water resources get drained down, owing to Goa's physiographical set up and ultimately joins the Arabian Sea. This results in sharp imbalance between the water supply and its availability during different months of the summer and monsoon season. The surface drainage is helped by a number of streams, nallahs, rivulets and rivers, which run across the State in a westerly direction and finally run into the Arabian Sea.

Surface Water Resources

The State has nine rivers, of which six rivers originate and flow exclusively within the State boundaries and do not have any interstate implications. However, Terekhol and Chapora rivers originate in Maharashtra while Mandovi river originates in Karnataka State. All the rivers except Sal in the South originates on the western slopes and subsequently meanders over falls and rapids into the coastal plains (during young/initial stage), from where they tend to become sluggish (during mature stage) and then ultimately joins estuary mouth and then into the sea (old stage). Most of these rivers are subject to tidal variations and salinity upto a distance of 20-40 kms upstream from their respective mouth regions as such these have been referred to as estuaries. Most of these rivers are excellent navigational channels and are used mostly for transporting ores through barges from ore loading jetties to Mormugoa Harbour for onwards export.

Out of nine rivers in the State, rough estimation reveals that basin-wise average runoff is to the tune of about 8436 million cubic meters (MCM). This entire water resource should not be mistaken as available for harnessing. As assessed by Central Water Commission (CWC), the water resource of Goa is 8570 MCM. Many of these rivers are tidal prone upto a distance of 20-40 kms, and utilizable part of the yield should necessarily be identified basin wise.

Table 1: Details of river basins in Goa

S. No.	Name of the River Basin	Length within the State	Length within the salinity Zone	Basin area	Average runoff
		(km)	(km)	(sq. kms)	(MCM)
1	Terekhol	26	26	71	164.25
2	Chapora	32	32	255	588.35
3	Baga	10	10	50	116.42
4	Mandovi	52	36	1580	3580.04
5	Zuari	145	42	973	2247.4
6	Sal	40	14	301	694.39
7	Saleri	11	5	149	343.04
8	Talpona	32	7	233	515.59
9	Galgibag	14	4	90	187.11
	TOTAL	362	176	3702	8436.59

Source: Master Plan for Madei / Mandovi River Basin A Report by The Panel of Experts, Vol. I, Vol. II

Groundwater Resources
The Central Ground Water Board (CGWB), Ministry of Water Resources has prepared "Master Plan for Development of Ground Water in Goa State" in March 1997. In this report, taking into account only the annual safe dynamic recharge, it has been stated that the ground water potential that can be exploited in the State is 150 million cubic meter (Mm3). The present use is 9 Mm3. Therefore the balance left for safe exploitation is about 141 Mm3 in the state.

During post-monsoon periods, river flow starts dwindling and very few rivers remain perennial throughout its entire length. This obviates the developmental water resources scenario thereby forcing the developmental authorities to focus on water storage measures for its optimal surface water utilization. The nature has bestowed abundant opportunity for groundwater recharge in the State by way of copious rainfall during the monsoonal period. As the overburden mantle is made up of laterites & lateritic soils along the foothills and plateau tops, the recharge is relatively faster. However, on the other hand, it enables effluent seepage through nallahs, creeks to lower the water table at a faster rate. The macro-level surveys indicate groundwater draft of about 525 MCM. Taking the water supply for domestic water requirements to be accorded top prioritization, as per the regulatory measures of about 15%, the rest is available for irrigation and other allied uses which work out to about 450 MCM.