Water Conservation and LHEP (Loktak Hydro-Electric Project)

Chabungbam Amuba Singh *

Loktak Lake - largest freshwater lake in North East India :: Pix - Bullu Raj

No one, not even the hardened critics, can deny that it is the launching of the LHEP (Loktak Hydro-Electric Project) that has saved the Loktak lake from the slow death inflicted by the natural processes of siltation and eutrophication, and infectious human activities. Had there been no LHEP and the associated conservation activities of the Loktak Development Authority (LDA), today we will be seeing a dying lake of less than a 100 sq km area instead of the present 236 sq km.

Such a scenario of a shrunken Loktak - with all the sister lakes in the valley dried up - would be fatal for the Manipur valley: some wise Saheb (Englishman) had reportedly said that without the Loktak, the Imphal valley would become a dust bowl.

The Loktak Multipurpose Project was conceived and planned six decades ago when the Imphal turel (Manipur river) was never known to show up dry bed as it has done, year after year, in the last one decade or so during the dry season. Those days, water was a commodity in abundance and the planners presumably felt no prick of conscience when they designed the LHEP to transfer 132450 hectare-metre of water annually to the Leimatak basin by lifting 185430 hectare-metre of water from the Loktak lake.

In plain words, the LHEP is designed to discharge 42 cumec of water into the Leimatak for power generation and lift another 16.8 cumec for irrigation. The amount of water the LHEP pours into the Leimatak river in a year is enough to flood the entire Manipur valley waist-deep in water. The question today is - can we afford to lose so much of surface water just to get a score of megawatt of electricity?

Such a question would have been dumped as irrelevant sixty-five years ago when school students (including the present author) were taught that Manipur of 8620 square miles area received 100 inches of rainfall annually; and the urgent concern of the state was the mitigation and control of flood, which were in fact not infrequent, rather than the supply of pipe water to its citizens (which was then considered an elitist urban delight only).

The concerns of the IFCD on prevention of flood were so profound that they once even considered blasting away of the Sugnu hump--an elevation in the bed of the Manipur river near its confluence with the Chakpi river. Thanks to the commissioning of the Ithai barrage, such a drastic idea was abandoned.

Precipitation (rainfall) is the only source of water in Manipur and hence it is the sole determinant of the quantity of water the state gets annually. To estimate the quantity of water the Imphal valley receives annually, we need to determine the catchment areas of the rivers and streams draining into the valley. According to a report webcasted by the Planning Department, Govt of Manipur, quoting 1984 IFCD source, the total catchment area of the Imphal, Iril, Thoubal, Sekmai, Heirok, Khuga, Manipur rivers upto Ithai and the Loktak lake through Khordak is 4651 sq km.

The data was presumably constructed on the basis of the Survey of India topo-sheets 1970 and could be considered authentic . [LDA, on its website, gives the catchment area of the Loktak lake to be 8221 sq km (Direct-1064 sq km plus Indirect- 7157 sq km), which is 36.8% of the total area of the state. It is obvious that the IFCD data is more reliable, at least, for my purpose).

No dependable study of the spatial distribution of precipitation over the state has been carried out, and hence there is no reliable estimate of the average rainfall over the catchment area mentioned above. As remarked afore, we have been misinformed for generations to believe that Manipur receives heavy annual rainfall of above 100 inches (2540mm). It is simply not true. There is a wide spatial variation in the precipitation pattern over the state.

For example, in some year when Tamenglong received more than 2000mm of rain, the Imphal valley (Lamphel and Raj Bhavan) received less than 1000mm. Googling for information on the rainfall in Manipur throws up the NEDFI Data Bank which quotes the Economic Survey Manipur 2008-09. Taking the data recorded at Imphal, Wangbal and Litan (averaged over 4/5 years), we arrive at 1085mm as the average annual rainfall in the catchment area of the Manipur River basin in central Manipur.

With the figures discussed above, we now have a crude estimate of the amount of water delivered annually by precipitation to the catchment area of the drainage system of the Imphal valley to be 504600 hectare-metre.

Not all the precipitation reaches the soil surface. A part of it is intercepted by the forest canopy and the dry leaves on the ground and returned to the atmosphere by evaporation. Ecologist call it interception loss. A study carried out by Asha Gupta of Manipur University in the Wangoi and Nambol areas shows that the interception loss can be as high as 44%. The interception loss, of course, depends on the quality of the canopy, the intensity of the shower and the temporal distribution of the precipitation. Asha Gupta's study covered a total of 201 rainfall events in the hydrological year 1999-2000.

Of the rainwater reaching the soil surface, a part will go underground to recharge the groundwater, a part will be collected as surface water storage like tanks, ponds and wetland stretches and still a part will be intercepted to sustain human activities like agriculture - all depending on the topography and land-use pattern in the catchment area. There is also the loss due to the perpetual process of evaporation.

Considering all these losses it may be safely assumed that only half the precipitation reaches the drainage channels. That leaves us with a figure of about 250000 hectare-metre of water annually available for channel flow, for the rivers and streams in the Manipur Central valley feeding the Loktak lake. This figure is critically too close to the volume of water the lake is transferring annually to the Leimatak basin to make any futuristic planner comfortable.

There is, however, no real cause for alarm. The estimate is, at best, crude and does not warrant to sound the alarm. But it does call for the need of a fresh look at the hydrological implications of the LHEP and for a deeper and realistic assessment of the hydrological status of the central Manipur River basin.