**4. Hydrological analyses**

The Loktak Lake has been the subject of study since 1950s with the primary objective of flood control and optimal use of water resources for accelerated economic development in the region. Water regime of Loktak Lake is determined by the inflow from various streams and direct precipitation on the lake surface. Overall 34 streams from the western hills and the Manipur river via Ungamel and Khordak channels drain into the lake. The hydrological analysis of the lake is investigated in detail viz., discharge and sediment etc.

Ecological Studies of Wetland Ecosystem in Manipur Valley from Management Perspectives 241

A total area of 63.50 km2 from different zones of Loktak Lake has been identified to be highly flood prone. The causes of flooding in the wetland system can be mainly attributed to unsystematic operation of Ithai barrage. Silted drainage system, developmental activities, change in land-use practices, loss of forest cover and presence of natural barriers like *Sugnu Hump* and alignment of Chakpi river with Manipur river further compounds the problem. Characteristics of floods in the northern zone are different from that of the southern zone. Many of the drains in the northern zone have heavily silted up and are choked with thick *Phumdi* resulting in reduction of their flushing capacity. This causes flooding in the

Water quality characteristics of Loktak Lake have been analyzed in detail. Water samples from six sites were collected seasonally i.e., rainy, winter and summer. All analyses were completed within ten days following standard method (Jain et al., 1999). The analysis carried out on various physico-chemical parameters reveals that the lake water of all the sites was circum-neutral to alkaline. The data reveals that pH of lake water ranges from 6.27 to 8.50. The lake water in different sites is severely affected and has also become considerably vulnerable to pollution with a wide range to contaminations. Dissolved oxygen concentration ranges from 6.80 to 9.32 (mg/l) and electrical conductivity in between 98 to 530 (µmoh/cm). Concentration of Chloride ranges from 8 to 30 (mg/l) and Phosphate-

Seasonal variation in water quality characteristics of Loktak Lake have been analyzed in detail (Table 6). Most of the selected parameters reflected the seasonal pattern showing higher values in the rainy season. The pH was recorded highest at Site MI during summer season and lowest at Site KI during winter seasons. Analysis of variance showed that the pH varied significantly at sites and seasons but their interaction was not significant (Table 7). Water temperature ranged from 9.7 C and 29.4 C in the temporal cycle, highest during summer and lowest during winter season. Variation in temperature was significant between seasons but not between sites. The turbidity of the lake at various sites ranged from 35 cm to 145 cm. The highest turbidity (145 cm) was recorded in the rainy season at Site MO. Variation in turbidity was significant between sites and seasons, and their interaction was also found to be significant {LSD (0.05) =1.9}. Alkalinity was lowest in the winter season at all sites**.** The total alkalinity varied significantly among the sites and seasons and their interaction was significant (Table 7) (LSD (0.05) = 2.4}. Electrical conductivity showed marked variation among seasons and sites with higher values ranging from 194 to 530 ( moh/cm) during rainy season, followed by summer (116 to 428 moh/cm) and winter season (98 to 150 moh/cm). It varied significantly among the sites and seasons, and their interactions was significant {LSD (0.05) =2.4}. Free carbon dioxide was highest in rainy season and ranged from 4.4 to 63.3 (mg/l) at various sites of the lake across the season. It varied significantly among the sites and seasons, and their interactions was also significant {LSD (0.05) =2.4}. Dissolved oxygen concentration range from 6.8 to 9.32 (mg/l). It varied significantly between sites and seasons and their interaction was also found to significant {LSD (0.05) =0.3}. Chloride concentration was highest in rainy season in all the sites and ranged from 8 to 30 (mg/l). It varied significantly among the sites and seasons and their interaction was significant {LSD (0.05) =4.1}. Phosphate-phosphorus concentrations fluctuate

phosphorous concentrations fluctuate in between 0.002 to 0.260 (mg/l).

**4.3 Flooding** 

upstream courses.

**5. Water quality analyses** 

#### **4.1 Water Inflow and outflow**

The annual average rainfall recorded in the lake watershed was 1392 mm during 2001. Total annual inflow of water into the lake was estimated about 1687 M cusecs. The surface inflow from 34 rivers/streams of the western catchment accounts for 52% of the total inflow into the lake. The total outflow of water from the lake was estimated about 1217 M cusecs. Water abstracted for the hydropower generation by National Hydroelectric Power Corporation (NHPC) accounts for 70% of the total outflow from the lake. Link channels account for 14% of water outflow, while loss of water through evapotranspiration and evaporation was estimated about 9.5% and 6.5%, respectively.

There has been a drastic change in the water exchange pattern between the Manipur river and Loktak Lake after construction of Ithai Barrage. The inflow was reduced to 91 M cusecs and outflow to mere 20M cusecs (Table 4).


Source: Loktak Lake Development Authority, Manipur

Table 4. Reduction in water exchange pattern between Loktak Lake and Manipur river

#### **4.2 Sedimentation**

Loktak Lake is gradually silting and the major contributor is sediment from the surrounding catchment. The annual average sediment input into the lake is estimated as 650,000 metric tones. Western catchment accounts for 65% of the total sediment inflow into the lake and the rest 35% from the link channels. The high amount of sediment from the western catchment is mainly due to land-use/cover change and *jhum* cultivation etc. Out of 45 microwatersheds of the western catchment, Thongjarok yields the maximum sediment load of 58 t/ha and the minimum of 2 t/ha by Merakhong (Table 5). Sedimentation has been observed in the peripheral areas of the lake especially at the mouth of the western streams and link channels, resulting in the reduction in the water holding capacity of the lake, and flooding in the peripheral areas. It has been observed that the silting rates have increased by 34.44% during 1993 to 2003 and by 69.94% during 1967 to 2003. High sediment deposition in the lake turns the lake turbid and deteriorating water quality.


Source: Loktak Development Authority (LDA), Manipur

Table 5. Sediment Yield during 2000-01

#### **4.3 Flooding**

240 Ecosystems Biodiversity

The annual average rainfall recorded in the lake watershed was 1392 mm during 2001. Total annual inflow of water into the lake was estimated about 1687 M cusecs. The surface inflow from 34 rivers/streams of the western catchment accounts for 52% of the total inflow into the lake. The total outflow of water from the lake was estimated about 1217 M cusecs. Water abstracted for the hydropower generation by National Hydroelectric Power Corporation (NHPC) accounts for 70% of the total outflow from the lake. Link channels account for 14% of water outflow, while loss of water through evapotranspiration and evaporation was

There has been a drastic change in the water exchange pattern between the Manipur river and Loktak Lake after construction of Ithai Barrage. The inflow was reduced to 91 M cusecs

> 2001-01 (M cum)

Reduction (%)

**4.1 Water Inflow and outflow** 

estimated about 9.5% and 6.5%, respectively.

and outflow to mere 20M cusecs (Table 4).

Source: Loktak Lake Development Authority, Manipur

lake turns the lake turbid and deteriorating water quality.

Sediment Load (t)

(MCM)

Source: Loktak Development Authority (LDA), Manipur

Table 5. Sediment Yield during 2000-01

(M cum)

Inflow 103 91 11 Outflow 315 20 93

Table 4. Reduction in water exchange pattern between Loktak Lake and Manipur river

Loktak Lake is gradually silting and the major contributor is sediment from the surrounding catchment. The annual average sediment input into the lake is estimated as 650,000 metric tones. Western catchment accounts for 65% of the total sediment inflow into the lake and the rest 35% from the link channels. The high amount of sediment from the western catchment is mainly due to land-use/cover change and *jhum* cultivation etc. Out of 45 microwatersheds of the western catchment, Thongjarok yields the maximum sediment load of 58 t/ha and the minimum of 2 t/ha by Merakhong (Table 5). Sedimentation has been observed in the peripheral areas of the lake especially at the mouth of the western streams and link channels, resulting in the reduction in the water holding capacity of the lake, and flooding in the peripheral areas. It has been observed that the silting rates have increased by 34.44% during 1993 to 2003 and by 69.94% during 1967 to 2003. High sediment deposition in the

> Area (ha)

Potsangbam 22 20752.18 1928.00 10.80 27 6 20 Awang khujairok 9 4229.91 843.40 5.00 54 8 41 Thongjarok 30 133440.83 2302.30 58.00 57 9 45 Merakhong - 12097.18 5947.20 2.00 - - - Nambol 104 65911.02 9540.00 6.90 243 34 198 Nambul 161 50204.90 19893.00 2.50 2338 270 2081

Sediment Yield (t/ha)

N (t/yr)

P (t/yr)

K (t/yr)

Parameters 1958-59

**4.2 Sedimentation** 

Rivers Discharge

A total area of 63.50 km2 from different zones of Loktak Lake has been identified to be highly flood prone. The causes of flooding in the wetland system can be mainly attributed to unsystematic operation of Ithai barrage. Silted drainage system, developmental activities, change in land-use practices, loss of forest cover and presence of natural barriers like *Sugnu Hump* and alignment of Chakpi river with Manipur river further compounds the problem. Characteristics of floods in the northern zone are different from that of the southern zone. Many of the drains in the northern zone have heavily silted up and are choked with thick *Phumdi* resulting in reduction of their flushing capacity. This causes flooding in the upstream courses.
