Sengupta, M. and Dalwani, R. (Editors). 2008. Proceedings of Taal2007: The 12th World Lake Conference: 703-716

Impact of Land Use on Water Quality of Phewa- Lake Pokhara,Nepal Rudra Bahadur Raya , Subodh Sharma Ph.D., Gopal Gurung SEC-Nepal, pokhara 5, Kathmandu University, Nepal, Programme officer, JICA-Nepal E-mail: rudra7raya@yahoo.com, waterrudra@hotmail.com, kuhimal@ku.edu.np, gopalgurung.np@jica.go.jp

ABSTRACT

The principle recent change in rural land use in the Pokhara valley of Nepal is that rural area are being absorbed into growing town and cities . this conversion of rural in to urban land has several impacts on lake water quality because of development such as industrial installations and so many hotel and restaurants which are located along the bank of environmentally vulnerable lake .

Phewa Lake which is a major tourist destination of Nepal is at present facing high human pressure at both its urban and rural watershed areas in the absence of proper sewage system in Pokhara city. The household waste water and sewage is directly discharged in to the Phewa Lake. Similarly sub surface seepage of septage from urban lakeshore area also pollutes the lake. Large amount of sediments load is carried in to the lake by different streams .flowing in to it, the major one being the Harpon stream. These streams and surface runoff from agriculture land also add nutrient load in to the lake. All this are causing pollution of the lake water making it beyond fit for recreational use. The nutrient load is causing eutrophication of the lake with proliferation of water hyacinth and algal bloom, increasing rate of fish mortality and destruction of Lake Ecosystem. High sedimentation rate of the lake has half reduced its size since 1956.

The prime objective of the water quality monitoring is to characterize the water quality of Phewa Lake over a long time frame. The water quality monitoring is mainly based on quantitative determination of selected physical and chemical parametric values that present in the water where various intervention have been implemented (discharge from hotel and restaurants ,fish farming ,outlet of Seti channel in the lake and tributaries running in the lake). Since the concentration of pollution level distribution at Phewa Lake is heterogeneous in nature, the stratified random sampling technique is adopted and to provide reliable data about sedimentation in the Phewa lake, which could then be used to estimate and predict sedimentation rate lake storage capacity, and its expected life span. The eco sounding, aerial photo and photo monitoring have given a clear picture of the sedimentation in the silt trap area. The survey indicates that the average sedimentation rate in the Phewa lake for the period from march 1998 to 2004 is about 180000 cu. M. and it has a silt trap area (I) it has a is about 94000 cu.m .annually. The total estimate average sediment contribution rate from the watershed was about 15 cu.m. for the sample period.

If this average annual sedimentation rate continues, 80% of the Phewa lake capacity would be silted up in the next 190 years virtually making the Phewa lake useless. The trap area depending on the situation of Harpon khola, will be comptely filled up in between 24 to 33 years reducing 16 %of the lake area .

Phewa Lake is mesotrophic to eutrophic status .most of the analyzed physo chemical parameters are destructive as found in natural surface water bodies.

In such situation ,if the lake continuous to be polluted and filled up by sediment at present rate ,its recreational and aesthetic value as well as national economy of Pokhara ,Nepal will be diminished .

Keyword: water quality, land use, lake pollution, sedimentation, water-shed ,eutrophic mesotrophic ,eco-sound ,aerial photo .

INTRODUCTION Background Nepal is renowned in the world on account of her natural beauty, geographical / biological diversity and cultural heritage. In adequate management and unwise utilization of these resources, despite there high potential has been undergoing several environment degradation. As a result, they may reach to a critically threatening point if adequate measures

are not taken. One of such important natural area is Phewa lake in Pokhara valley .The Phewa Lake is one of the most beautiful place in Nepal and attracts a large number of tourists from all over the world. By the virtue of its natural beauty, the lake contributes significantly in local and national economy through tourist industry.

The analysis of water is the major subject in the modern environmental chemistry. Lakes are one of the most important resources of water for the mountainous country like Nepal. Phewa Lake is

situated within one kilometer south west of pokhra city, kaski district at an elevation of about 793 m the area of lake is 4.6(Recent,2007) sq.km. (About 443hectares) and it holds approximately 53x106 cubic meter of water. At present, this lake is mostly used for hydropower generation (1KW capacity), irrigation (320 hectares) and recreation –boating and swimming. This lake and machhapuchhara (the Himalayan,fish-tail) have made Pokhara attractive tourists site. Harpon khola (stream) the main tributaries of Phewa lake is a perennial stream and from north western direction it is entering in the lake. The lake is surrounded by hills on two sides – north and south, village pame is situated on the north – western side and on the eastern side by Pokhara city it self. Pollution load in Phewa Lake is mainly from non point source.

There are hotel, restaurant and residential buildings that discharge there wastes directly in to the lake. On the northern side is the lake surrounded agriculture lands are touching. At the time of monsoon the rain water run from the hill side towards its bottom where there are agriculture fields and the manures and pesticides spread by the farmers on their agriculture field run in the lake (T.H.Tebbutt.1992). Another most important and severe problem of Phewa lake is the siltation in the lake from its tributaries and from its surroundings. In city like Pokhara, where lake water is used for recreational purposes such as swimming, boating and water withdraw from lake for hotel and restaurant uses. Water pollution on the lake impedes these activities. Also tourism, which is the major source of the countries income cannot flourish around polluted streams and lakes. Fishing and fish farming have been declining in this lake over the year. Fisherman, whose livelihood depends on the unpolluted streams and lakes, will be unable to sustain themselves and will have to look for other alternatives. The same kind of problems arouse for the boatmen for their livelihood.

The watershed area lies in a fragile physiographic region, which experiences intense monsoon rainfall events probably, it is the one of the highest rainfall–receiving watershed of Nepal (IWMP, 1991b). Intensive land use primarily in response to meeting their basic needs for food, fodder, fuel wood, fiber and shelter and development construction especially road without due consideration of the conservation measures integrated with high rainfall have been the major cause of the erosion process in the watershed, which has transported an enormous amount of sediment to the lake reducing its capacity. Sedimentation monitoring of the lake became utmost for the formulation of strategies for the soil conservation and watershed management and also for the management of the lake water for tourism, irrigation and hydropower production.

Objective of the study

1. To identified the concentration of pollution in the lake

2. To estimate and predict sedimentation rate, lakes storage capacity and its expected life span.

Problem statement However the lake and its watershed has been under immense and exhaustive pressure due to excessive human intervention since last couple of decade. This has resulted in various environmental problems as presented following 1. Lake of water quality deterioration making it unfit for recreational and aesthetic use.

• Discharge of domestic wastewater and sewer in to the lake through point source (Phirke khola, urban drain, seti channel etc)

• Nutrient /fertilizers from agriculture fields(non point source)

• Non point source: influenced by precipitation runoff during rainy season eg. Agriculture land, forest , urban area etc.

• Point sources: not influenced by precipitation discharge all year round eg. House , factories and domestic wastewater etc.

• Seepage and overflow of septage from septic tanks in to the lake.

• Direct discharge of toilet wastes in to the lake by lakeshore residents.

• Runoff carrying organic pollution load and solid waste in to the lake.

• Laudry washing by hotel and residents 2. Lake area shrinking at the rate of 2ha per year

• High sedimentation load from harpan ,andhari,sasurke khola (steams) and seti itrrigation channel

• Land slide and soil erosion at watershed areas

• Deforestation for fuel wood ,fodder,encroachment

• Inadequately developed infrastructure (egBaidham-pame road)at lake vicinity

• Soil erosion • Improper hill slope terrace cultivation • Over grazing by cattle

3. Lack of environment education

• Lack of incentive for improving daily habits that are detrimental to the lake.

• Lack of commitment and leadership

704

• Indifference of the rural people towards lake conservation as they do not get any benefit from the lake.

4. Lack of lake focused integrated

environmental conservation programme 5. Lack of resources to undertake such

programme 6. Lack of rural –urban linkage for collective

endeavor for environmental conservational of the lake

7. Lack of an active lake focused institution and sustainable utilization of resources

8. The HMGN supported Phewa lake area 9. Conservation committee does not have

representation of benefciaries hence more government dominant, and is grossly inactive.

10. Benefit from the lake not shared to rural community, which are equally responsible for its conservation

METHODOLOGY Study area The research area is situated around 900 m asl,Pokhara lies 200 km west from kathmandu , the capital city of Nepal Fig.1&2.The research area comprises the urban as well as rural watershed of Phewa Lake fig.3.

Phewa Lake, the touristically most important lake of Nepal is a stream fed dam regulated, semi- natural freshwater subtropical mountain lake (maximum depth 24m and mean depth 7.5 m), lying at an altitude of 742m asl, in Pokhara valley (28º 11’37” to 28º 17’ 26” North and longitude of 83º 48’ 2” to 85º 59’18” East). It occupies an area of 5.23 km2, watershed area of 110km2 (Rai et al., 1995) table 1. The lake has multiple uses such as hydroelectricity, irrigation, fishery and a boating facility. By land use pattern the lake features contrast in terms of forested with sparse rural settlement on southern side, agricultural land with dense urban areas on northern side, silt trap zone in western side and river channel zone in eastern side of the lakeshore. The watershed of the lake constitute

forest (44%), agricultural land (39%), urban and wetland area (5%), pasture and barren land (5%), lake area (4%) and shrub land (3%)(DSC, 1994).The population and mean annual rainfall of Phewa watershed accounts 0.14 million with annual growth rate of 7.4% and 35,000 mm respectively (CBS,1995). Trophically the lake changed from oligotrophic in ‘70s, to mesotrophic in ‘80s, and eutrophic by ’90s (Shrestha and Jananuer, 2001). Biodiversity richness accounts for 7 vegetation types in watershed area, 104 bird, 34 mammal, 16 fish, 14 reptile and 6 amphibian species (IUCN,1995a) plus 39 aquatic macrophytes including 23 hydrophytes

and 16 helophytes (Shrestha and Janauer, 2001). Number of tourists to Pokhara valley including the Lake Phewa accounted roughly 100,000 in 1995 (Banskota and Sharma, 1998). Method of water sampling Water samples were taken twice in a month, i.e first and last week, from 5 stations located in different part of lake namely Anadu, Khapaudi, Hallan Chowk, Inlet and Outlet. Water samples from Anadu were collected from 0m, 2.5m, 5.0m, 7.5m, 15m and 20m. Khapaudi site permitted samples were taken only from surface and 1m depths. Water temperature, pH and secchi disk validity were measured in situ. Water samples for dissolve oxygen was fixed at the site and measured by Wrinkler’s method. Nutrient analysis of water began immediately after collection of the samples in the laboratory. The standard methods involved in the nutrient analysis of water were: chlorophyll-a by SCOR/UNESCO (1960), ammonium-nitrogen (NH4-N) by Bower and Hansen (1980), nitrite +nitrate-nitrogen (NO2+ NO3-N) by Dowens (1978) and phosphate-phosphorus (PO4-P) by Murphy and Rilay (1966).

For phytoplankton, 100-ml portion of water was fixed with acid Lugol’s solution at a final concentration of 1% and a phytoplankton samples was concentrated by natural sedimentation. Cells of phytoplankton were enumerated with maematocytometer under a microscope. Water samples for zooplankton were collected with 70-micron plankton net having 20cm diameters. Samples were preserved in 1% procaine and % formalin. Zooplankton samples were concentrated and enumerated with Sedgwick-Rafter cell under a binocular.

Figure 1. Nepal

705

Figure 2: Pokhara valley

Figure 3: Phewa lake

Samples for bacterial count were fixed with

formaldehyde at 0.5% final concentration and stored 4.00C. Bacterial density was estimated using the Acridine orange direct count method (Hobbie et al. 1977): 3 sub samples were made by filtering 0.25 to 5.0 ml of water sample onto Nucleopore filters (0.2μm pore size). Bacteria were enumerated under an Olympus epifluoresecene microscope (1250x) with a B-excitation system (50W halogen lamp, IF 410-485 excitations filter). At least 300 bacterial cells were counted in each sample.

Aquatic plants coverage area is determined every fortnightly. Special attention has been given to water hyacinth coverage area in the lake. Every fortnightly the coverage area of water hyacinth is determined through eye estimation using the map of

lake for sketching the parts of the lake covered by water hyacinth. 2.3 Method of sedimentation survey 2.3.1 Echo sounding survey (Method I) The sedimentation survey of March 2006 is used as the base line information on the lake bed with reference to the high water level 794.15 m

The depth of the water in the lake is measured from a rowboat with an echo sounding instrument. The measured water depth is related to the reference water level (i.e.974.15m). the dates and water levels measured at Pardi damp during the survey are given in table 6 average water depth between benchmark to benchmark is computed for each cross –sectional profile using the reference water level (974.15m which is the designed highest water level).any decrease in water depth indicates deposition (sedimentation of the lake) and any increase in water depth indicates erosion of the lake bottom. The sediment deposit or erosion is computed by multiplying the mean of the average water depth of two cross-section by the area of the reservoir surface between those two cross-sections (method I) Sthapit, K.M., 1996. A bathymetric map was prepared. The gross capacity of the area of the reservoir surface between those two contour lines using the bathymetric 2.3.2 Map (method II). Table 1 back ground information of Phewa Lake.

Characteristics Phewa Elevation (msl) 742 Area (ha) 523 Catchments area (km2) 110

Water Depth (m) Maximum 24.0 Average 7.5 Volume of Water (m3) 393.2 * 105

Inlet stream Harpan Khola & Seti Khola Outlet Stream Phusre Khola

Source: (Rai et al. 1993 )

It is believed that method I is more realistic for estimating the sediment deposition as this will minimize the errors resulted other than the average water depth and lake surface areas . where as method II is consider to be more realistic for computing the gross capacity of the lake ,since there is a significant difference in gross capacity of the reservoir whenever the number of survey line changes.

The instrument used was a micro –processor –controlled depth recorder (eco-sounding) of the type ps-20r portable precision echo sounder. The

706

707

manufactured is kaijo dennico co ltd,japan . it continuously record depth to the bottom of the lake as boat moves along survey line. The echo sounder is operated by DC 12 v (10 -15v), 3.5 A the recorder works at a high frequency of 200 kHz

The positioning of the survey line is fixed by stretching a rope between benchmark so that measurement can be made along the same fixed line repeatedly. The rope is marked at every 25 meters interval and these positions are recorded in the chart during the echo sounding survey. This ensure the location of the measurement with reference to the benchmark. The sounding is carried out during calm (windless) weather to assure easy boat handling and accurate measurement along the straight line of the rope .

The distance and water depth were recorded manually from the sounding profile and process to estimate the average water depth using Microsoft excel spread sheet software .

When the water depth was less than 2-3 m and the profile was not clear , the water depth was measured manually using measuring rod or rope with stone and incorporated with the eco sounding graphs for the analyze . Aerial photo graph The survey utilizes also the aerial photo graph from 1983,1998,2001,2002 and 2004 to review the growth of the delta over a long time period. The photo graph help to understand the sedimentation process near the river mouth and over a longer time period, show visually the growth of the delta..

In the past, oblique photograph from a fixed point were taken from the near by hill top to understand the delta formation in the lake .some photo graph were taken during the survey. However, in the lake of the past photograph this could not be a success. RESULT AND DISCUSSION Water Quality of Sampling Sites Physical parameters of phewa lakes of Pokhara Valley Discussion Water quality Characteristics of Phewa Lake from pre-monsoon to post monsoon Some physical characteristics of Lake Phewa are given in Table-1. Details of the values of water quality determined from pre-monsoon to post monsoon are given in Table 2,3&4. The water temperature recorded minimum of 18.00C in post -monsoon to maximum of 26.8oC in pre-monsoon. The temperature at different station showed that there was change in water temperature at different stations.

The highest transparency of 3.1m was observed in pre monsoon from inlet source while later on in decreasing trend possibly because of sedimentation originating from its feeding stream. The highest visibility was observed at Anaudi in February with 5.9m, this seems this is one of the cleanest spot in the lake. In monsoon, in some station such as Inlet, H. chock and outlet the transparency could not measure because of the shallowness of the station.

Table 2. Water quality parameter of Phewa Lake at pre-monsoon period Parameters Observed value Inlet H.Chowk Outlet Khapaundi Anaudi Physical Water temperature (0C) 25.4 24.9 27.8 24.5 24.3 visibility(m) 3.1 3 2.9 4.1 5.9 Chemical Dissolve oxygen (mg/L) 8 6.5 6.6 7.5 7.3 Biological oxygen demand (mg/L) 2.9 3.5 4.6 4.5 4.3 Chlorophyll (mg/m3) 4.6 5.0 1.0 4.6 2.6 Nitrate nitrite (mg/L) 0.100 0.200 0.100 0.100 0.200 Ammonia (mg/L) 0.040 0.020 0.030 0.006 0.006 Total phosphorous (mg/L) 36.0 83.0 31.0 24.0 29.0 pH 5.0 8.1 6.0 5.7 5.8 Biological Heterotrophic bacteria (cells/ml) 4.435*106 Heterotrophic Nano flagellates(cells/ml) 0.832.103 Phytoplankton abundance (cells/ml) 227 1083 622 722 976 Zooplankton density (No/L) 51 233 374 478 836 Water hyacinth coverage (Eye estimate) 10% Season: Pre-monsoon

Table 3. Water quality parameter of Phewa Lake at monsoon period Parameters Observed value Inlet H.Chowk Outlet Khapaundi Anaudi Physical Water temperature (0C) 20.0 23.5 24.1 20.5 21.5 visibility(m) 2.8 2.9 2.1 2.3 2.7 Chemical Dissolve oxygen (mg/L) 7.0 9 8.7 7.6 8 Biological oxygen demand (mg/L) 4.5 3.2 3.8 4.2 4 Chlorophyll (mg/m3) 6.0 13.6 20.4 19.6 30.0 Nitrate nitrite (mg/L) 0.186 0.060 0.164 0.06 0.154 Ammonia (mg/L) 0.080 0.030 0.009 0.032 0.004 Total phosphorous (mg/L) 77.0 58.0 35.0 34.0 40.0 pH 6.5 7.6 7.3 6.8 7.0 Biological Heterotrophic bacteria (cells/ml) 9.02 x 106 Heterotrophic Nano flagellates(cells/ml) 0.9444 x 103 Phytoplankton abundance (cells/ml) 446 739 317 2091 414 Zooplankton density (No/L) 291 394 87 420 92 Water hyacinth coverage (Eye estimate) 8% Season: Monsoon

Table 4. Water quality parameter of Phewa Lake at post-monsoon period Parameters Observed value Inlet H.Chowk Outlet Khapaundi Anaudi Physical Water temperature (0C) 18.0 22.5 18.5 19 19.5 visibility(m) 2.7 2.2 2.4 3.4 4.4 Chemical Dissolve oxygen (mg/L) 7.7 9.69 7.0 7.4 7.3 Biological oxygen demand (mg/L) 4.2 3.7 4 3.9 3.5 Chlorophyll (mg/m3) 2.0 10.5 3.3 2.9 1.6 Nitrate nitrite (mg/L) 0.054 0.037 0.017 0.030 0.040 Ammonia (mg/L) 0.007 0.002 0.003 0.001 0.002 Total phosphorous (mg/L) 43.5 75.0 28.0 26.0 42.0 pH 5.6 6.5 6.1 6.2 6.2 Biological Heterotrophic bacteria (cells/ml) 10.33x106 Heterotrophic Nano flagellates(cells/ml) 0.738 x 103 Phytoplankton abundance (cells/ml) 379 1217 909 1443 1729 Zooplankton density (No/L) 738 685 271 458 218 Water hyacinth coverage (Eye estimate) 6%

The DO was not sufficient in all the sites. The dissolve oxygen (DO) concentration in water was within range from 6.5 to 9.9 mg/l. this amount of oxygen in different point shows the water is polluted and not suitable for drinking purposes.

The BOD was found in all site from 2.9 to 4.6 .this value indicate the water quality of phewa lake is organic polluted.

The pH range revealed acidic or alkaline nature of water. The pH ranged from 5 to 8.1 in Lake Phewa during sampling. The highest pH value was 8.1 in pre- monsoon period. Both nitrite + nitrate and ammonium nitrogen though did not show any definite pattern from pre-monsoon to post monsoon with the values ranging from 0.01 to 0.186 mg/l.

708

Table 5. Normal range of water quality parameters for different tropic status of Lakes Parameters Oligotrophic Mesotrophic Eutrophic Chlorophyll-a(mg/m3) <3 mg/m3 3-7 mg/m3 1-40mg/m3 Total Phosphorus (μg/L) 5—10 10*30 30-100 Total Nitrogen (μg/L) 0.0-4.0 4.0-1500.0 1500.0-5000.0 Heterotrophic Bacteria (cells/ml) 0.5-1 (x106) - 1-10(x106) Season: Post monsoon

However high amount phosphorus was detected from all sampled site. The highest concentration (83 mg/L) of total phosphorus (TP) was observed at Hallan Chowk in pre-monsoon. This might indicate that de-silting site and Harpahan Kola steam are the main source of nutrient input during the season of survey .Harpahan khola is passing through the agriculture land where excess use chemical fertilizer by farmer in this watershed area.About 16 species of phytoplankton were recorded from pre-monsoon to post monsoon period Phytoplankton is one of the important supplier of organic matter in lake ecosystem. The highest abundance of phytoplankton was observed in monsoon at Khapaudi site with a value of 2091 cells/ml with lowest in Inlet stream.

Heterotrophic bacteria were observed only at Anaudi site. The bacterial abundance was highest in post monsoon (10.33*106). Since the bacterial number did not vary in all the station and there was problems in observation of water samples from inlet stream and Harpan Khola. Therefore, heterotrophic bacteria from Anaudi were estimated. Heterotrophic nano flagellates was also estimated from single site Anaudi station.

Twelve species of zooplankton were identified during this period. The maximum density of was 836 No/1 in pre-monsoon from Hallan Chowk area.

The effects of different environmental factors on growth and flowering in water hyacinth have been previously studied. It showed that the optimum temperature requirement of the plant is 27-300C. The water temperature recorded minimum of 18.00C in post-monsoon to maximum of 27.60C in pre-monsoon which temperature is favorable for growth of water hyacinth at pre-monsoon season also the nutrient is high and growth of this plant in this season .

The growth of water hyacinth has been shown to directly relate to level of nutrient in water, particularly nitrogen and phosphorus. Phosphorus considered to be directly related with eutrophication process where water hyacinth and other macrophytes growth become boost. The highest concentration of total phosphorus (TP) was observed from Hallan Chowk in all sampled date followed by inlet and outlet source. This might indicate that de-silting site and Harpan Khola stream was still contributed as

input of organic materials as well as chemical fertilizer. Sedimentation Bathymetric map Bathymetric map of the reservoir was prepared based on the February 2006 data (because no any new data is found in Nepal) at the scale of 1:10,000 using 1 meter contour interval and is given in figure 4 with reference to the highest water level (794.15m), the area of the reservoir is about 439 ha Water depth The profile analysis for the February 2006 survey is given in annex I Annex II show the data computed for making bathymetric map with reference to the highest water level (794.15m) of the reservoir.

With respect to the reference water level (794.15m) the maximum and the average water depth measure in march 2000, may 2001,april 2002,December 2002 and January 2004 surveys are presented in table 7 and changes in average water depth are given in table 6 .

During the February 2006 survey, the maximum depth measured with respect to the reference water level of 794.15m in the lake was 23.62m, where as the estimated average water depth for the whole lake is about 9.60m. The change in the maximum and average water depth do not follow the uniform declining trend (table 6). The reasons might be one or more of the following:

• The accuracy of the instrument is not adequate for the slow sedimentation,

• The error in the measurement deviation in the survey line due to wind or not stretching the rope properly,

• The reference level at the Pardi Dam was not accurately recorded, since the marking in the scale is of 5 cm range,

• Error in reading the graph and / or • The section has sedimentation in one second

and erosion in another season due to change in under water current.

709

Table 6. Phewa lake survey, Maximum and Average water Depth in meters Phewa lake survey, Maximum and Average water Depth in meters

Maximum Depth in meters Average depth in meters Lines Mar’ 2000

May’ 2001

Apr’ 2002

Dec’ 2002

Jan’ 2004

Feb’ 2006 Mar’ 2000

May’ 2001

Apr’2002

Dec’ 2002

Jan’2004 Feb’2006

Silt Trap Area(I) 13 to 4 9.03 8.91 9.32 10.36 9.52 10.62 5.11 4.56 5.23 5.40 5.06 4.29 5 to 13 8.62 10.63 4.76 4.69 4.62 4.46 4.52 3.14 13 To 12 7.68 7.33 7.43 7.59 6.93 6.28 6.81 6.39 6.60 6.73 6.02 4.88 12 To 5

6.66 6.18 6.00 6.22 6.40 6.38 3.75 3.48 3.47 3.35 3.41 2.76

6 To 11 4.68 4.54 4.51 4.49 4.42 10.53 2.56 2.59 2.23 2.07 1.49 1.52 10To7 5.68 5.29 5.21 5.31 5.18 3.26 2.91 2.70 2.15 1.18 8 To9 3.58 4.04 3.97 3.77 4.23 4.07 2.50 2.83 2.78 2.66 2.72 2.50 9 To 10 5.79 5.68 5.83 4.46 4.45 4.24 9 To 21 5.91 6.07 5.95 4.59 4.56 4.53 22 To23 10.57 8.96 8.27 Main Reservoir (II)

14 to 3 21.85 22.13 22.86 22.56 20.06 14.74 14.50 15.07 14.82 14.67 16 To15 17.60 17.49 17.65 19.27 17.74 17.82 7.49 7.65 7.56 7.48 7.51 7.46 17 To18 9.45 9.51 10.01 9.37 9.51 7.35 7.13 7.57 7.18 6.95 2 To 20 14.53 15.36 14.38 14.51 8.82 9.01 8.64 8.79 3 To 20 14.77 14.71 9.53 9.26 14 To18 23.40 23.60 13.88 13.99 4 To 19 16.20 16.60 12.80 12.64 17 To 101

22.60 22.46 9.90 9.76

Table 7. Phewa lake survey, Depth changes in meter Phewa lake survey, Depth changes in meter

Depth change in meters Lines

Mar’2000 To may’ 01

May 01 to Apr’02

Apr’02 to Dec’02

Jan’04 to Dec’02

Jan’04 to Mar’2000

Feb’06 to Mar’2000

Feb’06 to Jan’04

Silt Trap Area 13 to 4 -0.55 0.67 0.17 -0.34 -0.05 -0.82 -0.77 5 to 13 -0.07 -0.07 -0.16 0.06 -0.24 -1.62 -1.38 13 to 12 -0.42 0.21 0.10 -0.68 -0.79 -1.93 -1.14 12 to 5 -0.27 -0.10 -0.12 0.06 -0.34 -0.99 -0.67 6 to 11 -0.16 -0.58 -1.07 -1.04 0.03 10 to 7 -0.35 -0.55 -1.11 -1.28 -0.17 8 to 9 0.33 -0.05 -0.12 0.06 0.22 0.00 -0.22 9 to 10 -0.01 -0.21 9 to 21 -0.12 22 to 23 -0.69 Main Reservoir 14 to 3 -0.24 -0.25 0.08 -0.07 -0.15 16 to 15 0.16 -0.09 -0.08 0.03 0.02 -0.03 -0.05 17 to 18 0.44 -0.39 -0.17 -0.40 -0.23 2 to 20 -0.37 -0.18 -0.03 0.15 3 to 20 -0.27 14 to 18 0.11 4 to 19 -0.16 17 to 101 -0.14 19 to 4 -0.16

710

711

Anything may happen and the correct

interpretation with out long term and closed monitoring is difficult. Also sediment distribution in the main reservoir due to it’s large size will so thin that annual change in the average water depth may not be significant here. Lake’s Storage Capacity The gross capacity of the reservoir is computed by multiplying the mean of the average water depths of two contour lines by the area of the reservoir surface between those two contour lines using the bathymetric map (method II). During the highest water level, the area of lake is 439 ha and the total capacity of the lake is estimated to be 42.2 million cubic meters (February 2006). About 74% of the coverage area and 87% of the water volume are in the main reservoir area(II). Similarly 10% of the coverage area and 8% of the water volume are in the

river channel (III). Only 16% of the coverage area and 5% of the water volume are in the silt Trap Area Table 8. Sedimentation Generally, Phewa Lake has broad and gentle sloped lake bottom except at the gorge portion. The sedimentation distribution is quite thin and in many places is beyond the accuracy of the instruction for the sediment computation on annual basis. The sedimentation up to January 2004 had been computed by the department of soil conservation. Therefore, here the sedimentation in the lake between the January 2004 and February 2006 is computed. The deposition or erosion of sediment is computed by multiplying the mean of the average water depths of two cross-section by the area of the reservoir surface between those two cross sections (method I).Table 9.

Table 8. lake storage capacity Lake Storage Capacity Contour Silt Trap Area(I) Main Reservoir

Area(II) River Channel Area(III) Total

Area Total

Interval Area in ha Volume in million cu. M.

Area in ha.

Volume in million cu. M..

Area in ha Volume in million cu. M.

Area in ha Volume in million cu. M.

0- 1 10.7 0.05 7.4 0.04 2.4 0.01 20.5 0.01 1 2.7 0.03 0 0.00 0 0.00 2.7 0.03 1- 2 10 0.15 7.4 0.11 2.4 0.04 19.8 0.30 2 - 3 12.1 0.30 10.4 0.26 2.4 0.06 24.9 0.62 3 2 0.06 0 0.00 0 0.00 2 0.06 3-4 11.1 0.39 12.8 0.45 3.1 0.11 27 0.94 4-5 5.2 0.23 10 0.45 3.9 0.18 19.1 0.86 5 4.8 0.24 0 0.00 2.7 0.14 7.5 0.38 5-6 2 0.11 13.6 0.75 2.4 0.13 18 0.99 6-7 1.1 0.07 15.7 1.01 2 0.13 18.8 1.22 7-8 1.1 0.08 15.1 1.13 3.6 0.27 19.8 1.49 8-9 3.3 0.28 19.9 1.70 1 0.09 24.2 2.06 9 1.1 0.10 1.7 0.15 0 0.0 2.8 2.25 9-10 1.2 0.11 31 2.95 1 0.09 32.2 3.16 10 0.4 0.04 0 0.00 0 0.0 0.4 0.04 10-11 10.7 1.12 10.5 1.10 21.2 2.22 11-12 10.7 1.23 0.2 0.03 10.9 1.25 12-13 30.5 3.81 1.7 0.21 32.2 4.02 13-14 22.4 3.03 2.7 0.37 25.1 3.39 14-15 39.7 5.76 0.6 0.08 40.3 5.84 15-16 12.8 1.89 0.8 0.13 13.6 2.11 16-17 7.3 1.21 0.1 0.01 7.4 1.22 17-18 7 1.23 7 1.23 18-19 7.4 1.36 7.4 1.36 19-20 7.6 1.49 7.6 1.49 20-21 6.2 1.27 6.2 1.27 21-22 8.4 1.80 8.4 1.80 22-23 2 0.46 2 0.46 23 8.7 2.01 8.7 2.01 Total 68.8 2.26 326.8 36.74 43.9 3..17 439.5 42.18 Percent 16 5 74 87 10 8 100 100

Table 9; Sediment calculation, Phewa lake Echo sounding survey, January 2004 to February 2006. Sediment calculation, Phewa lake Echo sounding survey, January 2004 to February 2006.

With reference to the highest water level(794.15m) Average water

pth in m. deVolume of water in ha-m.

Change in water volume in ha –m.

Cross section Surface area of lake in ha.

Jan-2004

Feb-2006

Jan 04 Feb 06 Jan -04 to Feb 06

Remarks

Main reservoir Line 15-16 and 17-101 35.5 8.7 8.6 309 305 -3.4

Line 17-101,17-18&14-18 95.2 10.3 10.2 982 974 -8.2 Line14-18,19-4&14-3 78.0 13.8 13.8 1079 1073 -5.2 Line19-4&3-20 41.5 11.2 11.0 463 454 -8.9 Line 3-20&2-20 8.5 9.1 9.0 77 77 -0.5 Line13-4&14-3 3.4 9.9 9.5 34 32 -1.6 triangle Line2-20 13.7 8.64 8.79 40 39 -1.0 Line17-18 23.9 7.18 6.95 57 55 -1.9 37.6 -2.9 299.7 78.9 77.8 3040.9 3010.2 -30.7 Silt Trap Line13_4&13_5 2.0 4.79 3.72 9.7 7.6 -2.2 Line 12_13,13_5&12_5 9.7 4.65 3.59 45.1 34.8 -10.2 22_23&12_13 6.8 7.49 6.58 51.0 44.8 -6.2 Line12_5&6_11 8.6 2.45 2.14 21.1 18.4 -2.6 6_11&7_10 13.4 1.82 1.75 24.4 23.5 -0.9 Line7_10,9_10&8_9 15.1 3.11 2.91 46.9 43.9 -3.0 Line9_10&9_21 3.4 4.5 4.39 15.5 14.9 -0.6 Triangle Line22_23 3.8 2.72 2.50 3.9 3.6 -0.3 Line8_9 2.3 4.56 4.53 3.5 3.4 -0.10 Line9_21 4.2 8.96 8.27 11.5 10.6 -0.9

The total area of the lake does not tally since the lake area downstream from 15-16 section is not included because of lack of survey lines. The sedimentation in the main reservoir is estimated as 30.7 hector meter and 27.1 in the silt trap area. Total sedimentation of the Phewa lake between January 2004 to February,06 is 57.8 -hectare meter.

Total 69.5 45.2 41.4 232.7 205.6 -27.1

GrandTotal 369.1 124.1 118.2 3273.6 3215.8 -57.8

The sediment deposited in the main reservoir is

calculated by multiplying the average depth of the cross sections and the area of the reservoir surface between those cross sections (method I). The total sediment deposited in the whole lake during 2000 to 01, 2000 to Apr 02, 2000 to Dec 02 and 2000 to Jan 04 were estimated as 177, 260;417, 600; 545, 330and 859,490 cu. m. respectively (after DSC, 2004). Therefore, average annual sediment deposition in the lake was estimated to be 213, 620 cu. m. for the period 2000 to 2004. Where as, the sediment deposited in the silt trap area is calculated by multiplying the average depth between the two contour lines and the area between the contour line by bathymetric map (method II). The sediment deposited during 2000 to 01, 2000to Apr 02, 2000 to Dec02 and 2000 to Jan 04 were estimated as 129, 020; 212,600; 267; 980 and 480,670 cu. m. respectively. (After DSC, 1994) therefore, average annual sediment deposition in the silt trap area is

estimated as 120, 170 cu. m. for the period 2000 to 2004.

Table 7 indicate that the total sediment deposited in the main reservoir and silt trap area are estimated as 307000 cu. m. and 271000 cu .m. respectively for a period of about four year(Jan 2004 to Feb 2006), and 578000 cu. m. is for the Phewa lake . Therefore the mean annual estimated sedimentation rate during January 2004 to Feb 2006 in the main reservoir and silt trap areas are about 76750 and 67750 cu. m. respectively and for whole Phewa Lake, it is about 144500 cu. m.

The total estimated average sediment contribution rate from the water save about 17 cu. m. per hectare for the period from 2000 to 2004(After DSC, 2004), about 12 cu. m. per hectare for the period 2004 to 2006 and 15 cu. m. for the period 2000 to 2006.

The average sedimentation rate in the Phewa lake for the period of March 2000 to Feb 2006 is

712

about 180000 cu. m. and in the silt trap area (1), it about 94000 cu. m. annually. Delta Formation Rapid sedimentation in the lake started the formation of the data at the mouth of the Harpan Khola . The delta formation is reviewed using the aerial Photograph of 1983, 2006, 2000, 2002 and 2004. The area of the delta exposed is very much dependent on the water level and no two aerial Photographs were taken with the same water level. The tentative water level of the lake during the month of the aerial of photography is given in table 7. The water level ranges from 792.75 in Feb 2004 to 793.7 in Dec 2006, with maximum difference of about one m. the water level in 1973 could not be known. Oblique photograph taken from the same point at different water level clarify how the water level affects the exposure of the delta area of the mouth of the Harpan Khola (Fig.4).

Photo no. 1 March 8, 2006. water level + 792.50 m. above mean sea level.

Photo no. 1.May 29,2006 water level + 793.50 m. above mean sea level. Figure 4.(exposure of the delta formation at different water level, Phewa lake).

The delta as expose in different aerial photograph is mapped at 1: 10,000 scale map showing basically silt trap area (Fig 5.). With the limitation of the different water level in the lake, about 25.5 hectares of delta is formed between 1983 to 2000. Similarly, the delta expansion continue by its formation of additional 12.9 hectares between 2002 to 2004 and 4.6 hectares between 2004 to 2006. On an average, delta formation is continued at the rate of about two hectares annually since 1983(Table 10). Table 10. area of delta formation in the Phewa lake. Area of delta formation in the Phewa Lake. Description Area in ha.

Delta formation (1983- 2000) 25.5

Delta formation(2000-2004) 12.9

Delta formation(2004-2006) 4.6

Total (1983- 2006) 43

Life spans of the lake. The capacity of the lake is estimated as 42.18 million cu. m. with highest water level of +794.15 m. if the lake is considered to be dead, when 80% of the silted up and if the average sedimentation rate about 180000 cu m. continue, the lake will be dead in about 190 years.

The sedimentation rate in the silt trap area for the period of 4 year (2000 to 2004) was about 120000 cu. m. annually for the period between 2004 to 2006, it was only about 67750 cu. m. there fore, and the average sedimentation rate for the period from 2000 to 2006 in the silt trap area is about 94000 cu. m.

With the annual average sedimentation of about 94000 cu. m. in the silt trap area about 68 ha. (I.e. 16 % of the lake area) it will be completely filled in 24 years.

However, the result depends on how the Harpan Khola shift with in the deltas. The aerial photograph of 1998 and 2001 indicated that the Harpan Khola was draining in the upstream part of the silt trap area. Similarly, aerial photograh of Feb 2004 indicated that the Harpan Khola started draining to the main reservoir area. Fig.7. therefore, prior to the 2004, the Harpan Khola was mainly draining to the silt trap area allowing it to silt up quickly. Since then the river was directly draining to the main reservoir near the end part of the silt trap area. This is one of the reason why the annual sedimentation of silt trap area between 2002-2004 was only 67750 cu. m. compared to 120,170 cu. m. for the period of 2000- 2004.

If the sedimentation rate of 67750 cu. m. continues in a silt trap area, it will be completely

713

filled only in the next 33 years. The growth of the delta at the rate of about 2 hectares per year also indicate that in about 33 years the silt trap area (I) of about 68 hectare will be completely silted up.

Therefore, the silt trap area (I) depending upon the situation of Harpan Khola, will be completely filled up in between 24 to 33 years, thus reducing the lake area by 16%.

Experience has shown that landslide and stream bank cutting contributes majority of the sedimentation in the lake, therefore landslide treatment and stream bank protection measures should be emphasized if the sedimentation in the lake is to be significantly reduced.

Figure 5: June 15 Water level 795.50m. Above mean sea level Figure Location of Harpan khola (stream) as seen in aerial photographs

Figure 6: Aerial photo graph-1998 Harpan khola draining to the upstream part of the silt trap area.

Figure 6: aerial photo graph -2001 harpan khola draining to the upstream part of the silt trap area showing signification sediment at the mouth

Figure 7: Aerial photo graph -2002 Harpan khola draining mainly to the downstream part of the silt trap area .

Figure 7: aerial photograph -2004, harpan khola draining mainly to the main reservoir area.

714

Photo; lab analysis of water sample by researcher

Photo; interaction about water hyacinth at lake by researcher CONCLUSION The water quality parameter of the three season in Phewa Lake suggest its eutrophic status. Among many station Hallan Chowk input highest amount of phosphorus. Next to it was the inlet stream. It is very difficult to state and recommend anything based on the collected data; however, efforts should be made to cut off the nutrient sources from Hallan Chowk and inlet stream of the lake. Values of limonological parameters area also relatively high at Khapaudi is the main site where plankton is gathered due to wind blowing direction. This was the reason that cage of plankton feeder fish has been kept there. Therefore, this finding should not be interpreted in other ways.

Erosion processes in the slope and deposition in the depressions is the natures’ law. Therefore, the lakes reservoirs are meant for the sedimentation and Phewa lake is no exception from this law of nature. Issues here are basically how the time frame of sedimentation of the lake or reservoirs could be lengthened for the betterment of the environment and

for the people and agencies to take necessary measures to lengthened the life span of the lake.

The echo sounding, aerial photo and photo monitoring have given a clear picture of the sedimentation of the silt trap area. The records from 2000 to 2006 indicate that the average sedimentation rate in the Phewa lake for the period from March 2002 Feb 2006 is about 180000 cu. m. and in the silt trap area about 94000 cu. m. annually . The total estimated average sediment contribution rate is about 17 km per ha. For the period from 2000 to 20044. and 15 cu. m. for the period from 2000 to 2006.

If the average annual sediment rate about 180000 cu. m. contain 80% of Phewa lake storage capacity will be silt up in the next 190 years virtually making the Phewa lake useless there fore the silt trap area depending up on the situation of Harpan Khola will be completely fill up in between 24 to 34 years reducing the 16 % of the lake area .

The annual change in the average water in the reservoir due to its size may not be significant . however for the reliable sedimentation record survey lines need to be added and poor benchmarks need to be replaced.

Sediment monitoring is a tool to guide the concerned agencies for the formulation of necessary strategy to protect the lake from sedimentation. Similarly study to understand the relationship between climate, erosion processes in the watershed and sedimentation in the lake are needed. ACKNOWLEDGEMENTS The author wishes to acknowledge the invaluable direction, support and advice provided by the JICA Project and Pokhara sub metropolitan city, Working Group. It is not an easy task to be compressive in acknowledge the help received from different persons from different field .the everyday life dimension during this study period is important to emphasize ,as this whole study has been interwoven in to a large mosaic of moments of happiness and stress. With no experience in interdisciplinary writing, I personally gained a lot from this opportunity to pursue this research. This study really provided with a wide range of knowledge in both my academic and professional field.

My in depth thanks go to Shree Prasad Dhaubhadel, Associate Professor, Institute of Forestry Pokhara, who enable me to think scientifically and come up with the scientific way of presenting my dissertation. His friendly, cooperative attitude and sense of humour will always be remembered. In spite of this busy and tight work schedule, he managed to give the sufficient time and relevant comments. I specially thanks to my research assistants Miss Pramila K.C.,Binita K.C. and rabin kadaraya (B.Sc forestry students) for her outstanding assistance during the field work and report preparation.

715

716

I am extremely thankful to mr Subodh Sharma (Ph.D.), Associated Professor Kathmandu University for his helpful assistance during my research period .my thanks goes to Bishomber Man Pradhan associated Professor of Institute of Forestry, for providing me with equipment facilities and moral support .I am thank full to miss Rosa Rawut for her assistance in chemical analysis of water parameter in ENPHO laboratory. REFERENCES Banskota, K., Sharma, B., 1998. Mountain tourism for

local community development in Nepal. A case study of Phewa lake side, Pokhara. Discussion paper 98/2, ICIMOD, Kathmandu

Bower and Hansen, 1980. Water quality assessment of lakes, Austria 13(2): 67-71

CBS (Central Bureau of Statistics), 1995. Statistical year book of Nepal. Kathmandu.

DECORE, 1991. Socio- Economic Baseline Survey of Phewa Tal watershed. Prepared for integrated watershed management project, Phewa Tal and Kulekhani watersheds.

Department of soil conservation, Babar Mahal, Kathmandu, Nepal.12(3): 141-146.

DSC,1994. Sedimentation survey of Phewa Lake, Jan 1994. Department of soil conservation, Barbar Mahal, Kathmandu, Nepal. 109-113.

ENPHO(1994)A review of limnological studies and research in Nepal .kathmandu Nepal 13(3): 65-71.

ENPHO(1996) A report on “ The water source monitoring prgramme for the shivapuri watershed by assessing water quality”19(4): 132-137.

Fosbeg,C.&S.O.Ryding (1980),Eutrophication parameters and trophic state indicate in 30 swedish was tereuiving lakes ;Arch,hydrobiol, 89;189-207.

IUCN, 1995a. Phewa Lake conservation Action plan. National Planning Commission in collaboration with IUCN-The World Conservation Union, Kathmandu.

IWMP, 1991b. Phewa Tal Siltation survey, base line date and the first results 1990-91, IWMP Technical paper no.2/1991. integrated watershed management project,

Phewa Tal and Kulekhani watersheds. Department of soil conservation, Barbar Mahal, Kathmandu, Nepal.

IWMP, 1992a. Phewa Tal watershed landuse map 1991. Integrated watershed management project Phewa Tal and Kulekhani watersheds Department of soil conservation, Barbar Mahal, Kathmandu, Nepal.

IWMP, 1992b. Integrated watershed management plan for Phewa Tal watershed, 1992. Integrated watershed management project Phewa Tal and Kulekhani watersheds Department of soil conservation, Barbar Mahal, Kathmandu, Nepal.

IWMP,1991a. Phewa Tal Siltation survey, manual for echo-sounding, June 1991, updated May 1992. Integrated watershed management project, Phewa Tal and Kulekhani watersheds. Department of soil conservation, Babar Mahal Kathmandu Nepal.

Rai, A.K, Shrestha, B.C., Joshi, P.L., Gurung, T. B., Nakanishi, M., 1995. Bathymetric maps of Lakes of Phewa, Begnas and Rupa in Pokhara valley, Nepal. Memoirs of the Faculty of Science. Kyoto University (Series of Biology) 16, 49-54.

SCOR/UNESCO (1960), surface water quality monitoring of lakes in Asian country 123-127.

Shrestha .R.R.(1990) Biological and physiochemical –investigation of the Bagmati river (with specific reference to water pollution ) M.Sc thesis .central department of botany T.U.kathmandu Nepal

Shrestha, P., Janauer, G.A., 2001. Management of aquatic macrophyte resource: A case of Phewa Lake, Nepal. In: P.K. Jha, S.R.Baral, S.B. Karmacharya, H.D. Lekhak, P. Lacoul (Eds.), Environment and Agriculture: Biodiversity, Agriculture and Pollution in South Asia. 99-107. Ecological Society (ECOS) Nepal.

Standard method for the examination of water and wastewater (APHA,AWWA.WEP) 19thedition 1995 201-209.

Sthapit, K.M. 1996. Sedimentationmonitoring of Kulekhani reservoir. Paper presented in the international conference on reservoir sedimentation, held on Sep 9-13-1996, fort Collins, Colorado, USA.

T.H.Tebbutt. 1992 Principles of water quality controlled 4th edition 124-128.