Ecosensitive Watershed Management as a Solution to the Power Crisis

Sri Lanka is rich in water resources. Therefore the economic well being of the country is closely associated with the water resources development. Most of the technically feasible and economically viable hydropower sources have already been tapped. On the other hand, hydroelectric power development needs comparatively high capital cost and long construction periods. However, electricity demand is rapidly increasing while alternative power sources such as liquid fuel and coal power sources are not acceptable due to high operational cost and negative environmental effects.


Introduction
"A single drop of water that falls from the sky should not be allowed to flow into the sea with out serving man. " -King Parakramabahu the Great The concept of water management has been completely described in the above statement.
However, water resources cannot be fully utilized for human needs mainly due to the associated environmental problems. Therefore the management of the existing water resources projects is becoming more important.
Even though three fourth of the Earth's surface is covered by water only as little as 2.5% of this is pure and fresh. Much of the water resources [70%] are in the form of icebergs in the arctic and other regions of extreme cold. As such, less than one percent of the total volume is suitable for human consumption.
A thousand cubic metres of water per year is the minimum need of each human being. The United Nations Development Programme [UNDP] says anything less than 1000 Cubic metres indicate an acute shortage of water.
Sri Lanka has a land area of 65,000 sq km. This is only one twothousands of the Earth's ground surface. The world's annual average rainfall is about 800 mm while Sri Lanka receives 2000 mm average rainfall. A minimum of 600mm rainfall is available in the dry zone while the central hill area of the country receives more than 3000 mm per annum. Let us assume only 400 mm of rain can be retained for human needs. Each Sri Lankan citizen would be entitled to 1300 cubic metres of water, which is well above the minimum requirement.
A huge volume of water is used for irrigation, which requires only a marginal head for conveyance. More than eighty percent of irrigation water cannot be reused. Therefore, per capita irrigation demand is nearly 700 cubic metres per annum. An average man needs 80 cubic metres of potable water for domestic purposes leaving another 280 cubic metres of water for other unclassified applications. Even after harnessing the available energy in the water it can be used for some other purpose such as irrigation. Therefore water used for hydropower generation is non-consumptive. On the other hand Irrigation, Domestic water supply and hydropower development can be integrated with each other .The socio economic development of the country can be mainly attributed to the multipurpose water development that has taken place in the country.
The earth is surface receives massive energy in the form of solar radiation. Fifteen percent of the solar energy is consumed by the Hydraulic cycle. Finally, less than five percent of the energy in the hydraulic cycle can be extracted in the form of hydroelectric power. The total hydropower development potential of the country is more than 2500 MW while only 1270 is exploited so far with an annual energy output of 4756 GWh. The remaining Hydropower sources cannot be exploited mainly due to environmental reasons, bureaucratic and political reasons and not being economically viable. The present hydropower system is capable of generating 1 kWh per person per day. This can be further increased by 20% , if proper watershed management is adopted.

Hydropower Development in Sri Lanka
The geographical formation of the country has favorable effects on hydropower development. The massive Mahaweli project contributes nearly fifty percent of the national demand while the Kelani system contributes nearly thirty percent of the demand. Samanalawewa and Kukule Ganga are the other major projects, which contribute significant amounts of energy to the National grid.
Upper Kothmale, Moragahakanda, Uma oya, Broadlands, Belihul oya, Madulu oya and Gin Ganga are the remaining major hydropower projects. A large number of medium and^minor hydropower schemes are now being constructed by the private sector. The details of the hydropower projects are given in the annexure.

Reasons for the present power crisis
During the initial stage of hydropower development, electricity was considered as a luxury or industrial oriented and only 10% of the population enjoyed the benefits of hydroelectric power' On the other hand individual con sumption was as low as 0.5 kWh per day.
After completion of the accelerated Mahaweli project, nearly twenty years ago, electricity supply was more than the demand and consumers were requested to go on electricity. However, 200 MW thermal plants were installed in Kelanitissa and Sapugaskanda for peak load operations. During the last two decades, electricity demand has increased at the rate of twelve percent per annum.
Today, peak demand is about 1400 MW while maximum hydropower is limited to 1200 MW. Under normal circumstances only 1000 MW is available in the form of hydropower while another 400 MW has to be generated by the thermal plants. During severe droughts hydropower capacity reduces up to 500 MW causing power cuts. Now, all peak load thermal power plants are being operated even for the base load. The lifetime of the thermal plants will be drastically reduced due to continuous operation.

Aim
The aim of the study is to find out a cost effective and environment friendly solution to the energy problem so that we will be able to overcome the present power crisis.

Methodology
Hydrologic Behavior of the Mahaweli watershed up to Rantambe reservoir [Some 3000 sq km] is modeled under the virgin forest condition, prevailing condition and after restoration. By comparing outcome of above three conditions necessary recommendations will be drawn.
Under the second part of the analysis, the effects of the proposed upper Kothmale reservoir on entire Mahaweli project will be considered. Mahaweli system will be compared with the most. effective Kehelgamu oya and Maskeli oya combined system and the conclusion will be made by considering theoretical, practical, and socioeconomical aspects.

Mathematical model to represent the Hydrological behavior of upper Mahaweli catchment.
It is very difficult to model the natural things while man-made things can be modeled very easily. Therefore modeling of a watershed may not give hundred percent accurate results.
Results from the mathematical model will be calibrated by means of available data.
Hydrological behavior of a catchment is a function of so many variables. The slope of the terrain, soil type, nature of the forest cover and the rainfall intensities are the governing factors. It can be shown that any catchment produces it's maximum efficiency under the virgin forest condition. However, considerable deforestation has taken place, especially during the last two centuries, thus reducing the watershed performance drastically.
River flow consists of rapid component surface run off and slow ground water flow through a long period. Flood flow is causing certain difficulties to man while the Ground water flow makes our living more comfortable. Natural watersheds are capable of retaining more ground water, which could be released slowly during the dry period.
There are several methods to assess the ground water recharge and surface run off. Curve Number method was selected for this analysis.
Technical data pertaining to the Upper Mahaweli Catchment is given below.

Results
Tables 5, 6 & 7 and the graphs in the annexure shows the distribution of the ground water flow and flood flow through out one year period. Capacity of the virtual ground water reservoir can be increased by 200 MCM by proper Watershed Management. Table 8 shows the hydropower generated by each power station after the proposed restoration.

Sedimentation of Existing tanks
Large reservoirs are normally designed for a hundred year lifetime. In most of the cases an annual sediment yield of 0.25% was assumed in initial designs. [Even after the active lifetime 75 % of the capacity should be available for storing water. However, actual rate of sedimentation of tanks seems to be very much higher than predicted values.

Table5
Reservoir Sediment If the rainfall intensity is more than twenty five millimetres per hour those rains are called erosive rains. However, thick forest cover over a watershed area can provide a high resistance for soil erosion. Sedimentation of large teservoirs can be reduced by more than fifty percent by proper watershed management.

Comparison of Kelaniand Mahaweli Systems
The Kehelgamu Oya and Maskelioya system is capable of generating 1600 GWh per annum with a combined catchment area of just 320 sqkm while the highest output of 2400 GWh is obtained from the Mahaweli system with a catchment area of 3200 sqkm.
As far as the Kelani system is concerned, the Hydropower benefits from one hectare of watershed land is nearly worth four hundred thousand Rupees.[Net income of a properly managed one hectare of tea land in this area is about two hundred thousand Rupees]. Higher productivity of the Kelani system can be attributed to the higher rainfall received, low Evapo-Transpiration and mainly due to construction of two reservoirs at the maximum possible elevation of 1100 metres above Mean Sea Level. These reservoirs are capable of supplying a regulated flow for the rest of the system. The other important feature of the Kelani system is Utilization of nearly 1000 metres effective head starting from major reservoirs and ending at Samanala pond.
However, the same efficiency is not available in the Mahaweli system. The entire Mahaweli system utilizes an effective head of 500 metres.  Therefore, new hydropower projects should not be encouraged, unless the existing projects are properly managed or the new project has significant favorable effects on the existing system. Perhaps effective hydropower system management requires certain modifications to the old system. However, this involves little capital investment, compared to the initial project cost, which could be recovered during a short period.
The productivity of the Kelani Hydropower system is six times than that of the Mahaweli. Theoretically, it should be possible to extract 7,000 GWh from the Mahaweli system, The analysis shows that it is possible to obtain 3850 GWh from the Mahaweli network, with the combined effect of watershed management and construction of the proposed Upper Kothmale powerhouse. However, construction of Upper Kothmale project will be a multi-dimensional optimization problem subjected to so many constraints.
Global warming has become an environmental hazard especially for the tropical countries.
Scientists have correctly pointed out that the emission of carbon dioxide is the main reason for global warming. In 1990, at Koyoto, Japan, a set of emission reduction targets were determined for the 39 worst carbon dioxide emitting countries. It was also decided that most of the developing countries are not required to reduce the GHG emission as they were well below the emission levels of industrialized nations. There are several flexibility mechanisms that allow the countries to trade their obligations with others. Clean Development Mechanisms allow developed nations to trade their obligations with a country like Sri Lanka, which is not under any obligation to reduce emissions.
Under the prevailing conditions 1-Ha of forest land in the upper watershed area can provide hydropower benefits worth 200,000 to 400,000 Sri Lankan Rupees.
When the Koyoto Protocol becomes law, reforestation of one Ha of bare land or leaving a forest land as it is will be able to absorb 100 tons of Carbon dioxide worth 20,000 US$ annually. Now, it is the right time to give up harmful activities in the central hill areas. Nature will reward us for living with nature. Otherwise, we will have to spend billions of Rupees for importing oil while leaving the poor people of the country for ever in darkness. Week Number