Economic Benefits of Water by Assessing the Comparative Advantage of Water used Activities. A Case Study at Kaltota Irrigation Scheme Associated with Geoinformatics Techniques

The term water use can be classified into consumptive use and non-consumptive use. In Sri Lanka, the highest volume of water is used for irrigated agriculture for paddy cultivation and hydropower generation. In hydropower generation the total amount of water does not undergo a state change i.e. non-consumptive use, whereas in irrigated agriculture only a portion of the released water undergoes a state change i.e. consumptive use. By nature, the balance portion is used for the sustainability ofthe environment including social life. There was no reliable technique to quantify the consumptive use of water in irrigated agriculture. Therefore ,as a common practice the total amount of water released has been accountedfor the event of crop production. If irrigation water is not released for a season, the affected outcome is the immediate loss of crop production. If this is continued for consecutive seasons, several unfavorable situations related to the social life and the environment could be experienced. Hence, one has to identify the different water use activities related to irrigated agriculture and apportion the released amount of water among such activities. Techniques cf Satellite Remote Sensing can now be used to estimate the consumptive use of water irrespective ofthe type of vegetation cover over the land surface. This paper describes the authors work of estimation ofthe amount of water consumed by the paddy crop at the Kaltota Irrigation Scheme using Geoinformatics techniques. The economic benefit of water for paddy production is compared with the same for hydropower generation at Samanalawewa Hydropower Station which is situated on the same water course at the upstream of the Kaltota Irrigation Scheme.


Introduction
Assessing the economic benefits of water by comparing the market prices of "water used products" for which water is used as the main component during the production process or computing "amount of rupees generated by a drop of water" has now become a common practice.In this context, water, as a commodity Manuscript received: 30.09.2004 Revised manuscript received: 03.10.2004Manuscript accepted: 10.10.2004 will receive a relative value than an absolute value because, unlike gold, there is no way to assess the absolute value of water.On the other hand, nature can survive without gold, but not without water.The total amount of fresh water available does not vanish in the global context.But the pattern of receiving water has been hampered by man made activities.And, also, with the increasing population, water is treated as a scarce resource.
In irrigated agriculture, water is used for several activities such as evaporation, transpiration, percolation, surface drainage etc.
Once the water is released, it is distributed over a large area and physically disappears throughout the season.The amount of water used for the crop growth (actual evapotranspiration or ET_act) i.e. the amount of water transpired from the crop canopies including the amount of water evaporated from the land surface, will undergo a state change and added to the atmosphere, which is a consumptive use.The portion which is discharged as surface drainage will be (re)used by the downstream users and the environment in the vicinity.By measuring the drainage discharges, this amount could be estimated.But the quantification of the balance portion, which infiltrated as deep percolation to recharge the ground water table, is rather difficult.In order to compute the water productivity over irrigated agriculture, the amounts of water used for said activities have to be estimated.
When water is used for hydropower generation, the amount of water used will usually 'be discharged to the same watercourse at a further downstream location.The amount of water does not undergo a state change.In other words, it is a non-consumptive use.The amount of water used and the units of hydropower produced could be easily quantified.During the last decade, scientists have shown increasing interest in using data from earth observation satellites to obtain information on land surface parameters which undergo spatial and temporal changes.Information on such parameters related to soil, water and vegetation is of prime importance for managing natural resources.Remote Seinsing produces the spectral measurements that provide the biophysical input data needed to determine evapotranspiration, crop water stress, soil moisture at root zone depth, and biomass growth, etc. Area representative measurements instead of point measurements and time series analysis of data acquired in shorter intervals will improve the quality and the accuracy of the results.High frequency satellite measurements can fulfill both requirements.This research study estimates the actual evapotranspiration of crop growth using satellite remote sensing and computes the water productivity over said activities.And it also computes the water productivity over hydropower generation to assess the comparative advantage of water.The following objectives have been established in this study.

•
To carry out a water use study on a

Fig. 2 Water use types within the process of irrigated agriculture
Irrigation water is released to the Walawe Ganga from the Samanalawewa reservoir through its bottom outlets.There exists a permanent water leak from the reservoir.
which also accumulates to the irrigation release.The amount of water leaking is also accounted for while releasing the irrigation demand.

Remote sensing and GIS approach to compute actual ET of crop
It is required to identify the water use types before estimating the quantities.In this study the amount of water received from rainfall is not considered.The water use types within the process of irrigated agriculture are shown in   water has also provided security against droughts, especially in areas where irrigation is provided with surface water resources, as in dry years like year 2000 in Sri Lanka.
All the said benefits have a direct impact on the social life and health security of the rural population and the sustainability of the ecosystem.But it is rather impossible to make an assessment of all the benefits in monetary terms, other than the crop yield which has a regular market price.One can think of carrying out a survey by withholding the release of irrigation water to the Kaltota Scheme for a few years continuously.On humane grounds such an exercise would be highly undesirable.
However, in one cultivation season, farmers in the Kaltota Scheme received financial compensation equivalent to the 100% risk free value of their seasonal harvest assuming that a harvest is the only benefit gained by irrigation water.The amount of water to be released for irrigation was used to produce hydropower.But, this cause of action compelled the farmers to face grave consequences.Traditional farmers couldn't change their life style from farming activities to any other activity.Easy money began to corrupt their life style.The social and cultural system started to collapse by threatening human health, social life and peace.The new system was not repeated and therefore the ecosystem was protected without any permanent damage taking place.

Assessment of water productivity over hydropower and agriculture
For

•
In order to assess the overall productivity of water released for irrigated agriculture, the economic benefits of water, which have not been consumed by the crop, have to be estimated.
• Unlike other Hydropower Projects in Sri Lanka, the Samanalawewa Project discharges the used water further downstream, bypassing the Kaltota Irrigation Scheme.Therefore, the effects are much more significant.

Conclusion
According to this case study, the economic benefits of water can be assessed in terms of water productivity under two alternative scenarios; irrigated agriculture for paddy and hydropower.The remote sensing approach has opened an avenue to estimate one major component of the water use study i.e. the actual amount of water consumed by the food crop during any particular time period.The common practice of accounting the total amount of irrigation water only to paddy production is not acceptable.Performance indicators have to be developed to assess the economic benefits of the portion of water not consumed by the crop i.e. the indirect benefits.Most of such benefits are directly related to the social life, culture and the ecosystem.Irrigated agriculture protects the sustainability of both the social system and the physical environment and is the very core of rural life.The age-old concept "Wewai, Dagebai, Gamai, Pansali (Reservoir, Degaba, Village, Temple)" reveals that the social system of the village life is based on the irrigation system.Therefore, irrigated agriculture is not merely a food producing exercise.The indirect benefits are much more valuable than the direct pricing of the seasonal harvest.However, available water resources have to be caref'lly managed because water is not a free commodity.
through two irrigation canals at right and left banks of the weir.The location map of the system is shown in Fig 1.

Fig. 1
Fig. 1 Location map of Samanalawewa Reservoir and Kaltota Irrigation Scheme

Fig. 2 Fig. 3
Fig.2 and thereby the water use equation can be expressed as;

1 .
the economic development of the country, industrial development is also necessary and as important as the agricultural development.In this context, energy, in the form of electric power, is one of the major factors of production.Hydropower and Diesel-power plants are used to produce electric power.On the other hand, rice is the staple food of the country where water and land become major factors of production.The following two scenarios were considered for the computation.In the 1999 Yala season, if irrigation water (47.65 MCM) was not released to the for the year 1999 and average CEB rates) * Source of data -Weerasinghe et al. 2002

Table 1 . Estimation ofET_act using remote sensing approach 10 day intervals of Yala season, 1999 Actual Evapotranspiration of irrigated paddy at Kaltota Scheme during Yala 1999 10 day intervals of Yala season, 1999
1.