Till Sun and Moon Shines [Part 11]

By Eng. (Dr.) Chandana Jayawardana

Infiltration

Infiltration is the flow of water through the soil surface. The rate and volume of the infiltration depend on the conditions on the soil surface, soil properties such as texture, structure, and chemical peculiarities and soil moisture content. In current hydraulic engineering perspectives, infiltration is considered as a loss of irrigated water as it reduces the amount of directly usable water for human activities. Such perception should be a direct consequence of extracting, producing, consuming and decomposing linage of water utilization strategy as any flow to outside of this linage is considered to be a loss. But in our ancient system, some consciously introduced measures could be identified to induce infiltration, rather than reducing.

Turnour commenting on the terrains of the ancient canals notes even under the most favorable circumstances canal length is double, and in some cases is equal to four or five times the direct distance. The reason assigned is that the constructors were unaware of the use of locks for the elevation or depression of the levels and provision of aqueducts and embankments for abridging the length of canals.  Brohier, limits the validity of above statement as a conjectural opinion formed in the absence of any remains of ancient aqueducts. However he admits the excess lengths of canals and attributes the reason for such extended lengths as the limited scope of apparatus available for taking levels over the deeper depression. Brohier further exploring the terrains of some ancient canals, notes the respective paths followed “sinuous” and menders in “easy curves”.

Although explained in different terms, all above observations prove a general feature of ancient canals that they are “too” long with “unnecessary” curves.  This was a negative aspect of ancient canals according to above authorities, whose approach was in line with modern hydraulic engineering perspectives. But our ancient view of the same subject would have been the mere opposite, as according to Samanthapasadika, straightening canals was a punishable offence, and to be avoided during the construction. Therefore it could be safely assumed that the curved terrains noted by Turmenr and Brohier is a feature consciously adopted during ancient canal constructions.

Extended lengths, low gradients, more curves integral with kaliyas (internal pools) associated with ancient canals, reduce water conveyance speed and increase the water retaining period within the canal. According to Dharmasena, the structure called godawala, a shallow depression, is located at the top ends of the side slope streams blocked to form olagama (storage) tanks.

Considering above features, it appears that some structural elements have consciously been incorporated to the small tank cascade systems to retain water for a longer period before conveying to next tank in line, thus inducing infiltration to some extent. In other words infiltration of water might be considered as a more important process than the conveyance. According to this utilization philosophy, water in one point is conveyed to a remote point only after enriching the locality. Water conveyance was a process highly related with soil, rather than separation from soil. This water enrichment strategy, against the water conveyance strategies adopted in current hydraulic engineering, would have maintained the ground water table ensuring the localized water cyclicity. This is a feature totally neglected in the examples discussed in section 3.0, and may be a major contributing factor for their un-sustainability.  

The inducement of infiltration could be treated in terms of interchangeability between blue water and green water entities also. In today’s context, water management is mainly focused on blue water, the water available in liquid form in water bodies. While blue water is important as a resource in social production, household activities and industry, green water resource as moisture in the soil sustains biomass production. The ultimate destination of blue water would be the ocean which is a remote drain point from the origin, while green water is used in immediate locality. By allowing surface blue water resource base to infiltrate into the unsaturated soil, the green water resource base would be enriched. This in turn enhances the tendency towards localized water cyclicity.   

References

Brohier, R. L., The Inter-Relation of Groups of Ancient Reservoirs and Channels in Ceylon, Journal of Royal Asiatic Society Ceylon, Vol. XXXIV, No. 90,  Colombo, 1937, p. 69.

Brohier, R.L., Maps of Ceylon Down the Ages, Ceylon Geographical Society Bulletin, Vol. VI, No. 4 and Vol. VII, No. 1, Colombo, pp. 67-71.

Dharmasena, P.B., Small Tank Heritage and Current Problems; Small Tank Settlements in Sri Lanka, ed. M.M.M Aheeyar, Hector Kobbekaduwa Agrarian Research & Training Institute, 2004, pp. 31-40. 

Turnour, G., Epitome of The History of Ceylon, Cotta Church Mission Press, 1836.

Eng. Chandana Jayawardana has earned his first degree in Electrical Engineering from University of Moratuwa and then, post graduate qualifications in Industrial Engineering and Buddhist Studies. He is currently working as Design Manager, Balfour Beatty Ceylon (Pvt) Ltd, Katunayake.