25 September, 2021


Partitioning Water Between Agriculture & Hydro-Power To Maximize Sri Lanka’s Clean Energy Output

By Chandre Dharmawardana

Dr. Chandre Dharmawardana

The largest drain on Sri Lanka’s foreign exchange earnings is in purchasing  fossil fuels for power and transport,  and in feeding  the 22 million people who depend on a mere million hectares of arable land, as compared to 44 million hectares  in Canada with a population of just 37 million. Much of agricultural land  in the “dry zone”  depends on irrigation water. This  is provided by a network of reservoirs that tap the heavy rains of the hill country to deliver the water to the agricultural regions via rivers and irrigation canals. 

In  previous articles (e.g., Island 7-Aug-2021, Island and Colombo Telegraph ) I restated what I had stated in many articles extending back into two decades, namely, that just by CUTTING DOWN EVAPORATION  from our hydro-electric tanks, we can boost our clean energy production to such an extent that the targets of 70% production of clean energy become possible. Here I point out the possibility of  boosting power production EVEN FURTHER by optimal partitioning of water between agriculture and hydro-power production, by shielding the water in the IRRIGATION tanks from EVAPORATION, and in NOT USING WATER to control weeds in paddy fields.

Figure 1.  Schematic of the main Mahaweli hydro-power and agricultural irrigation network.[Credits. Thushara de Silva et al., Vandebilt University.]

In ancient times, small manually built tanks (“weva”) supplied water to small hamlets (“gama”) where people lived at a precarious subsistence level (see Prof. Siriweera’ study of ancient food security).  The  temple or Kovil was the only spiritual, cultural and educational resource. That ancient hydraulic system held pride of place in the ancient world. But it did not tap the heavy rainfall of the hill country.  The total population  sustained by the whole Land even at its best times was probably less than today’s population in  Colombo.

In contrast, today’s hydraulic system not only provides irrigation water, but also electric power that drives modern technologies, hospitals, electrified transport, and every aspect of daily life at levels of culture and leisure that were not available even to elites.

The attached figure (Figure 1, credits: Thushara de Silva et al., 2019, Vandebilt  University, USA) shows how hydro-power stations (red squares) of the Mahaweli system are also  associated with the various irrigation schemes (green hexagons)  denoted by  A, B, C, … H, MH etc.  The head water of the Mahaweli is diverted at Pollgolla towards “the north” via Bowatenna mainly for agriculture, while another branch supplies the Minipe agricultural regions via Randenigala and Rantambe power stations.

However, if more water is sent to irrigated agriculture, there is less available for power production. Agriculture needs water in large amounts at specific times, while power production uses water far more steadily. The water that is used for hydro can be used in agriculture when stored downstream and released at the required time.

Figure 2.  Agricultural water requirements in various Mahaweli Systems in millions of metric tons per hectare.  [Credits: Thushara de Silva et al., Vanderbilt University, USA]

Figure 2 shows the monthly water use  in the two planting seasons. The upstream reservoirs have to provide enough water to the irrigation reservoirs at a time to be  ready to supply the irrigation water as soon as needed.  I find that it sufficient to model the water-requirement by just two Gaussians, when analytic calculations can be done. We find that water requirement can be cut down by beyond a FATOR OF TWO using two simple provisions:

(A) the irrigation tanks are covered by floats to prevent evaporation. This also prevents algae and water weeds, and improves the aquatic environment. The floats may also carry solar cells and connected to the central grid using smart switching technologies that are now standard, even in small islands like Hawaii.

(B) WATER IS NOT USED for weed control in paddy planting. 

Instead, weeds should be controlled using safe herbicides like glyphosate.  Given socio-political resistance to “chemicals” entrenched in occult beliefs in “revelations by God Natha”, or due to baseless propaganda against “agro-chemicals”, the so-called “System Rice Intensification” (SRI) methods  may be used.  SRI has been tested out in India and Madagascar.  It  is said to increase yields by over 30%, i,e.,  4-5  tonnes per hectare instead of at most three tonnes per hectare obtained with reduced use of “chemical” fertilizers, while also using 40% LESS water than conventional methods. If chemical fertilizers and humus are used together, the yields become 8-10 tonnes per hectare, with even less soil erosion. However, a pilot project  to test a new idea is needed before extensive adoption.

The steps (A) and (B) or similar ideas are not included in the usual studies on optimizing water management in multipurpose reservoir  systems even though quite complex models like RIBASIM, WEAP (e.g., see,  Louckes and van Beek 2017) etc., have been used  by engineering researchers. Such models, though complex, are only as good as the ideas incorporated into them. Ideas can be tested more transparently using simpler analytic models of the sort used in theoretical physics. There are also losses in seepage that are not included in standard engineering models, or in my calculations, as no simple engineering approach is currently available to reduce seepage from the bottom of the reservoirs.

All this suggests  that if reservoir capacity permits, the water available for hydroelectricity can be easily doubled while providing more than adequate irrigation water to the agricultural schemes of  Lanka and providing ALL THE NEED POWER using only hydro power, for at least the next decade. If so  much water can be saved, it makes good sense to expand reservoir capacity or bring into service the abandoned small tanks that are found in many parts of the dry zone – but they too will need evaporation shields. Furthermore, the possibility of raising the Kothmale dam to increase capacity by 20% has been proposed by Engineer Kenderagama (5-Nov-2020, Island newspaper). If this is in fact  feasible, the hydro-power output will also increase by almost 20%. Thus we see that  lack of a “power research Institute” similar to, say, the TRI for the eta sector, hampers in the evaluation and analysis of new ideas in such a vital national endevour like power production, as the CEB alone is not equipped to deal with such matters.

The projections of power needed by Sri Lanka given by the CEB are INCORRECT and are a gross under-estimate as the CEB does not seem to have considered that most motor vehicles will use electric power within a decade.

In any case, the proposed steps are a means of establishing a solid shield against global warming and persistent drought that will be part of the future weather patterns. Then the mechanisms put in to prevent evaporation or deal with sudden excess water will become  “God Sent” provisions.

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Latest comments

  • 0

    Oh wow….AFTER the waters have flowed through the hydropower plants, then channel the waters through underground Water-Pipelines to the Agriculture fields. Then water will take its natural course and evaporate from fields to come down as rain in the hill country, and seep through the soil into the ground water that eventually goes back into the rivers and lakes. Zero water loss. Then we can get rid of glyphosate forever. But it’s a good idea to cover irrigation tanks with floats.

    • 1

      I don’t believe that agriculture in Sri Lanka needs so much water. Tamil Nadu has less rainfall and no ancient irrigation system but still produces enough to even export. The price of diesel in India is 225 LKR.
      We must drop our foolish pride, look round, and learn from others.

      • 1

        The only justification for flooding the paddy field (six foot deep overall per crop) was weed control. It is bound to dilute and drain out applied fertilizer in the process.
        We were willing to accept hybrid seed, synthetic fertilizer, weedicides and pesticides, but nor the healthy options that went with them.

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