When the tanks (magnificent ancient reservoirs of rural Sri Lanka) are full, their serene beauty and abundant wildlife belie an inconvenient fact: they are all different. A foray through Dry Zone tanks the other day, when they are at their lowest point before the rains, highlighted this truism.
For example the Ulankulama Wewa, a medium-sized tank that we chose for student research, has a wide, gently sloping upper bed. It is covered with low C-4 grasses that cover a variously pitted soil. The soil is dry and crumbly and the mat of grass above it must be several inches. This grass, I was told by Professor Bandara and other members of our reconnaissance team, comprises the perahana.
The perahana acts as a mechanical filter, removing silts that are suspended in the water. It is supposed to act as well as a habitat for invertebrates, small fish, and birds. The perahana also provides fodder for grazing cattle. We saw it. At the same time it softens the compaction of their hooves as they cross. It preserve and enriches the tank bed soil at the same time as protecting the lower tank bed from silting.
The perahana at Ulankulama was wide, thick, and low-lying, in striking contrast to the perahana in photos sent to me by MUA Tennakoon of SAP SRI. The perahana of the tank at Alisthana is composed of tall reed beds, a distinct, narrow band of vegetation. The reed-bed perahana plays many of the same roles of habitat preservation and soil protection. But with a major difference. According to Dr. Tennakkoon, perahana have disappeared due to overgrazing and encroachment into the upper tank by land-hungry farmers. Is this the same perahana that Bandara describes? Inconveniently, it is not. Is it the "real" perahana, supplanted by the low-lying crabgrass of the perahana at Ulankulama? Maybe. Is there room for both forms of perahana in a healthy functioning tank? Perhaps.
Geographically, topographically, biologically, morphologically, and culturally each tank is different. Yet each tank performs the same roles, primarily storing water for irrigation but also maintaining the water table, ameliorating local climate, and providing a livelihood and a source of water for local human settlements.
We are tempted to construct a typology for tanks and tank ecosystems. But as an evolutionist I am more than hesitant to do so. The tank systems evolved along with the communities that created and nurtured them. In co-evolution with biotic communities the tanks took on a myriad of forms. There can be no one "right" way to do tanks.
There's another, practical rationale behind this. In terms of rehabilitating tanks, it's essential that we remain aware of the setting, the components, and the demands of each tank. Trying to impose a one-size-fits all model will perpetuate the destructive mistakes that have characterized tank rehabilitation since British times.
So, with some 30,000 tanks covering the landscape, about half of which are still in use, the question arises how we can characterize them. I presume there are trends. It's not a snowflake-type once in a million. But instead of constructing typologies and defending our models at all costs, maybe we should admit collective defeat and in so doing, join forces to find a modulated approach that addresses each tank's needs.