Inflow to Deduru Oya New Reservoir

The inflow to the Deduru Oya reservoir is determined based on monthly inflow data obtained from the feasibility study. The observed inflows exhibit significant variability, and the "Normal year" represents the flow amount with a 50% probability.

In contrast, the inflow to the ancient irrigation tanks connected to the LB canal is only around 2% of the normal inflow to the Deduru Oya reservoir. As a result, the revitalization of ancient tanks primarily offers cultural, environmental, and societal benefits, along with empowering the local communities at the village level. The attraction towards restoring these tanks is driven mainly by non-water-related advantages.

canal map
Thiessen polygons for estimating rainfall coverage area over the LB canal region

 

In order to understand the role the ancient tanks can play, we considered the normal year flow conditions and extreme dry weather condition flow. The dry condition is assumed to constitute of 20% (80% non-exceedance) monthly flows and these normal and dry estimates are shown in below figure;

graph 1
Deduru Oya inflows to reservoir during normal and extreme dry climatic year

 

A combined system can provide resilience

  • A combined system of ancient reservoirs and the LB canal provides resilience.
  • Special attention is required for water management in the combined system, considering water allocation and available resources.
  • Under normal operations, the irrigation department is responsible for issuing water at distribution canals managed by farmer associations.
  • In ancient reservoirs, a designated villager is responsible for allocating water based on community norms.
  • Water management within each village tank's command area is the focus, without optimization among tanks in a cascade.
  • Downstream tanks rely on spillage, catchment inflow, seepage, and return flows to supplement water supply.
  • If the LB canal is directly linked to a tank, responsibility lies with farmers to ensure sufficient flow downstream.
  • Farmers need to be trained in quantification and managing their allocated water.
  • If necessary measures are not implemented, the irrigation department may have to take over water management from ancient tanks, undermining the purpose of their revival.