A Composite Irrigation Water Quality Index and Leaching Requirement for Groundwater of Durg Block, Chhattisgarh, India

Abstract

Irrigation water quality is conventionally reported as a set of separate indices: electrical conductivity, sodium adsorption ratio, soluble sodium percentage, residual sodium carbonate, Kelly's index, permeability index and magnesium adsorption ratio, each classified against its own limit. A water that is acceptable on every index taken singly may nevertheless be close to several limits at once, and that condition is invisible to the index-by-index reading. This study combines the seven indices into a single composite irrigation water quality index and converts the measured salinity into the extra water needed to keep the root zone below a target salinity, for ten observation wells in Durg block, Chhattisgarh. The composite index ranged from 11.8 to 58.3, placing seven wells in the no-restriction class, two in low restriction and one, Maroda, in moderate restriction. Maroda is not the most saline well in the block; its electrical conductivity of 0.79 dS m⁻¹ is below the block mean, and it passes every index individually, yet it stands simultaneously at 76, 83, 91 and 88 % of the permissible limits for soluble sodium percentage, Kelly's index, permeability index and magnesium adsorption ratio, and is the only well with a positive residual sodium carbonate. It is the sodicity profile rather than the salinity that sets it apart, and only the composite treatment brings that out. The leaching requirement, computed from the salinity of each well, ranged from 3.8 to 11.9 % of the applied water for a target soil salinity of 3 dS m⁻¹, the threshold for paddy, and from 1.9 to 5.6 % for 6 dS m⁻¹, the threshold for wheat, so a leaching fraction of about one-eighth of the applied depth is sufficient across the block for its principal crops. The two most saline wells, Durg and Anda, require the largest leaching fractions and are also those where a shift towards more salt-tolerant crops would be most economical.