Irrigation water distribution and long-term effects on crop and environment

The response of three water delivery schedules, representing various levels of flexibility, on crop production, water saving, soil salinization, drain age volumes and watertable behavior was examined. A physical-based transien t soil water and solute transfer model, Soil-Water-Atmosphere-Plant (SWAP), was used as a tool. The evaluations were made for un-restricted and restri cted water supply situations considering three different watertable conditi ons prevailing in the fourth drainage project (FDP) of the Punjab, Pakistan . From the simulation results it is apparent that on average the effect of irrigation schedule flexibility on crop yields is not very significant. How ever, compared to a fixed schedule provided un-restricted canal water suppl ies are available, the Productivity of irrigation water supply (Y-act/I-IT) , is up to 30% higher for the on-demand schedule. The on-demand schedule ca pable of complying with the temporal variations in climate is also more eff ective in water saving, reducing drainage volumes and controlling rising wa ter-tables if fanners follow guidelines and do not over-irrigate. In the pr esent water deficient environment of the Indus basin, the benefits of the o n-demand schedule and a fixed schedule are comparable. In the absence of su fficient canal water supplies, infrastructure and a well-designed and effec tive monitoring and communication system, moving towards the on-demand syst em will be un-productive. For the longterm sustainability of the irrigation system, improvements in the performance of the present water allocations a nd on-farm water management practices seems to be more necessary. (C) 2001 Elsevier Science B.V. All rights reserved.