Multi-scale Environmental Sustainability Modeling in Large Irrigation Systems

Unsustainable land and water management practices that violate the system’s carrying capacity constraint over long periods can impose significant costs in terms of lost opportunities in farm production and regional development, say by causing waterlogging and salinity. In fact, the productivity of major irrigation areas in the semi arid and arid regions of the world is facing challenges of waterlogging and secondary salinisation of landscapes (Ghassemi et al., 1995). It is estimated that more than 60 million ha or 24% of the all the irrigated land is salinised (World Bank, 1992). A small fraction of deep percolation (leaching fraction) under crops is necessary to leach out excess salts from the root zone to maintain productivity (Hoffman, 1990). Excessive irrigation of crops, seepage losses from channels and storages result in groundwater recharge to the unconfined aquifers (Rushton, 1999). If the groundwater recharge is greater than the groundwater leakage to the deeper aquifers and lateral regional groundwater flow, the watertables will start rising. When the watertable is less than 2 m from the soil surface, the root zone of the plants becomes restricted and capillary flows from the watertable start accumulating salts in the root zone and at the soil surface causing reduction in crop yields (Kijne et al., 1998). In situations where the shallow groundwater is of good quality, it is possible to tap and re-use the groundwater recharge by using horizontal tile drains or vertical tube wells (Keller et al. 1996, Khan and Rushton 1997, Rushton 1999, Seckler 1996) or by adopting appropriate cropping and tree plantations. The waterlogging and salinisation situation is complex if low quality water exists in the superficial aquifers consisting of slowly permeable materials such as medium and heavy clays. In these aquifers shallow groundwater pumping is possible only in limited locations and re-use or disposal of saline groundwater poses a major problem. These typical conditions that exist in the Murray Darling Basin irrigation areas in Australia.