As outlined at the beginning of this chapter, groundwater management is often treated as if it took place in areas distinct from surface irrigation schemes. However, when looking at many such schemes, ranging from India to Pakistan and Mexico, farmers actually use surface and groundwaters in conjunction. This implies that groundwater use is probably even more widespread than it seems. Many times farmers use groundwater because surface water schemes are not functioning, not delivering water on time or not timely enough to grow sensitive (and often high-value) crops. If groundwater were managed better -and surface water more effectively - significant benefits could be achieved.
One of the key disadvantages of unmanaged conjunctive use is that without control, groundwater use is usually concentrated at the tail ends or around the margins of surface water irrigation areas. This is suboptimal because excessive groundwater abstraction here often aggravates natural salinity problems, and meanwhile excessive groundwater laminae in the main riparian areas can cause rising water tables and water logging. Planned conjunctive use would optimize the situation by spreading both uses.
Obstacles to managed conjunctive use include distortions between surface and groundwater abstraction costs. Why would farmers upstream - where they receive abundant surface water through their irrigation canals - want to irrigate with groundwater, which would be far more expensive than the highly subsidized surface scheme only to benefit the tail-end farmers? Often there are also legal impediments to doing so. Therefore, the incentive structure needs to be examined in order to move towards more sustainable management of the physical system.
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