Chapter 21: Conjunctive management of water resources in agriculture
Worldwide irrigation consumes the bulk of renewable freshwater resources, accounting for over 70 percent of total water withdrawal (Siebert et al., 2010). While irrigated agriculture is practiced on 16 percent of total cultivable land (257 million hectares in 2007), it produces 44 percent of agricultural output (Alexandratos and Bruinsma, 2012). About 41 percent of the area is irrigated by water derived conjunctively from ground and surface sources (Siebert et al., 2010). While irrigated area is expected to continue to expand by an additional 200 million hectares by 2050 (Alexandratos and Bruinsma, 2012), natural water resources available in a sustainable fashion will at best remain constant (on average) and in many regions are likely to decline due to climate change.2 And the supply of freshwater available for irrigation will further decline as water is reallocated away from irrigation to satisfy the needs of ever-growing urban sectors. Moreover, average annual water recharge is only a partial index of natural water supply – variability of water supply is often not less important. It has been widely accepted that the pronounced impact of climate change so far shows up in the form of larger weather fluctuations, such as erratic precipitation both within and between rainy seasons, and increased frequency of extreme events, such as prolonged droughts (Stern, 2007 and IPCC, 2014). This means that surface water, which is more vulnerable to ongoing weather conditions, is likely to become more erratic, both inter-and intra-temporally.
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