as seen in some Sub‐Saharan Africa case studies (Van Koppen et al., 2005). Subsidizing the cost of water for irrigation has become common practice in developed regions like Europe and the US as a means to encourage agricultural development, though many of these policies are currently being reformed due to their environmental impacts (such as over‐extraction and water pollution, see Wichelns, 2010).

# _ZeroInvest_

_ZeroInvest_ is a scenario where there is no new investment in irrigation and therefore no expansion of irrigated areas beyond 2010 levels in developing regions. There are no investments to improve the water application efficiency of existing irrigation systems. Water used for irrigation is limited to the quantity that is physically available after domestic and industrial demand has been satisfied.

_ZeroInvest_ is used as a reference scenario but not a policy scenario.

# _Invest_

_Invest_ is a _moderate public support_ scenario where the irrigation equipment investment costs needed to expand irrigated areas or upgrade existing areas to more efficient irrigation systems in developing regions are supported by public investments, leading to 32% more irrigated area by 2050 in developing regions. Farmers are responsible for the operations and maintenance of the systems, which have been included in the model as production costs. The price of water that farmers face for irrigation use reflects the on‐farm capital costs to utilize water as well as the relative scarcity of water due to the increasing demand for water for other sectors. _Invest_ reflects a mixed‐cost sharing policy that is similar to the one used in the Fadama Project in Nigeria in which a portion of the parts and materials were subsidized (Foster and Briceno‐Garmendia, 2009).

In _Invest_ the underlying improvement in the efficiency of water application, “more crop per drop” improves by 1.5% each decade. Efficiency can be achieved through upgrades or conversion of existing flood/gravity irrigation systems to more drip and sprinkler systems or by improving the overall efficiency of flooding/gravity irrigation systems. Water used by irrigation is limited to the remaining portion of physically available water at the pixel level after domestic and industrial demand has been satisfied.

# _MaxInvest_

_MaxInvest_ is a _high public support_ scenario where ambitious investment in expanding and upgrading irrigated areas in developing regions are supported by public investment, leading to an increase of irrigated area by almost 70% by 2050 in developing regions. As with _Invest_ , farmers are responsible for the operations and maintenance of the systems, which have been included in the model as production costs. Additionally, the price of water that farmers face is subjected to scarcity impacts although some of the on‐farm capital costs are subsidized making the per unit price of water less expensive. The main difference between the _Invest_ and _MaxInvest_ scenarios is that _MaxInvest_ reflects a policy where priority is given to increasing the accessibility of water by fully subsidizing it.

In _MaxInvest_ , the underlying improvement in the efficiency of water application, “more crop per drop” improves by 3.0% each decade. Efficiency can be achieved through upgrades or conversion of existing flood/gravity irrigation systems to more drip and sprinkler systems or by improving the overall efficiency of flooding/gravity irrigation systems. Water used for irrigation is limited to the quantity that is physically available after domestic and industrial demands have been satisfied.

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