Abstract
Abstract. Water supply sources for irrigation (e.g. rivers and reservoirs) are critically important for agricultural productivity. The current rapid increase in irrigation water use is considered unsustainable and threatens food production. In this study, we estimated the time-varying dependence of irrigation water requirements from water supply sources, with a particular focus on variations in irrigation area during past (1960–2001) and future (2002–2050) periods using the global water resources model, H08. The H08 model can simulate water requirements on a daily basis at a resolution of 1.0° × 1.0° latitude and longitude. The sources of irrigation water requirements in the past simulations were specified using four categories: rivers (RIV), large reservoirs (LR) with a storage capacity greater than 1.0 × 109 m3, medium-size reservoirs (MSR) with storage capacities ranging from 1.0 × 109 m3 to 3.0 × 106 m3, and non-local non-renewable blue water (NNBW). The simulated results from 1960 to 2001 showed that RIV, MSR and NNBW increased significantly from the 1960s to the early 1990s globally, but LR increased at a relatively low rate. After the early 1990s, the increase in RIV declined as it approached a critical limit, due to the continued expansion of irrigation area. MSR and NNBW increased significantly, during the same time period, following the expansion of the irrigation area and the increased storage capacity of the medium-size reservoirs. We also estimated future irrigation water requirements from the above four water supply sources and an additional water supply source (ADD) in three future simulation designs; irrigation area change, climate change, and changes in both irrigation area and climate. ADD was defined as a future increase in NNBW. After the 2020s, MSR was predicted to approach the critical limit, and ADD would account for 11–23% of the total requirements in the 2040s.
Highlights
Irrigation is crucial to satisfying increasing food demands (Bruinsma, 2003; De Fraiture et al, 2007)
In the anthropogenic water withdrawal sub-model (Fig. 1), irrigation water requirements from various water supply sources were abstracted in the following order of priority to meet the irrigation water requirement” (IR): a. the river flow (RIV), which is a naturalised flow regime; b. large reservoirs (LR), which are determined by subtracting the river flow including large reservoirs with a storage capacity greater than 1.0 × 109 m3, from those without large reservoirs; c. medium-size reservoirs (MSR) with storage capacities ranging from 1.0 × 109 m3 to 3.0 × 106 m3; and d. non-renewable non-local blue water (NNBW), which comprises the remaining demand and can be determined by assuming an unlimited water supply source
NNBW increased under the conditions of increased irrigation area because RIV and mediumsize reservoirs (MSR) could not fulfil the required supply of water needed for the increased IR
Summary
Irrigation is crucial to satisfying increasing food demands (Bruinsma, 2003; De Fraiture et al, 2007). Food production requires intensive levels of water withdrawal for irrigation, which can deplete water supply sources or even cause them to run dry. When these sources are depleted, a decline in future food production is expected (Hanjra and Qureshi, 2010). It is highly unlikely that we will be able to depend on existing irrigation water sources (e.g. rivers, reservoirs and aquifers) in the future, for two main reasons. The Yellow River experienced a persistent decline in observed annual runoff from 1960 to 2000 (Piao et al, 2010), which was largely attributed to water use for irrigation (Tang et al, 2008). Diversion of water to support cotton plantations via an inefficient irrigation system has led to the retreat of the Aral Sea (Peachey, 2004)
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