There are indications that increased vapor flows through irrigation can alter local and regional climates
cover and its management (McCulloch and Robinson 1993; Bosch and Hewlett 1982; Bruijnzeel 1990).
General work on the influence of vegetation, climate, and land cover on the water balance of a system has shown that there are vegetation-specific changes (L’vovich 1979; Calder 2005). Management of plant production that redirects blue water to green water can reduce the amount of water to downstream systems (Falkenmark 1999). For example, replacing crop or grasslands with forest plantations can decrease runoff and streamflow (Jewitt 2002). e South African Water Act classifies forest plantations as a “streamflow reduction activity,” and forestry companies have to pay for their water use since less of the precipitation reaches the river.
Moisture recycling. Clearing land for agriculture and increasing use of irrigation have modi- fied green water flows globall , reducing them by 3,000 cubic kilometers through forest clear- ing and increasing them by 1,000–2,600 cubic kilometers in irrigated areas (Döll and Siebert 2002; Gordon and others 2005). e ability of changes in land cover to influence climate through changes in green water flow has been increasingly recognized. It has been suggested that large-scale deforestation can reduce moisture recycling, affect precipitation (Savenije 1995, 1996; Trenberth 1999), and alter regional climate, with indications of global impacts (Kabat and others 2004; Nemani and others 1996; Marland and others 2003; Savenije 1995).
Pielke and others (1998) conclude that the evidence is convincing that land cover changes can significantly influence weather and climate and are as important as other hu- man-induced changes for the Earth’s climate. However, the models employed do not deal explicitly with green water flows, but rather with the compounded effects of changes in al- bedo, surface wind, leaf area index, and other indicators. Nevertheless, regional studies in West Africa (Savenije 1996; Zheng and Eltathir 1998), the United States (Baron and others 1998; Pielke and others 1999), and East Asia (Fu 2003) have illustrated that changes in land cover affect green water flows, with impacts on local and regional climates. Likewise, biome-specific models of land cover conversions from rainforest to grasslands have shown a decrease in vapor flows and precipitation as well as effects on circulation patterns (Salati and Nobre 1991) and savannahs (Hoffman and Jackson 2000). ere are also indications that increased vapor flows through irrigation can alter local and regional climates (Pielke and others 1997; Chase and others 1999). e conversion of steppe to irrigated croplands in Colorado resulted in a 120% increase in vapor flows (Baron and others 1998), contributing to higher precipitation, lower temperatures, and an increase in thunderstorm activity (Pielke and others 1997).
Whether these changes can trigger rapid regime shifts (box 6.9), which in many cases may be irreversible, and changes to which farmers need to adapt is still speculative. In the Am- azon the clearing of land has reduced moisture recycling, resulting in prolonged dry seasons and increased burning, and may have triggered an irreversible regime shift from rainforest veg- etation to savannah (Oyama and Nobre 2003). ere is also increasing concern about changes in the African and Asian monsoons, including weakening of the East Asian summer monsoon low-pressure system and an increase in irregular northerly flows (Fu 2003). Likewise, modeled vegetation changes for agricultural expansion in West Africa have shown potentially dramatic impacts on rainfall in the African monsoon circulation (Zheng and Eltathir 1998).