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Most PET method-GCM combinations result in increasing annual runoff, with the notable exception of the NCAR GCM, for which all PET methods lead to a decrease in runoff (Table 3). The 0.9–6.1% decreases for NCAR represent an absolute decrease of 7.5–50.4 mm from the 832.8 baseline mean annual total. IPSL results in the largest increases in annual runoff (9.4–12.6%, or 78.2–105.2 mm). The range in annual runoff change between PET methods under each GCM is between 1.7% (HadCM3) and 3.6% (CCCMA) (Table 3). The range between GCMs for each PET method is far larger: 13.9% (Hamon) to 15.5% (Hargreaves and Penman). The two physical PET methods are generally associated with relatively high increases in annual runoff for each GCM (or relatively low decreases in the case of NCAR), but there are no obvious similarities between the two radiation- or temperature-based methods. The lowest increases in runoff generally result from the Hamon method.

Table 3

Annual percentage change in runoff for each PET method. The numbers in bold type are the mean of the range across GCMs (final column) and PET methods (final row)

CCCMAHadCM3IPSLMPINCARRange
H-S 1.5 1.4 9.4 5.3  6.1  15.5 
Ham 0.1 0.0 10.0 4.3  3.9  13.9 
J-H 1.2 0.1 10.7 4.8  3.5  14.2 
P-T 0.9  0.3 10.2 4.4  3.8  14.0 
Pen 1.7 1.0 12.5 4.7  3.0  15.5 
Gran 3.7  0.3 12.6 6.7  0.9  13.5 (14.4) 
Range 3.6 1.7 3.2 2.4 5.2 (3.2)  
CCCMAHadCM3IPSLMPINCARRange
H-S 1.5 1.4 9.4 5.3  6.1  15.5 
Ham 0.1 0.0 10.0 4.3  3.9  13.9 
J-H 1.2 0.1 10.7 4.8  3.5  14.2 
P-T 0.9  0.3 10.2 4.4  3.8  14.0 
Pen 1.7 1.0 12.5 4.7  3.0  15.5 
Gran 3.7  0.3 12.6 6.7  0.9  13.5 (14.4) 
Range 3.6 1.7 3.2 2.4 5.2 (3.2)  

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