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HK Project in a Global Context

It is important to place our selection of the Whitewater Basin in a broad hydro-climatic context that includes global river basins. To this end, we define a dimensionless aridity index, B=En/?P, where P denotes annual precipitation, En net radiation, and ? the latent heat of vaporization. Here En/? is the water equivalent of annual net surface radiation. The parameter B enables us to introduce a coupling between water, energy, vegetation and river basins in terms of (1) the annual water balance, R/P=1-(ET/P), where R is the mean annual runoff; and (2) the “Budyko relation” ET/P=f(B) (Budyko 1974), where ET is evapotranspiration and f(·) is a semi-empirical function. Moreover, B=1 defines the threshold between water-limited and energy-limited environments.

Budyko and his co-workers introduced B to classify global biomes across a hydro-climatic gradient spanning water-limited and energy-limited environments on an annual time scale. Budyko’s pioneering work spanning nearly four decades continues to guide modern investigations of land-atmosphere-vegetation interactions on annual and interannual time scales (Choudhury 1999; Koster and Suarez 1999). Results by Milne et al. (2002) suggest that the Budyko relationship can be obtained from invariance in coupled biophysical processes under a change of spatial scale. These results suggest the new hypothesis that B is a natural parameter for scale-invariant extrapolation of floods in different hydro-climates across Strahler stream orders. The parameter B is directly connected to predicting floods in gauged and ungauged basins (PUB; Sivapalan et al. 2003), due to changing hydro-climatic conditions at annual, interannual and longer time scales. Poveda et al. (2006) have obtained the first set of results on this theme. A long-term goal of the HK project is to develop a physical basis of SSSON in floods at annual, interannual and longer time scales.

In the Whitewater Basin, near the 100th meridian, the value of B is near 1. Therefore, due to the effects of interannual variability in ET/P, the Whitewater Basin experiences both water- and energy-limited conditions. Thus, it provides ideal experimental conditions for understanding how this variability impacts SSSON in peak flows and RET; another long-term goal of the HK project. The HKP3 proposal of Milne (Table 2) is developing the necessary observational base using a new biological allometric theory for tree-ring analysis that would enable us to make progress on this important long-term goal.