REPLY TO DISCUSSION by David L. Verbyla1

Abstract

We appreciate Verbyla's comments; he points out some of the complexities involved in predicting the effects of climate change on water resources. As he indicates, and as we intended, this study was a firstcut approximation of how climate change may affect seasonal soil moisture and runoff in the Delaware River basin. Studies of the effects of atmospheric carbon-dioxide enrichment on plants in controlled environments have suggested that stomatal resistance to transpiration increases as carbon dioxide concentrations increase (Rosenberg et al., 1990). As Verbyla indicates, some studies of the effects of climate change on water resources have included the direct effects of increasing atmospheric carbon dioxide on stomatal resistance to transpiration (Aston, 1984; Idso and Brazel, 1984; Wolock and Hornberger, in press). These studies have shown that increased stomatal resistance to transpiration can counteract the effects of increased temperature on soil moisture and runoff. A large amount of uncertainty exists, however, concerning the net effect of increasing atmospheric carbon dioxide on plants. Little is known about the responses of plants to increasing concentrations of atmospheric carbon dioxide, and it is uncertain whether the responses observed in controlled greenhouse environments will be observed in the real world (Rosenberg et al., 1990). Despite uncertainty regarding the effects of increasing carbon dioxide on plants, sensitivity analyses can be performed by using a range of possible effects, as Verbyla suggests. We have performed a number of analyses to assess the effects of prescribed climate changes on the water resources of the Delaware River basin that include a range of prescribed increases in stomatal resistance to transpiration. Results indicate that increases in stomatal resistance to transpiration do influence runoff, and that increases in stomatal resistance can counteract the effects of increases in temperature on runoff. Our preliminary results (unpublished) suggest that a 3to 5-percent increase in stomatal resistance to transpiration can counteract the effects of an increase in mean annual temperature of 1°C. These results appear to support Verbyla's comments and illustrate the importance of including the effects of increases in stomatal resistance to transpiration in sensitivity analyses of the hydrologic effects of climate change. Increased stomatal resistance to transpiration, however, is only one of the many possible responses of plants to increased atmospheric carbon dioxide; Verbyla lists many of the possible responses in his discussion. For example, Rosenberg et al. (1990), suggests that the effects of increased stomatal resistance to transpiration can be counteracted by increased plant growth. Plants exhibit many complex responses to changes in atmospheric carbon dioxide that are not understood. We are not confident that, by adding a few of these responses to a study, we will increase the accuracy of the results. In our study (McCabe and Ayers, 1989) we reasoned that we should start with simple scenarios of climate change (changes in temperature and precipitation) and try to understand the effects of these scenarios on hydrologic variables before adding more complexity to either the hydrologic model or to the climate-change scenarios. Although we agree with Verbyla that the direct effects of increasing concentrations of atmospheric