Can we model the hydrological impacts of environmental change?

Received 6 July 2007 Accepted 9 July 2007 Natural and anthropogenic changes constantly impact the environment surrounding us. Available moisture and energy change due to variability and shifts in climate, and the separation of precipitation into different pathways on the land surface are altered due to wildfires, beetle infestations, urbanization, deforestation, invasive plant species, etc. Many of these changes can have a significant impact on the hydrological regime of the watershed in which they occur (e.g. DeWalle et al., 2000; Porporato et al., 2004; Milly et al., 2005; Xu et al., 2005; Poff et al., 2006; Oki and Kanae, 2006; Hayhoe et al., 2007; Weiskel et al., 2007). Such changes to water pathways, storage and subsequent release (the blue and green water idea of Falkenmark and Rockstroem, 2004) are predicted to have significant negative impacts on water security for large population groups as well as for ecosystems in many regions of the world (e.g. Conway and Toenniessen, 1999; Falkenmark, 2001; Johnson et al., 2001; Sachs, 2007). The growing imbalances among freshwater supply, its consumption, and human population will only increase the problem (Vorosmarty et al., 2000). A major task for hydrologic science lies in providing predictive models based on sound scientific theory to support water resource management decisions for different possible future environmental, population and institutional scenarios. But can we provide credible predictions of yet unobserved hydrologic responses of natural systems? Mathematical models of the terrestrial hydrological cycle are the vehicles that (potentially) enable us to make such predictions (Ewen and Parkin, 1996). These models consist of two elements important for this discussion: (1) model equations (or the model structure), which are the mathematical descriptions of the underlying physical processes; and (2) model parameters, which are the descriptors of the specific physical characteristics of a particular natural system. While most model structures are applicable to a range of systems (e.g. watersheds) with similar dominant processes, most model parameters are specific to a certain system at a certain location (and potentially even at a certain time-period). Assuming, for simplicity, that our knowledge is generally sufficient to select a reasonable model structure to represent a specific natural system (though there might be more than one reasonable choice), then the main task left to the hydrologist is to decide on appropriate model parameters to represent the system at hand. In the case of environmental change impacts, the task is to decide which parameters will change and by how much to reflect the new characteristics of the altered system. This decision requires an understanding of how watershed characteristics relate to model parameters: