In watersheds of the western United States hydrologic non-stationarity is becoming increasingly apparent. In these watersheds, non-stationarity comes in two forms: allogenic, associated with changes in precipitation, and autogenic, associated with land use practices that alter precipitation-runoff relations and flood storage and conveyance. As discussed in several talks in this session, traditional structurally-based flood engineering methods may be a contributor to autogenic non-stationarity. The impacts of these methods are exacerbated by current policies, and a rigid adherence to standardized probabilistic methodologies, regardless of their effectiveness. While well-intentioned, the National Flood Insurance Program and the National Economic Development evaluations have inadvertently promoted floodplain development and non-stationarity by linking land use planning incentives and benefit/cost ratios to annual exceedance probabilities of particular flows. The most egregious of these is the over-reliance on flows with annual exceedance probabilities of .01 as a regional land use planning tool. These flows typically have no geomorphic, ecologic, or hydrologic significance and can be removed from most floodplains using the traditional structural methods that increase non-stationarity.

While exacerbating autogenic non-stationarity, current engineering and policy approaches to flood management and land-use planning are perhaps the least adaptable to hydrologic change. The unspoken assumption in flood frequency analysis is that the near-past is a predictor of the immediate future. Agencies and politicians are reluctant to acknowledge hydrologic change, and face numerous disincentives to develop painful and expensive strategies to adapt to it. Superimposed on this is the inability of the public to understand, accept, and respond to changes in flood risk. There remains the widespread public perception that flood projects are permanent, effective and certain. This is compounded by the flood memory half-life, which reflects various strategies that individuals use to suppress the memory of recent flooding disasters. Institutional inertia, public policy and personal perceptions make adaptation to hydrologic non-stationarity uniquely difficult to accomplish.