River Basin Management
Given the broad range of possible climate change impacts, water utilities are searching for new approaches to respond to an uncertain future. In addition to infrastructure modifications, the Intergovernmental Panel on Climate Change states that operations will also need to be revised to accommodate climatic shifts and build resiliency. Dynamic reservoir operations (DRO) have great potential to help utilities respond to the uncertainty of climate change. These operating rules can be designed to meet multiple objectives by using current conditions, historical data, and forecasts to provide a more integrated and sustainable approach to water management.
Water management planning has traditionally been done using the historical record of hydrology and evapotranspiration as the best available representation of potential future conditions. Tree ring data and recent increases in the frequency and magnitude of extreme events remind us of the uncertainty of the future and suggest that additional information should be considered in planning. But the arbitrary use of Global Circulation Model (GCM) output, for example, can mislead or paralyze.
Whether or not climate change is influenced by man-made activities, GCMs are one source of potential future scenarios. HydroLogics has been at the forefront of the appropriate use of general circulation model (GCM) output in planning. Dan Sheer, HydroLogics' founder, co-authored an award-winning research paper in the early 1990s on the use of GCM model output in California water resources planning. At that time, the spread in GCM output made it usable in snowpack-driven systems only. While that is changing, there are still pitfalls that HydroLogics helps their clients avoid:
Bias correction. The most common approach to using GCM output in water resources planning is to produce flow traces at a basin or sub-basin scale using precipitation and temperature output. Existing rainfall-runoff models frequently fail to produce reasonable flows during drought because they have not been adequately calibrated for low flows. The data can be post-processed to account for these biases.
Realistic daily flows. If water management decisions matter on a daily time step, realistic daily flow patterns are essential. These can be achieved by altering historical flow traces or using synthetic hydrology with the representative set of statistical properties.
Reliability of the precipitation output and the assumptions that go into downscaling that output. HydroLogics is exploring the use of other GCM outputs for use in scenario development. In collaboration with a climate change researcher in Canada, we sampled years of historical hydrology based on sea surface temperature anomaly, which correlates strongly with runoff in Alberta, Canada. In this region, inter-annual to decadal low flow periods can occur, and this methodology allowed evaluation of longer drought periods than those for which current reservoir operations have been designed.
Ensemble approach. One risk with cherry-picking a few GCM traces to use in planning is that the solutions derived under these scenarios may not perform well over the wide range of potential hydrology. Our OASIS software is set up to work with ensembles of input, such as large sets of inflow and evaporation traces, allowing the user to see the range of system responses in the model output. It should be noted that these ensembles are not equally-likely, thus complicating the analysis.
For example, if short-term demand restrictions during drought is one potential management strategy to address climate change, it may be very helpful to know how often these reductions are put into place over the range of GCM model runs. An example of this output from our Water Research Foundation DRO/climate change report is shown below (along with the cautionary note on interpretation).