Scenarios Consistently Overestimate Consumptive Water Use in the Upper Colorado River Basin
Jian Wang and David E. Rosenberg
Future consumptive water use is difficult to predict because underlying factors that influence consumptive use—future population growth, irrigation technology and practices, climate, agricultural commodity prices and other factors—are individually difficult to forecast. Future consumptive water use cannot be forecast as a point value within a range, nor with probabilities. Instead, future consumptive water use can only be authentically described using a set of scenarios that represent possible future conditions.
Since the 1980s, Bureau of Reclamation and the Upper Colorado River Commission have developed many scenarios of possible future consumptive use for the Upper Colorado River basin (Figure 1, dashed colored lines). Each scenario describes one possible trajectory of future consumptive water use out to the year 2040, 2050, and 2060. When averaged over the long term, each scenario of future consumptive use over-estimated the observed consumptive use reported by Bureau of Reclamation in its regular Colorado River Consumptive Use and Loss Reports (Figure 1, solid black line). Many other water systems also consistently overestimate their demands (Heberger, 2016; Kindler and Russell, 1984). Additionally, the trace of observed use has many more year-to-year variations due to changing precipitation and out-of-basin exports than the forecast scenarios. The disparity between the scenarios of possible future consumptive use and observed use show the inherent difficulties and uncertainties to estimate future demands and develop scenarios that span all possible future conditions. Thus, it is better to develop more scenarios to cover a wider range of future conditions. However, including more scenarios also increases the computational burden of analysis.
The question then is: How should Colorado River stakeholders manage the future Colorado River in the face of these water demand and other uncertainties? For insight, read our white paper “Managing the Colorado River for an Uncertain Future.”
Heberger, M. (2016). "21st Century Water Demand Forecasting." Pacific Institute. https://pacinst.org/rethinking-future-water-demand-blog/.
Kindler, J. and Russell, C. (1984). Modeling Water Demands. London: Academic Press. ISBN 0-12-407380-8 http://pure.iiasa.ac.at/id/eprint/2392/