Scientific Theme: Advanced Modeling and Observing Systems

Complementary Research: Innovative Research Program
Is the Ground Drying up in South Platte?
Investigators: Balaji Rajagopalan (CIRES, University of Colorado) and Harihar Rajaram
(Department of Civil, Environmental and Architectural Engineering, University of Colorado)
Background and Objective. The South Platte Basin is home to a population of about 3 million. Despite substantial
urban and suburban growth, there are more than a million acres of irrigated land in the basin. Water management in
the South Platte Basin is complicated by the complex interactions between surface and groundwater, and the steep
gradients in hydroclimatology between the mountainous regions, the Denver metropolitan area and Eastern
Colorado. The sustained groundwater flow of about 515 acre ft/year (Figure 1) into the South Platte River helps to
maintain in-stream flows for sustaining riparian ecosystems and water quality. However, continuous declines of
groundwater levels in portions of the basin (Figure 2) have led to considerable difficulties in maintaining in-stream
flows, requiring flow augmentation from reservoirs. During the severe drought that impacted the region in 2002,
water managers faced substantial difficulties in providing water to agricultural users (from groundwater sources)
while maintaining in-stream flows. This led to the recognition of a new objective in groundwater management – i.e.
maintaining groundwater levels not just from a resource standpoint, but also in the context of sustaining the riparian
ecosystem. While this region is no stranger to dry periods, the recent dry spell coupled with increasing population,
economic growth and land-use change is placing significant stress on the groundwater resources. Despite the
important role of groundwater in the basin, there are virtually no efforts to understand long-term behavior of the
groundwater system within the context of climate variations.
Year-to-year variability of hydroclimate in the Western U.S. has been linked to large-scale climate features in the
Pacific Ocean, in particular El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). Hoerling
and Kumar [2003] use climate model simulations to present tantalizing evidence that the recent long dry spell could
be a result of cooler than normal tropical Eastern Pacific and warmer than normal tropical Western Pacific (a La
Niña pattern) and Indian Oceans. This long break from the El Niño-active period of 1980s and 1990s could be a
strong factor for the dry spell. Which then raises the questions: Why now? Is climate change responsible in any
way? Adding to these woes are recent indications [Regonda et al., 2005] that the annual cycle in precipitation is
shifting early in the Western U.S., perhaps caused by global climate change. In particular, the spring warmth is
tending to occur early with decreased winter snowpack. This implies reduced inflow into the rivers, as a majority of
them is driven by snowmelt and groundwater recharge. Given the high stakes, the key questions are:
1. What does the future hold, especially under global change and interannual variability?
2. With climate fluctuations and change and land-use changes, will S. Platte groundwater dry up?
3. Even if declines in groundwater levels are not as drastic as implied by 2., will they decline to where the
groundwater inflows that sustain in-stream flows are severely reduced?
Insights into these research questions will have significant impacts on current water resources management and on
the future socio-economic development of the Western U.S.
Research Plan. Although there are several decision support systems such as the CDSS (Colorado Decision Support
System) and SPMAP (South Platte Mapping and Analysis Program) that aid the State Engineer‘s office and Water
Resources Agencies in managing the basin, few efforts have been made to incorporate long-term climate change
scenarios into these systems. Our objective in this project is to evaluate the response of groundwater levels in the
basin to long-term climate change. We plan to do the following:
1. Compile
i. the space and time data of groundwater levels from the basin. This will be obtained from the State
water resources office.
ii. long records of precipitation (P) and temperature (T) from the basin from http://www.ncdc.noaa.gov/
iii. climate change scenarios for the basin from the recent IPCC (Inter-governmental Panel on Climate
Change) projections from several climate models runs by different modeling groups around the
world and available at (http://ipcc-ddc.cru.uea.ac.uk/asres/sres_climate/sres_home_climate.html)
2. Develop and test a stochastic P and T generator for the basin.
3. Test and refine a well-established groundwater flow model for the basin (e.g. from the State Engineer‘s
office) to ensure that the representation of surface-groundwater interaction and coupling of groundwater to
recharge are adequate for capturing future scenarios (e.g. where hydraulic disconnection between the
aquifer and stream may occur).
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