Introduction

The Colorado River basin covers about 637,000 km² in the southwestern United States and Mexico. Much of the basin is arid, and river flows mainly derive from the high elevation snow pack over the Rocky Mountains, which contributes about 70% of the annual runoff. A secondary source of water for the basin is from summer monsoon precipitation, especially in the lower basin. The Colorado River system is one of the most heavily regulated rivers in the world, and it provides drinking water, irrigation, flood control, and hydropower to a large area of the U.S. Southwest. Our research involves estimating intra- and inter-annual variability of water storage so that the basin can be managed sustainably.

Figure to left shows location of the Colorado basin and river network. Red dots represent the outlets of sub-basins.

Objectives:

1. Estimation intra- and inter-annual variability of water storage in the basin.
2. Determine monthly atmospheric and terrestrial water balances using different forcing datasets (ERA-40, NARR and ECMWFop).
3. Compare results with hydrologic modeling (VIC) and remote sensing (GRACE).
4. Correlate river flows with different climate variability indices (ENSO, PDO, AMO)

Three atmospheric datasets are used in this study:

The ECMWF European Re-Analysis (ERA-40) data spans a period from 1958 to 2002 and has spatial resolution of 125 km and temporal resolution of 6 hours (http://dss.ucar.edu/pub/era40/).

The ECMWF operational analysis (ECMWFop) is available in near real-time with a spatial resolution of 40 km (http://www.ecmwf.int/).

The NCEP North-American Regional Re-analysis (NARR) data spans a period from 1979 to 2005 and has a spatial resolution of 32 km and temporal resolution of 3 hours(http://www.emc.ncep.noaa.gov/).

In addition we used a land surface dataset compiled by Maurer et al. (2002) which spans 50 years (1950 to 2000) of data at a temporal resolution of 3 hours and a spatial resolution of 0.125 degree. To force the land surface model (VIC), seven variables are used: precipitation rate, surface temperature, vapor pressure, air pressure, downward shortwave radiation, downward longwave radiation and wind speed. From NARR 8 variables were used to force VIC: precipitation, temperature, air pressure, specific humidity, downward shortwave radiation, downward longwave radiation, zonal wind and meridional wind.

Land and Atmosphere Water Balance:

We estimate monthly terrestrial water storage variations from water balance computations using water vapor flux convergence, atmospheric water vapor content and river runoff (Figure Left; Seneviratne et al., 2004).

Land Surface (VIC) Model:

We estimate terrestrial water storage changes and runoff using the land surface model VIC at 0.25 degrees (Variable Infiltration Capacity), forced with observed land surface variables (Maurer et al., 2002) and re-analysis data (NARR). Output variables are baseflow, surface runoff, evaporation, transpiration, sublimation, soil moisture, and snow water equivalence. Discharge at specific locations is computed using the VIC routing model. (http://www.hydro.washington.edu/Lettenmaier/Models/VIC/
VIChome.html)

Figure to the right shows the VIC Model Schematic: Layer and fluxes representation

Gravity-based Total Water Storage Changes:

For the period 2002 to 2005 we have estimated monthly water storage changes based on GRACE (Gravity Recovery and Climate Experiment) derived gravity anomalies over the Colorado Basin. We used averaging kernels from Swenson and Wahr (2006) to derive basin-scale gravity variations.

Results:

ERA-40 based BSWB estimates of total storage change at Imperial Dam have negligible closure error (see Figure 6). The corrected annual storage change at that site shows a clear decreasing trend since 1985. The same trend is present in the Lees Ferry estimates (Figure 12) and coincides with positive PDO. Interesting to note is that at Lees Ferry the beginning of the analyzed period is characterized by increasing storage, and corresponds with negative PDO.

(Click here to download results figure to right)

Acknowledgements:

Part of this research is funded through a Water Sustainability Program (WSP) Technology and Research Initiative Funds (TRIF-UA) sponsored project entitled "Estimating Arizona's Water Reserves From Space-borne Gravity Observations".

References:

Maurer, E.P., A.W. Wood, J.C. Adam, D.P. Lettenmaier, and B. Nijssen, 2002: A Long-Term Hydrologically-Based Data Set of Land Surface Fluxes and States for the Conterminous United States, J. Climate, 15(22), 3237-3251

Seneviratne, S.I., P. Viterbo, D. Lüthi, and C. Schär, 2004: Inferring changes in terrestrial water storage using ERA-40 reanalysis data: The Mississippi River basin. J. Climate, 17 (11), 2039-2057

Swenson, S., and J. Wahr, 2006: Estimating Large-scale Precipitation Minus Evapo-transpiration from GRACE Satellite Gravity Measurements, J. Hydrometeorology, 7(2), 252-270