Competing Interests and Concerns in the Rio Grande Basin: Mountain
Hydrology, Desert Ecology, Climate Change, and Population Growth
Introduction
Importance of Snow for Runoff in the Rio Grande Basin
The Rio Grande is a major basin in North America and the world.
The basin encompasses two countries and eight states. Total river
length is 3058 km making it third longest in the US and 21st longest in
the world. Its drainage area is 870,236 km2 and includes the major
tributaries of the Pecos and Rio Conchos. The lower two-thirds of the
basin only receives 175-380 mm of rainfall annually making it an arid
drainage. Yet, because the Rio Grande is a mountain basin
throughout, significant snowmelt runoff is produced in the headwater
region resulting in reliable streamflow (Figure 2).
Snow remains in cold storage for most of winter and melts and runs off in a short time
(several weeks to months). The snowmelt runoff process is more efficient than rainfall runoff
because of the compressed snowmelt period. In the Rocky Mountains, some basins have 90%
of total runoff resulting from snowmelt. In some high mountain basins around the world, the
amount of runoff derived from snowmelt is as much as 99%.
Major snowmelt area
MODIS snow distributions compared favorably to both finer
resolution Landsat data and coarser resolution NOAA-AVHRR data.
In order to forecast snowmelt runoff, two additional steps are
necessary:
• Construct snow cover depletion curves
• Input to the snowmelt runoff model (SRM) and generate forecasts
Snowfall makes up to as much as 50+ % of the total precipitation, however, in the upper Rio
Grande in Colorado, an average of about 75% of streamflow results from snowmelt in the
mountains. Even when you move into New Mexico, snowmelt in the mountains provides a
large percentage of the discharge. Even near Elephant Butte Reservoir which is located in the
desert, a small part of streamflow originates as snowmelt. Although the Rio Grande runs for
the most part through desert, it is a stream that originates with snowmelt and as you move into
the lower basin, it is a stream that maintains itself with rainfall.
Pecos
Figure 6. Importance
of snow reflected in
SNOTEL data.
Rio Conchos
Figure 1. Rio Grande location.
Figure 2. Upper Rio Grande
Valley scene.
There are some common misconceptions about the basin by those
living in or nearby it. They might ask “how can snow be important, I
live in the desert?” This belief is common because the Rio Grande
runs right through the largest North American desert, the Chihuahuan
Desert. As can be seen in Figure 3, the major population areas are
located where the Rio Grande Valley and the Chihuahuan Desert
coincide. Most population is in the desert portion of the basin as seen
by city size in Figure 3.
Figure 10. Snow cover depletion curves for 2002 for the Rio Grande at
Del Norte, CO.
Figure 7. Importance of
snow gradient from
north to south.
Measuring the Snow Resource and Forecasting the Runoff
In the Rio Grande there are three major ways to measure the snowpack:
• Manual snow surveys
• Automated SNOTEL measurements relayed with meteor burst systems
• Analyze daily satellite snow extent images
Figure 11. Snowmelt Runoff Model (SRM) flow chart.
Figure 3. Juxtiposition of
the Rio Grande Basin and
the Chihuahuan Desert.
So, very few people live in the high mountains where snow is
dominant and where a significant amount of the flow of the Rio
Grande is produced (Figure 4).
Figure 4. Upper Rio Grande
sparsely-populated snow accumulation areas.
Figure 5. Cities in the
desert, snow is not a
common occurrence.
Results From the Snowmelt Runoff Model (SRM)
Figure 8. MODIS 250 m SRM Processing System
MODIS 250m resolution channels 1 (0.62-0.76µm) and 2 (0.725-1.0µm) seem to possess
characteristics of an ideal snow cover area accumulation and depletion sensor. They are:
Moderately high resolution
• Frequent coverage (daily)
• Overpass time with less clouds (morning-Terra) and a backup (afternoon-Aqua)
• Rapid processing
• Low cost – FREE !
These data can be obtained from the NASA DAAC or the Oregon State University
subscription service. Figure 9 shows examples of MODIS snow mapping in the Rio Grande at
Del Norte (3,415 km2).
• The Snowmelt-Runoff Model (SRM) is designed to simulate and
forecast daily streamflow in mountain basins where snowmelt is a major
runoff factor and to evaluate the effect of a changed climate on the
runoff regime.
• To date the model has been applied by various agencies, institutes and
universities in about 100 basins situated in 25 different countries.
• The largest basin where SRM has been applied so far is about 917,444
km², although originally it was used on small basins.
• SRM is a simple degree-day model that requires remote sensing input in
the form of basin or zonal snow cover extent. Model variables are
derived from actual observations of temperature, precipitation, and snow
covered area.
• For the Rio Grande Basin above Del Norte, Colorado, there have been
several studies performed using SRM. For a ten-year period from 19731979 and 1982-1984, simulations showed that R2 averaged 0.86 and Dv
was 1.7%.
If snow occurs in Juarez, El Paso, Las Cruces, or Albuquerque, it is
considered more of a nuisance rather than an important resource.
Further contributing to the confusion is the fact that the snow
contributing area (white area in Figure 1) makes up only 50,673 km2,
or about 15% of the entire basin. In this mountain rim region as much
as 635 mm of precipitation can occur which is two to three times the
amount in the southern more desert portions of the Rio Grande. In the
northern areas, snow can make up about 75% of total annual
precipitation.
Figure 9. Comparison of
MODIS snow mapping in
2001, 2003, and 2004.
Figure 12.
2003 SRM
forecasts for Rio Grande at
Del Norte, CO.
Albert Rango, USDA/ARS/Jornada Experimental Range, P.O. Box 30003, MSC 3JER,
New Mexico State University, Las Cruces, NM 88003 alrango@nmsu.edu
U53A-0713
SRM Used for Evaluating Effects of Climate Change
Evapotranspiration Estimation
SRM has an algorithm that will automatically produce hydrographs for future
years based on the impact of climate change scenarios.
Most evaporation loss from open water occurs in the Rio Grande’s largest
reservoir – Elephant Butte. Changes in surface area are related to the evaporation
loss and reservoir releases.
False color images from
ASTER sensor on board
NASA’s TERRA satellite
Figure 15. Satellite observed
changes of Elephant Butte.
January 14, 2002
Figure 13. Climate change simulation on the Rio Grande at Del Norte.
July 5, 2003
Quantifying regional ET is being attempted using satellite data and a regional
evapotranspiration model (REEM). The results are very promising.
Uncertainties Impact Water Management in the
Rio Grande
1. Forecasts need to be readily available to balance all the potential uses of
water.
•
•
•
•
•
•
•
•
Flood regulation
Irrigation demands
Municipal and industrial demands
Indian water rights
Compact and treaty obligations
Water quality parameters
Riverine and riparian habitat protection
Endangered and threatened species
protection
• Recreational uses
• Hydropower generation
LST
+
NDVI
+
ALBEDO
= Evapotranspiration
Figure 16. Close comparison of measured and calculated ET using Regional
Evapotranspiration Model.
Regional Evapotranspiration Model (REEM):
Measured & Calculated ET, PECAN-2002
9
Figure 14. Elephant Butte reservoir
and power station.
ET,Measured
ET, Cal. REEM
8
2. Demands dictated by Compact and Treaty have a great impact.
Most water comes from Colorado and Northern New Mexico. The Rio
Grande Compact (1938) and the Treaty with Mexico (1906) attempts to see
that downstream users in southern New Mexico, Texas, and Mexico get their
fair share of water.
ET, mm/day
7
6
5
4
3
2
1
0
Table 1. Apportionment of water under the Rio Grande compact in an
average year.
Total Flow
Colorado
New Mexico
Texas
Delivery
Required
Available for
Depletion
975,000 af
308,480 af
666,100 af
1,194,000 af
887,000 af
307,000 af
887,000 af
n/a
707,000 af
3. Consumptive use amounts in different parts of the Rio Grande.
0
50
100
150
200
250
300
350
Julian Day
Figure 17. Model vs. measured evapotranspiration results.
Conclusions
• The Rio Grande Basin is a large and complex basin in need of improved
forecasting and simulations to aid in water management.
• In addition to climate inputs, SRM requires critical satellite remote sensing
inputs to operate.
• In the snowmelt region, MODIS is the optimum remote sensing data source.
In Colorado:
Irrigation 99%
Domestic <1%
In New Mexico, it is more complex. From Cochiti Dam to south of
Albuquerque:
Irrigation 28%
Domestic 25%
Riparian ET 28%
Open Water Evaporation 16%
Others 3%
Estimates for the entire Rio Grande Basin:
Irrigation 80%
Domestic 15%
Recreation, Endangered Species, and Others 5% (and growing)
• Although still in the early stages of the Rio Grande project we have been able to
forecast early seasonal volumes and peaks, as well as daily flows for the six
month period starting on April 1.
• The forecasted flow data have compared very well in the years attempted so far.
We will do additional testing of the snowmelt forecasts and integration of the
models to result in a comprehensive water resource management system
employing meteor-burst (SNOTEL) and remote sensing (MODIS) technologies.
• Satellite data is also useful when combined in regional ET models to estimate
evapotranspiration loss.
• Public misconceptions about the importance of snow in sustaining water flow in
the Rio Grande need to be corrected.