A procedure is described whereby the correlation between: (1) satellite derived snow-cover depletion and (2) residual snowpack water equivalent, can be used to update computerized residual flow forecasts for the Conejos River in southern Colorado

Leaf, C. F.

English

NASA Technical Reports Server

N 76-16 5 74
APPLICATIONS OF SATELLITE SNOW COVER IN COMPUTERIZED SHORT-
TERM STREAMFLOW FORECASTING V
C. F. Leaf, Fort Collins. Colorado
ABSTRACT
A procedure is described whereby the correlation
between: (a) satellite derived snow-cover depletion
and (b) residual snowpack water equivalent, can be
used to update computerized residual flow forecasts
for the Conejos River in southern Colorado.
INTRODUCTION
In the Rocky Mountain West seasonal or total flow fore-
casts are generally made utilizing regressions between snow
accumulation and runoff and annual surveys of peak snowpack
water equivalent. Extensions of these early-spring forecasts
to a short-term basis using statistical methods is extremely
difficult since precipitation and meteorological conditions
during the ensuing melt season vary widely from year to year.
One index of runoff, which has been found useful for im-
proving residual flow forecasts is areal snow cover. In the
United States, aerial surveillance of the snow cover was
proposed more than 30 years ago. More recently the areal extent
of snow has become an important parameter in the growing em-
phasis on applications of remote sensing in the water resource
field (Rango 1975)• Advanced space technology and several new
satellite snow mapping procedures (Barnes and Bowley 197^  > Foster
and Rango 1975) promise to make satellite snow cover observations
available on a dependable and routine basis. These satellite
snow cover estimates used in combination with conventional snow
survey data as inputs to computerized simulation models should
make the problem of short-term residual volume forecasting less
difficult in the very near future.
This paper shows how satellite imagery can provide key in-
formation for making residual volume streamflow forecasts in the
Rio Grande basin using a computerized simulation model recently
developed by the U. S. Forest Service. Mapped snow cover esti-
mates and model output are summarized for the Conejos River, a
282-square-mile tributary of the Rio Grande in southern Colorado.
I/This study has been funded by the USDA, Soil Conservation
Service, Snow Survey Unit, Denver, Colorado.
175
https://ntrs.nasa.gov/search.jsp?R=19760009486 2020-03-22T18:08:20+00:00Z
The demonstration project is one of four currently being
sponsored by NASA to determine the effectiveness of satellite-
derived snow-cover estimates in operational streamflow fore-
casting (Rango 1975).
JUSTIFICATION
THE RIO GRANDE DRAINAGE BASIN
Waters of the Rio Grande are administered under the Rio
Grande Compact between three States and by an international
agreement with Mexico. Accordingly, it is imperative that
accurate short-term forecasting procedures be developed so that
water is dispatched fairly and efficiently throughout the Basin.
In Colorado, the Rio Grande provides irrigation water to the
San Luis Valley. To provide maximum delivery of water to these
users and still meet the Compact obligations, accurate fore-
casts are required on a continuing 10-day basis starting April
15 of each year (Qazi and Danielson 1973).
Cone.ios River
The area selected for this study is drained by the Conejos
River, a key headwater tributary of the Rio Grande River. Figure
1 is a map of this basin, and Table 1 summarizes pertinent geo-
graphic characteristics of subunits selected for the simulation
analysis discussed subsequently in this report. Annual water
yields, corrected for Platoro Reservoir, which regulates flows
from approximately ^ square miles of the drainage basin, are
summarized in Table 2. These data were obtained from surface
water records published by the U.S. Geological Survey, As seen in
Table 2, runoff during the past 17 years has varied from a low of
7.9 inches in 1959 to a high in 1965 of 22.1 inches.
METHODS
THE SUBALPINE WATER BALANCE MODEL
In order to make the best use of satellite imagery and to
provide the most flexibility for updating forecasts, it was
decided to use the "Subalpine Water Balance Model" (Leaf and Brink
1973a, 1973b) to simulate flows at 10-day intervals on the
Conejos River. This model simulates winter snow accumulation,
the shortwave and longwave radiation balance, snowpack condition,
snowmelt and subsequent runoff on as many as 25 subunits. Each
subunit is described by relatively uniform slope, aspect, and
forest cover. Results from each subunit are compiled into a
"composite overview" of an entire drainage basin. Figure 2 is
a generalized flow chart of the system.
176
\
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--, A
•^ «; v ' \\
C O L O
\ 10
\
\
'V
scale-miles
Figure I - Conejos River Base Map Showing Subunits (lmi.=1.6l km),
177
Table 1- Geographic Description of Conejos River Drainage Basin
Subunit
1
2
3
4
5
6
7
8
9
10
Area I/
(ni.2) (fan2)
25
13
37
30
24
40
16
9
52
32
>4
• 5
• 5
.2
.0
.0
.1
• 3
.7
.7
65
35
97
78
62
103
41
24
136
84
.8
.0
.1
.2
.2
.6
.7
.1
• 5
.7
Average Elev.
(ft.) (m)
11
11
10
10
11
11
,000
,500
,500
,500
,000
,000
10,500
10
9
9
,300
,400
,500
3,
3,
3,
3,
3,
3,
3,
3,
2,
2,
355
507
202
202
355
355
202
141
867
897
Average
Aspect
SE
NNVf
E
SW
ESE
NNE
ENE
SW
NE
SW
Avg. Slope
(percent)
34
34
28
33
25
23
27
35
15
15
I/ Total: Forest and Open
178
MATBAL
1
SNOWPAC
SUBROUTINE
SNONVAP
RECH:
REQ. SATISFIE
BY MELT.
RAIN
Figure 2 -Flow chart of "Subalpine Water Balance Model"(Leaf and
179
Calibration of Model to ConeIPS River
Daily temperature extremes in each of the response units
were estimated by extrapolating observed temperatures at the
base station Wolf Creek Pass IE. Because reliable long-term
radiation data are not available in the Conejos drainage, short-
wave radiation input to the model was generated from potential
solar beam radiation adjusted for temperature and the slope/
aspect characteristics of each subunit. In order to compute an
index of the radiation received, potential radiation was re-
duced by means of a thermal factor. This factor was determined
by degree-day relationships which vary according to season of
the year.
Peak snowpack accumulation on the Conejos was estimated by
extrapolating snow-course data published by the Soil Conser-
vation Service. To ensure the proper snow accumulation in each
response unit, base station precipitation (at Wolf Creek Pass IE)
was adjusted until the specified peak-water equivalent on each
subunit was reached.
The Conejos River Basin was divided into 10 subunits as seen
in Figure 1. Further division of forest and open areas resulted
in 20 response units used for the simulation analysis.
RESULTS
RESIDUAL VOLUME FORECASTS (1958-1972)
Figures 3 and 4 show the comparison of simulated vs. actual
residual runoff as of the first day of April, May, June, and
July for 8 years. The close agreement between observed and
simulated flows indicates that the model is a useful tool for
continuous forecasting of short-term flows in the Conejos River
for the full range of runoff observed during the past 15 years.
TEN-DAY RESIDUAL VOLUME FORECASTS (1973 and 197 )^
Figure 5 summarizes observed and simulated residual flows
on a 10-day basis for 1973 and 1974. Also shown in Figure 4 are
satellite-derived snow cover estimates. Again, agreement is good
even when residual volume forecasts are made at 10-day intervals.
The superimposed snow cover estimates reveal a high correlation
with simulated runoff and suggest that satellite snow cover data
can provide key information for updating model input in a real
forecasting situation. Just how this can be accomplished is
discussed next.
INCORPORATION OF SATELLITE SNOW COVER OBSERVATIONS FOR FORECASTING
Table 3 compares mapped snow cover estimates on the Conejos
River with residual water equivalent and streamflow for 1973 and
197 .^ In keeping with previous research, Table 3 and Figure 4
suggest that depletion vs. runoff relationships vary depending
180
Table 2 - Annual Runoff Corrected For Platoro Reservoir, Conejos
River Basin.
Year Runoff Platoro Res. Adjusted Runoff
Change in Storage (in.) (mm)
1958 -,/
59*"^
60
61*
62
63*
64*
65
66
67
68*
69
70
71
72*
73*
74*
— incr
16.6
9.9
-
13-3
16.8
8.7
10.3
20.2
15.8
15.0
15.4
17.6
15.8
11.7
8.0
19.7
10.7
les
-1.02
-1.98
-
-0.27
+0.60
-0.70
+0.31
+1.89
-1.24
-1.08
+0-55
-0.46
+0.25
-0.43
+0.01
+2.14
-1.20
15.6
7-9
-
13.0
17.4
8.0
10.6
22.1
14.6
13-9
15-9
17.1
16.0
11-3
8.0
21.8
9-5
396.2
200.7
-
330.2
442.0
203.2
269.2
561.3
370.8
353-1
403.9
434.3
406.4
287.0
203.2
553-7
241.3
I/ * Used in simulation analysis
Table 3 " Mapped Snow Cover vs. Residual Water Equivalent and
Streamflow. (Conejos River, Rio Grande Drainage Basin)
Year
1973
June 22
1974
May 12
May 30
Snow Cover
(percent)
31
42
18
Sim. Residual
Water Equiv.
(in.) (mm)
3 76.2
4 101.6
1 25-4
Observed Residual
Streamflow From Snqwmelt
(acre-ft) (m3)
1.379xl05 1.700x10®
n
7.015x107; 8.649x10'
5.333x10 6.576x10'
!/ Personal communication with Jack Washichek, Soil
Conservation Service, Denver, Colorado
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184
on the magnitude of the snowpack and precipitation through the
snowmelt season (Leaf and Haeffner 19?l). For example, in Table
3, 31 percent snow cover on June 22 corresponds to a residual
flow of 137,900 acre-feet in 1973—a big snow year, whereas on
May 12 in 197^—a l°w snow year—i*O percent snow cover represents
a residual flow of only 70,150 acre-feet. However, if one looks
at residual water equivalent, one finds that a direct relationship
exists between this variable and the areal extent of snow cover.
The stable correlation between snow-cover depletion and
residual water equivalent is independent of precipitation input
and can be utilized in combination with direct snowpack measure-
ments through the melt season to revise model estimates of stream-
flow. In most areas, satellite imagery would provide the prim-
ary basis for updating streamflow forecasts so long as the drain-
age basin is partially snow covered. Streamflow forecasts prior
to the onset of snowmelt and during those times when the water-
shed is completely covered with snow would rely on direct snow-
pack measurements.
CONCLUSIONS
Satellite snow cover data used in combination with the
recently developed Subalpine Water Balance Model can provide a
sound physical basis for making continuous short-term streamflow
forecasts in the Upper Rio Grande Basin. Reconstitution studies
of a 15-year streamflow record indicate that the model is ade-
quate for making residual- volume forecasts at time intervals as
short as 10 days. A comparison of model results with mapped snow
cover estimates indicates that an apparently stable relationship
between satellite derived snow cover and residual water equiva-
lent can be the primary basis for modifying short-term forecasts
and ultimately delivery schedules to ensure the maximum amount
of water for Colorado users while at the same time meeting
Compact commitments downstream.
185
REFERENCES
Barnes, J.C., and Bowley, C.J.
1974. Handbook of techniques for satellite snow mapping. ERT
Document No.O^O?-A, 99 p. Prepared for NASA/Goddard Space
Flight Center, Greenbelt, Maryland.
Foster, J.L., and A. Rango.
1975-A method for improving the location of the snowline in
forested areas using satellite imagery. NASA Rep. X-910-
75-41, 8 p. Goddard Space Flight Center, Greenbelt, Mary-
land.
Leaf, Charles F., and Glen E. Brink.
1973& Computer simulation of snowmelt within a Colorado sub-
alpine watershed. USDA For. Serv. Res. Pap. RM-99, 22 p.
Rocky Mt. For. and Range Exp. Stn., Fort Collins, Colo.
Leaf, Charles F., and Glen E. Brink.
1973b Hydrologic simulation model of Colorado subalpine forest.
USDA For. Serv. Res. Pap. RM-107, 23 p. Rocky Mt. For. and
Range Exp. Stn. Fort Collins, Colo.
Leaf, Charles F. and Arden D. Haeffner
_1971• A model for updating streamflow forecasts based on areal
snow cover and a precipitation index. West. Snow Conf.
(Billings, Mont., Apr. 1967) Proc. 39:9~l6.
Qazi, Razig A. and Jeris A Danielson.
1973- A predictive model for upper Rio-Grande index flows. Water
Resources Bulletin, Vol. 9, No. 2, p. 284-290. Amer. Water
Resources Assoc. Urbana, 111.
Rango, Albert.
1975. Applications of remote sensing to watershed management.
NASA Rep. X-913-75-86, (for presentation at the Symp. on
Watershed Management, ASCE, Aug. 11-13, 1975, Logan Utah),
Goddard Space Flight Center, Greenbelt, Maryland.
186


Applications of satellite snow cover in computerized short-term streamflow forecasting

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