References and Publications

1
Precipitatioin Characteristics in the Core Region
of the North American Monsoon and Their
Relationship to Large Streamflow Events
David J. Gochis1, Juan-Carlos Leal2, W. James
Shuttleworth3, Christopher J. Watts2, Jaime
Garatuza Payan4
1NCAR/ASP/RAP (E-mail gochis_at_rap.ucar.edu)
2IMADES, Hermosillo, Son. MX 3HWR, Univ.
Arizona 4ITSON, Obregon, Son. MX
ABSTRACT
The Streamflow Regime in NW Mexico
Fig. 3
Principal Components Analysis (PCA)
The mountain and coastal plain regions of the
northwestern Mexico lie under the influence of
the North American Monsoon System (NAMS). Recent
analyses have shown this region to exhibit among
the highest interannual variance in precipitation
and in streamflow in the world. However, no
clear relationship between large-scale
teleconnective forcing (such as sea-surface
temperatures) and regional hydrometeorology has
yet been established. Precipitation
characteristics such as intensity-frequency-durati
on relationships from a new event-based raingage
network are presented. Correlations between the
various precipitation characteristics and
streamgage measurements in 15 unregulated basins
in western Mexico are explored. The relationship
between topography and precipitation structure
are also be examined. In future studies,
particular attention will be focused on the
relationship between catchment topographic
features and soil structure (e.g. soil type and
depth) and estimates of runoff fractions.
Fig. 2a
The streamflow regime in NW Mexico is dominated
by a strong summertime signal concomitant with
summer monsoon rains (Fig. 2a). All basins show
a distinct summer maximum in monthly flow volume.
Interannual monthly streamflow variability
(quantified as the coefficient of variation of
monthly flow volume, Fig. 2b) is lowest during
the summer months indicating that the higher
flows are a persistent feature. There is marked
flow variability in the low flow season in the
spring and also in the fall. The increased
variability in the late fall and early winter are
likely due to land-falling tropical storms. In
most basins, the months with maximum flow volume
are summer months. This is not universally true
though as some basins show fall and wintertime
maximums.
Figure 6
A principal components analysis was performed on
both the JAS streamflow volumes and basin
averaged precipitation. Loading factors were
then spatially interpolated to reveal regions of
coherent variability. Both streamflow and
precipitation exhibit a N-S dipole structure in
the first two components. This feature has been
found by Brito-Castillo et al. (2002) in their
analysis of reservoir inflow volumes in the same
region. These features suggest there are two
distinct modes which influence the precipitation
and runoff regime in the NAM region. The third
principal component of streamflow variability
also suggests that basins on the east-side of the
SMO differ from the basins on the west. (The
third component of precip. was not discernable
from higher components and therefore not deemed
significant.
Fig. 2b
However, the clear signal in the annual cycle of
streamflow does not necessarily imply increased
predictability in summertime streamflows. Figure
3 shows the 1 month lag autocorrelation values
for the 15 test basins. While low flow months
tend to have a strong serial correlation the
transitional months of May and Jun and the summer
months of June, July, August, and Sept. each
possess comparatively low correlation values with
the respective preceding months streamflow.
Fig. 2c
The North American Monsoon Region Selected Test
Basins
Precipitation Characteristics in the NAM Region
Figure 1
The core region of the North American Monsoon
region is characterized by steep topographic and
precipitation gradients, which, in turn, result
in strong gradients in vegetation. For the
streamflow analysis portion of this study we have
selected 15 basins which drain the Sierra Madre
Occidental mountains in western Mexico (2 of
which drain to the east). Drainage areas range
between 1,000 and 10,000 sq. km. All basins are
also unregulated by impoundments or large
diversions with the exception of the San Lorenzo
basin (11) Other basin characteristics are
provided in the Table 1.
Figure 4
Using data from 2002, there is evidence that
supports a maximum in precipitation occurrence
along the western slope of the SMO. While
precipitation is less frequent at lower
elevations (0-500 m) events here tend to possess
higher intensities.
It has been suggested that runoff in the NAM
region is controlled more by precipitation
characteristics (intensity, duration and
frequency) than by seasonal precipitation totals
(Gochis et al. 2003a). We examine this first by
looking at precipitation features obtained from a
new event based raingage network recently
established in NW Mexico. (Gochis et al. 2003b)
Rainfall Runoff Correlation Structure
Table 2
Table 4
The similar patterns revealed from the PC
analysis above suggest that there may be a
correlation between JAS precipitation (P) and
streamflow (Q). Pearson correlation
coefficients for all 15 basins are given in
Table 4. Statistically significant correlations
are present in 9 out of 15 basins. It is
noticed that all of the southern basins and
those that drain to the east are significantly
correlated while only 2 basins from the
northwestern part of the domain are. Also
shown in Table 4 are correlation coefficients
between the annual southern oscillation index
(SOI) and JAS precipitation and streamflow. It
is evident from these values that only basins in
the south are significantly correlated with the
SOI. It is, therefore suggested that principal
component 2 revealed above is more strongly
related to tropical forcing than are basins
further north.
The diurnal cycle of wet day rain rates (Fig. 5)
also show a general relationship with elevation.
Precipitation usually begins earliest at the
highest elevations and progressively later at
lower elevations. The lowest elevation band
(0-500m) also shows evidence of nocturnal
activity. Disregarding the 1000-1500 m band, the
0-500m band has the highest wet-day hourly rain
rates. (Note The strong maximum in the
1000-1500m el. Band is currently felt to be an
artifact of poor sampling in this region.)
Runoff Coefficient Analysis
Boldface indicates correlation significant at 95
level.
Table 3
To obtain an understanding of the rainfall runoff
relationship in the NAM region we calculate the
runoff coefficient (discharge/precipitation) for
the 15 test basins. Lacking a long time series
of the event data presented above we used the 1
degree CPC gridded daily precipitation product
(Higgins et al. 1996). This product is gridded
from available historical gage precipitation
measurements from the climate observing network
across Mexico. Basin average precipitation
values were calculated for each basin and used in
calculation of the runoff coefficient. We used
JAS (July-August-September) values of both
precipitation and streamflow. Various
combinations of months and 1 month lag periods
were also calculated but the results changed
little. As can be seen from Table 3, mean
values of the seasonal runoff coefficient varied
from 9-43. Smaller values of the runoff
coefficient (between 9 and 20) appear to be
confined to the northernmost basins. Standard
deviations, however, were often in excess of 50
of the mean values indicating a large uncertainty
in the mean values. This could indicate a large
interannual variability in runoff coefficient
values, but, in this case, more likely stems from
problems in the precipitation data used. The two
red values in the range column indicate that
runoff coefficients for some periods exceeded 1.0
which is unlikely in this semi-arid region where
groundwater inputs are not suspected to be
significant.
References and Publications Brito-Castillo, L.,
A. Leyva-Contreras, A.V. Douglas, D. Lluch-Belda,
2002. Pacific decadal oscillation and the filled
capacity of dams on the rivers of the Gulf of
California continental watershed. Atmosfera
(15), 121-138. Gochis, D.J. and W. J.
Shuttleworth, 2003a The hydrometeorological
response of the modeled North American Monsoon to
convective parameterization. In Press, J. of
Hydrometeorology. Gochis, D.J., J.-C. Leal,
C.J. Watts, and W.J. Shuttleworth, 2003b
Preliminary diagnostics from a new event-based
precipitation monitoring system in support of
NAME. In Press, J. Hydrometeorology.   Acknowledg
ements Support for this work was provided in part
by the NOAA Joint CLIVAR/PACS-GEWEX/GAPP North
American Warm Season Precipitation Initiative
Contract No. NA16GP2002 and by the Advanced Study
Program at the National Center for Atmospheric
Research.