David J. Gochis, Nat’l Center for Atmos. Res. Luis Brito-Castillo, CIBNOR, Guaymas, Son. MX

1. Mechanisms for Interannual Variations in Summer Streamflow from Headwater Catchments in Western Mexico David J. Gochis, Nat’l Center for Atmos. Res. Luis Brito-Castillo, CIBNOR, Guaymas, Son. MX Presented: NOAA Climate Diagnostics and Pred. Workshop, Oct. 27, 2005

2. Acknowledgements • Balaji Rajagopalan and Katrina Grantz (CU-Boulder) • Grantz, K., B. Rajagopalan, M. Clark, and E. Zagona, Spatio-Temporal Variability of the North American Monsoon (submitted), Journal of Climate, Special issue on the North American Monsoon, 2005. • http://civil.colorado.edu/~balajir/ publications • CNA • NOAA-OGP GEWEX Americas Prediction Project: NA16GP2002

3. Objectives… • Provide a basic overview of the hydroclimatology of the headwater region in the Sierra Madre Occidental • Characterize rainfall-runoff relationships governing streamflow on intra-seasonal to inter-annual timescales • Explore interannual variability of streamflow associated with the North American Monsoon, and the relationship between cool season and warm season flows

4. Data: • NCEP/Climate Prediction Center • 1x1 gridded daily precipitation • uniformly disagg. to 0.1 • monthly averaged across basins • Timeseries of monthly streamflow • 15 headwater catchments • BANDAS dataset, courtesy CNA

6. Water Management Issues:Gila River Gila River Basin Arizona and New Mexico

7. Basic Hydroclimatology

8. The “Monsoon” Water Year • Monthly mean cycle dominated by warm season rainfall associated with the N. American Monsoon (NAM) • Jul-Aug-Sep flow volume accounts for 50-85% of total annual flow • Minor appearance of secondary max in Dec-Jan in response to synoptic transient systems • Maximum recorded flows occur during monsoon, late fall or early winter

9. Previous Findings…(Dettinger and Diaz, JHM; Gochis et al., 2003, JHM; Grantz et al., 2005 (submitted J.Clim) • Latitudinal transition to a summer-dominated regime proceeding southward from Mogollon Rim.

10. Regionalization of Precipitation and Streamflow (Gochis and Brito-Castillo, J. Hydrol. in press) • VARIMAX rotated EOF analysis of • seasonal (JAS) streamflow reveals • three distinct regions of coherent • streamflow variability: • EOF1 – north • EOF2 – south • EOF3 – east • Explain ~ 71% of the JAS variance • Nearly identical EOF analysis of • seasonal (JAS) precipitation reveals • three very similar regions • Explain ~ 86% of the JAS • variance

11. Jul-Aug-Sep Jul Oct Regionalized Rainfall-Runoff Analyses: Runoff Coefficient (Qr=Q/P) 21% 19% 34% • Seasonal increase in Qr peaking in Oct • Reductions in precipitation in greater proportion than reduction in streamflow result in the marked increase in Oct values • Distinct sub-regional differences in this evolution

12. Precipitation-Runoff Processes: Runoff Fraction • Relationship between JAS P and Q is non-linear and the runoff fraction (Qr=Q/P) exhibits substantial interannual variability • Tendency for Qr to trend towards the 1:1 line (increase in value) with higher precipitation values • This feature is most pronounced in EOF2(south) which receives the most rainfall • Possible indication that in-basin abstractions are being met so that basins produce runoff with higher efficiency with greater seasonal precipitation

13. StreamflowDiagnostics:Precipitation – Streamflow Relationship • Precipitation – streamflow relationship is non-linear • High rainfall  very little infiltration  high streamflow • Decaying influence of in-basin abstractions

14. Interannual Relationships

16. Streamflow Diagnostics:Volume Analysis (summer) • San Francisco River at Clifton, AZ • July, Aug: decreasing trend • Sep, Oct: increasing trend

17. Annual Streamflow Center of Volume All Basins • No consistent long-term trend in CoV date though EOF2 has had more frequent early peaks since mid 1990s • Marked increase in CoV date variability since 1976-1977 climate shift most evident in individual basins • Effect is to increase the occurrence of early season peaks prior to DOY 200 (7/19) • Only EOF 1 (North) exhibits a weakly sig. correl. w/ ENSO • Shift due possibly to shift in onset, decreased summer flows or increased cool season flows EOF Comp. (Procedure by Stewart et al., 2004)

18. JAS Total Q Correlation to ENSO: SOI S S • Clear evolution towards stronger correlations at ‘decreasing’ lags • Peak correlation occurs with EOF2 and concurrent SOI (sig. @ 95% level) • Sign of correl. indicates that higher flows occur during La Nina (high SOI) and low flows occur during El Nino (low SOI) S – Significant @ 95%

19. S S S S S S S S S S S S S S JAS Total Q Correlation to ENSO: TNI & Nino 3.4 • Character of correlation is markedly different with respect to regions • EOFs 1(North) and 3(East) exhibit modest negative correlations with TNI and N3.4 at all nearly all • EOF2(South) exhibits no long term correl. with TNI or N3.4 TNI – ‘Trans-Nino Index’ of Trenberth and Stepaniak, 2001, J. Clim

21. Influence of ENSO on NAM Water Resources • Using composite • El Niño/La Niña years from SOI: • Spatially, it is seen that La Niña’s • influence is enhanced in • EOF2(south) • Reason for large influence of La Niña is combined factor of more summer precip. and reduced winter precip. especially in southern basins during La Niña • years: El Niño: Pct Change in JJAS Flow La Niña: Pct Change in JJAS Flow Large sensitivity in EOF2 EOF1 decreases During El Nino

22. Interannual Variability of the Runoff Fraction (Qr): • Compositing EOF Qr values by strongest correlate (SOI-EOF2, TNI-EOF1 and 3) it is shown that higher JAS runoff fractions tend to occur with La Nina than with El Nino • Difference in composite means significant @ 90% level in EOFs 1 and 2 • ENSO composite averages bound the all year averages

23. JJAS Composite Difference MapsNCEP/NCAR Rean.: • Subtracting SOI-based El-Nino from La-Nina years yields: • Enhanced Pacific-North American continent SLP gradient • Weaker E. Pac. Trade winds south of Mexico occur during La Nina compared to El Nino along with increased meridional component… • Increased meridional winds into Mexico during La Nina present up to mid-levels (though more confined to the coast)

24. Flux out of Mx. decreased Flux into Mx. increased

25. JJAS Composite Difference MapsNCEP/NCAR Rean.: • Modestly enhanced PW field over entire NAM region (except E. Chih) w/ enhanced differences in southern region

26. JAS Total Q ENSO Correlation Modulated by PDO: S S S • Brito-Castillo et al showed that correlations between ENSO and streamflow are significantly modulated by PDO (~ 76-77 shift) • Correls. between concurrent SOI and Q increase in EOFs 2 and 3 during low PDO phase (79-99) compared to full record and compared to PDO high phase (45-76) • Correl. between concurrent TNI and Q are significantly negative in EOFs 1(North) and 2(south) in high PDO phase (45-76) • Low PDO phase exhibits weaker correl in EOF1, no correl in EOF2 but stronger correl in EOF3(East) S S S S

27. Conclusions • The North American Monsoon System exerts significant influence on regional streamflow both in the U.S. and in Mexico with effects being much more pronounced in Mexico • Headwater systems in the Sierra Madre obtain, on average 50-85% of their annual flow from July-Oct in response to monsoon rains, compare to ~25% for Gila R. • 3 coherent regions capture a significant majority of the spatial coherence in seasonal precipitation and streamflow (North, South and East) • Monthly and seasonal rainfall runoff correlations and runoff fraction exhibit sub-regional behavior that is reasonably well encapsulated by the EOF composites; southern basins appear to achieve a ‘conditioned state’

28. Conclusions • SOI impact on EOF2 (south) streamflow seems quite clear • Significant positive correl. • La Nina (+ SOI) occurs with increased JJAS flow while El Nino (- SOI) occurs with decreased JJAS flow • Reduced cool season flows also occur during La Nina  results in JJAS having possessing large fraction of annual flow • Mechanisms for increased streamflow seems to be that SOI favors the northward transport of tropical moisture into southern MX resulting in higher PW values • Impact of SOI on northern regions is not as well resolved • ENSO as defined by TNI or Nino3.4 indices exhibits neg. correl. with EOF1 streamflow (and less so with EOF3, perhaps Atlantic influenced?) • Relationship mostly expressed through reduced JJAS flow during El Nino events and NOT as substantial increase of JJAS flow during La Nina as with SOI • Broadly consistent with findings of Grantz and Rajagopalan • PDO effects: • SOI:Q correlation in EOF2 increases during low PDO phase (1977-1999) compared full record while TNI:Q correlation in EOF1 is greater during high PDO phase (1945-1976). • Nearly all basins exhibit higher JJAS fractions of total annual flow prior to 1977 than after 1977 though differences are on the order of 5-10%. • Change in JAS fraction of annual flow is explained by all basins exhibiting greater than or equal cool season flows post 1977 compared to pre 1977. Increase in cool season flows result in shift in ‘center of volume’ of annual streamflow since 1977. • Changes in JJAS flows across 1976-77 climate shift are regional dependent with EOF1 possessing increases in JJAS flow post 1977, EOF2 flows generally decreasing since 1977 and EOF3 flows showing mixed signals

29. NAME research is funded by the NOAA-OGP with contributions from NSF, NASA and the Mexican Government

30. Publications: • Gochis, D.J., L. Brito-Castillo, W.J. Shuttleworth, 2005: Hydroclimatology of the North American Monsoon region in northwest Mexico. J. Hydrol, In press. • Grantz, K., B. Rajagopalan, M. Clark, and E. Zagona, Spatio-Temporal Variability of the North American Monsoon (submitted), Journal of Climate, Special issue on the North American Monsoon, 2005.

31. PDO Effect on JJAS Streamflow East North South Low Elev.

32. JAS Standardized Streamflow Anomalies - Low - High East Northwest South Low Elev. 1976-77 Climate Shift (Graphical Assist. by Todd Lane - NCAR)