In the previous comment, Legates et al. express concern about the statistical reliability of the positive runoff–temperature relationship presented by Labat et al. We are grateful for this opportunity to respond to these concerns. As Legates et al. correctly points out, the effect of temperature on runoff is a complex relationship, which involves precipitation, evaporation, anthropomorphic affects, among others. As such, the effect of increased temperature on runoff is strongly dependent on the identity of the watershed of interest. For example, a watershed located in a glaciated region, such as Iceland, exhibits a strong positive correlation between runoff and temperature, whereas a watershed located in a arid climate, such as the Sahara desert, exhibits a negative correlation; often there is no run off at all during the summer months in such watersheds

Goddéris, Yves

Guyot, Jean-Loup

Labat, David

Probst, Jean-Luc

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Labat, D.

Godderis, Y.

Probst, J. L.

Guyot, J. L.

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Reply to comment of Legates et al.

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Reply to comment of Legates et al.D. Labat *, Y. Godderis, J.L. Probst, J.L. GuyotLaboratoire de Me´canisme de Transferts en Ge´ologie, UMR CNRS/UPS 5563, 14 Avenue Edouard Belin, 31400 Toulouse, FranceIn the previous comment, Legates et al. [12] expressconcern about the statistical reliability of the positiverunoff–temperature relationship presented by Labatet al. [10]. We are grateful for this opportunity to respondto these concerns. As Legates et al. [12] correctly pointsout, the effect of temperature on runoff is a complex rela-tionship, which involves precipitation, evaporation,anthropomorphic affects, among others. As such, the ef-fect of increased temperature on runoff is strongly depen-dent on the identity of the watershed of interest. Forexample, a watershed located in a glaciated region, suchas Iceland, exhibits a strong positive correlation betweenrunoff and temperature, whereas a watershed located in aarid climate, such as the Sahara desert, exhibits a nega-tive correlation; often there is no run off at all duringthe summer months in such watersheds.These differences make it extremely challenging toestimate the relationship between temperature and run-off at the global scale. What these differences tell us,however, is that any studies limited to a single wa-tershed, or even a single continent are inadequate todetermine the global affects of temperature on runoff.Labat et al. [10] presented the first attempt to deter-mine the relationship between temperature and runoffat a global scale. They observed a small positive correla-tion; runoff was found to increase 4% for each increasein 1 C of temperature increase. If true, this result hasmajor implications on the feedback between tempera-ture and continental weathering in global geochemicalmodels. It is important to emphasise, as pointed outby Legates et al. [12], that the correlation presented byLabat et al. [10] has a significant uncertainty and de-pends strongly on the quality of the database; a changein the data considered in the correlation could either in-crease or decrease the computed effect of temperature on* Corresponding author. Tel.: +33 561 332612; fax: +33 561 332560.E-mail address: labat@lmtg.obs-mip.fr (D. Labat).runoff (although which would remain positive as we dis-cussed below in the technical reply). This observationillustrates the critical need for improved data on theconnection between temperature and runoff, that is notlimited to a single watershed or continent, but on a glo-bal scale. Such data are essential to refine this correla-tion to better constrain this important relationship at aglobal scale.1. Technical reply1.1. Use of discharge records that reflectnon-climatic trendsWe refute the assertion that ‘‘Labat et al. [5] specifyonly the rivers for which data were acquired, rather thanthe specific streamgauging stations that were used’’.Effectively, all gauging stations are clearly identified bylatitude and longitude in Tables 1–3 and Fig. 1.Indeed, they are located at the mouth of the water-sheds, thus integrating all the climatic and anthropo-genic effects over the watersheds. Legates et al. [12]insist in general on the Assam Dam influence on Niledischarges whereas this example has been clearly identi-fied and clarified by Labat et al. [5].Ref. [1] in Legates et al. [12] deals only with highstreamflow events in relation with high precipitationevents. This cannot be compared to our study since wedeal with complete annual runoff timeseries. We alsonote that Ref. [9] in Legates et al. [12] argues that theUnited States are getting wetter, in agreement with ourconclusions based on a large watershed study with gaug-ing station located at the mouth.Legates et al. [12] claim that ‘‘Labat et al.s use ofsuch records violates a long-accepted and well-docu-mented practice in hydroclimatic research’’. Labatet al. [10] is the first study dealing with global runoff.Fig. 1. Top: Runoff–temperature relationship with the positive slope.Bottom: Value of the slope as a function of the starting year of the timeseries. We show, using this statistical approach, that the slope ismaximum when series start around 1925 and then decreases to 1.5–2%but remains positive. The up and down lines corresponds to theconfidence interval of the slope value for each starting years.Of course, small catchments (i.e. below 20,000 km2) maybe strongly perturbed by anthropogenic activities, andshould not be used for studies that attempt to link globalclimate and runoff. But, a quick overview of the litera-ture supports the idea that large watershed hydrologicalresponse allows an identification of climatic oscillations([1–9,11,13,15,18–20]; among others).In conclusion, Legates et al. [12] suggest that the lar-ger the watershed, the larger will be the anthropic ef-fects on the hydrologic signal. We argue that thelarger watershed, the most clear will be the climatic sig-nal (as previously demonstrated by Probst et al.[16,17,15]).1.2. Documented lack of a relation between streamflowand air temperature in previous studiesWe note that the entire discussion in Legates et al.[12] focuses on the United States, where they argue thatno climatic signal can be observed in the streamflows.The systematic extrapolation of these results obtainedfor the US to all continents is clearly doubtful.We never claim that precipitations are not the maindriving force of change in runoff. The relationship iden-tified between air temperature and runoff was found atglobal scale, but this do not preclude any cross relation-ship between air temperature, precipitations and runoff.Furthermore, the study of Karl and Riebsame [8] dealswith much smaller watershed than Labat et al. [10],working at continental scale. As we have shown in thepaper [10], the relationship between air temperatureand runoff at continental scale differs from the relation-ship identified at global scale.1.3. Inappropriate estimation of data to fill gapsin long-term streamflow recordsConcerning the data-reconstruction method, Legateset al. [12] claim that wavelets are inappropriate (1) toisolate climatic effects from anthropogenic effects; and(2) to estimate data when missing for long time periods.Nakken [14] shows that wavelets have already beenused to isolate climatic effects from anthropogeniceffects.In Labat et al. [10], most time series extend over morethan 10 years. We acknowledge that short time serieshave been used for some minor rivers. Extrapolating10- or 20-year hydrological time series to centenial scalesis statiscally correct since the longest climatic oscilla-tions correspond approximatively to 30 years. There-fore, the original 10- to 20-year series already reflectsthe long-term climatic response.Furthermore, the method used in Labat et al. [10] isproved to be correct, since we show that the correlationcoefficients between observed and reconstructed annualrunoff are equal to 0.8 (which is quite significant).Then, Legates et al. [12] also argue that removing asingle point of the runoff-temperature relationship tendsto unvalidate the statistical validity of this relationship.First, our study deals, for the first time with global data,and large dispersion of the points is of course expected.But, this is not by itself a justification for removing out-liers. All available data must be considered.Furthermore, the outlier corresponds to the year 1926and removing this point i.e. calculating slope starting in1927 leads to a 0.25 slope coefficient but in no way to anull coefficient (Fig. 1).The important point is that the slope significantly dif-fers from 0 and is positive. We show in Fig. 1 the valueof the slope as a function of the starting year of the timeseries. Using this correct statistical approach, the slopeis maximum when series start around 1925 and then de-creases to 1.5–2%, but remains positive. We acknowl-edge that the 4% slope claimed in Labat et al. [10]must be considered as a maximum but we refute the Le-gates et al. [12] assertion of a null-slope relationship be-tween runoff and temperature.1.4. Selection of the time period analyzed and lackof explanation for relationships before 1925The 1925–1994 period was selected so that our resultsmay be directly compared to the study [16], which is upto now the only data-based study of global runoffchange linked to global climatic change.Legates et al. [12] indicate that ‘‘Fig. 4 is inconsis-tent with the IPCC and confounds their argument ofa strong air temperature/runoff relation’’. Fig. 4 inLabat et al. [10] is certainly consistent with the IPCC.Legates et al. [12] refers to the behaviour of thecontinental air temperature alone, which displays a dis-tinct behaviour for the 1890–1925 period. In fact, weused global averaged temperature shown in Fig. 2.8,p. 115, 2001 IPPC report (Jones et al. 2001). This ser-ies include both continental air temperature and seasurface temperature. The global averaged temperatureis the only reliable global temperature measurementwhen dealing with changes in the global hydrologiccycle.1.5. Regression and the presence of an influence pointThe 15 year shift observed between temperature andrunoff response was based on a visual observation,mainly focusing on the peak observed in the 50s for run-off, while an apparent similar peak is observed in tem-perature 15 years earlier. We think that this shiftmight be insignificant.Legates et al. [12] claim that ‘‘there are 69 years ofpairwise comparisons, but it is inappropriate to assumethat there are 68(n  1) degrees of freedom because thedata are temporally autocorrelated’’. The T-studenttest is commonly applied in hydrological studies, de-spite the existence of correlations, as long as this corre-lation is rather weak. Indeed, we are not working withthe 3 year mobile average of the temperature signal (asprobably suspected by Legates et al., and which dis-plays a strong auto-correlation), but with the annualaverage. This annual average is characterized by alag-1 correlation coefficient equals to 0.4, and a lag-3correlation coefficient reaching 0.18. This demonstratesa weak autocorrelation of the signal, validating the useof the T-student test and the use of 68 degrees of free-dom in the T-student test.The T-test clearly shows the statistical significanceof the runoff-temperature relationship. We also men-tion in Fig. 1 the up and down bounds of the confi-dence interval of the slope regression coefficient. Aswe already mentioned earlier, we acknowledge thatthe 4% slope claimed in Labat et al. [10] must be con-sidered as a maximum but refute the Legates et al. [12]assertion of a null-slope relationship between runoffand temperature.2. ConclusionLegates et al. [12] argue that no climatic signal can beobserved in the streamflows which is clearly wrong (seeSection 1). They systematically extrapolate results ob-tained over the United States continent to all continentswhich is clearly doubtful.We think that Legates et al. [12] are wrong when dis-cussing the anthropogenic impact on the continentalrunoff. Of course, small catchments (<20,000 km2) maybe strongly perturbed by anthropogenic activities, andcannot be used for studies that attempts to link globalclimate and runoff. But, a quick overview of the litera-ture demonstrates that this is not true for large scalecatchments, but are able to record global climaticchanges despite anthropogenic disturbances. We alsosystematically refute all the statistical issues mentionedby Legates et al. [12].For all these reasons, we strongly claim that we effec-tively provide the first data-based positive relationshipof the runoff-temperature relationship and that this po-sitive relationship has profound implications in ourunderstanding of climate changes.References[1] Berri G, Ghietto MA, Garcia NO. The influence of ENSO in theflows of the upper Parana river of South America over the past100 years. J Hydrol 2002;3:57–65.[2] Bischoff S-A, Garcia NO, Vargas WM, Jones PD, Conway D.Climatic variability and Uruguay river flows. Int Water ResourAssoc 2000;25:446–56.[3] Bjornsson H, Mysak LA, Brown RD. On the interannualvariability of precipitation and runoff in the MacKenzie drainagebasin. Clim Dynam 1995;12(1):67–76.[4] Depetris PJ, Kempe S, Latif M, Mook WG. ENSO controlledflooding in the Paranas river 1904–1991. Nature 1996;83:127–9.[5] Eltahir EAB. El Nin˜o and the Natural Variability in the Flow ofthe Nile River. Water Resour Res 1996;32(1):131–7.[6] Garcia N, Vargas WM. The temporal climatic variability in theRio de la Plata basin displayed by the river discharges. ClimaticChange 1998;38:359–79.[7] Jury MR, Melice J-L. Analysis of Durban rainfall and Nile riverflow 1871–1999. Theor Appl Climatol 2000;67:161–9.[8] Karl TR, Riebsame RW. The impact of decadal fluctuations inmean precipitation and temperature on runoff—a sensitivity studyover the United States. Climatic Changes 1989:423–47.[9] Krepper CM, Garcia NO, Jones PD. Interannual variability in theUruguay basin. Int J Climatol 2003;23:103–15.[10] Labat D, Godde´ris Y, Probst JL, Guyot JL. Evidence for globalrunoff increase related to climate warming. Adv Water Resour2004:631–42.[11] Labat D, Ronchail J, Calle`de J, Guyot JL, De Oliveira E,Guimara˜es W. Wavelet analysis of Amazon hydrological regimevariability. Geophys Res Lett 2004;31:L02501.[12] Legates DR, Lins HF, McCabe GJ. Comments on ‘‘evidencefor global runoff increase related to climate warming’’, thisissue.[13] Marengo J. Variations and change in South American streamflow.Climatic Change 1995;31:99–117.[14] Nakken M. Wavelet analysis of rainfall-runoff variability isolatingclimatic from anthropogenic patterns. Envrion Modell Softw1999;14:283–95.[15] Pekarova P, Miklanek P, Pekar J. Spatial and temporal runoffoscillation analysis of the main rivers of the world during the 19th-20th centuries. J Hydrol 2003;274:62–79.[16] Probst JL, Tardy Y. Global runoff fluctuations during the last 80years in relation to world temperature change. Am J Sci1989;289:267–85.[17] Probst JL, Tardy Y. Long range streamflow and world continen-tal runoff fluctuations since the beginning of this century. J Hydrol1987;94:289–311.[18] Rao AR, Hamed K. Multi-taper method of analysis ofperiodicities in hydrologic data. J Hydrol 2003;279(1–4):125–43.[19] Robertson AW, Mechoso CR. Interannual and Decadal Cycles inRiver Flows of Southeastern South America. J Climate 1998;11:2570–81.[20] Williams GR. Cyclical variations in the world-wide hydrologicaldata. J Hydraul Div 1961;6:71–88.

Analysis of Durban rainfall and Nile river ﬂow 1871–1999. Theor Appl Climatol

Climatic variability and Uruguay river ﬂows. Int Water Resour Assoc

Comments on ‘‘evidence for global runoﬀ increase related to climate warming’’, this issue.[13] Marengo J. Variations and change in South American streamﬂow. Climatic Change

Cyclical variations in the world-wide hydrological data.

De Oliveira E, Guimara ˜es W. Wavelet analysis of Amazon hydrological regime variability. Geophys Res Lett

El Nin ˜o and the Natural Variability in the Flow of the Nile River. Water Resour Res

ENSO controlled ﬂooding in the Paranas river 1904–1991.

Evidence for global runoﬀ increase related to climate warming.

Global runoﬀ ﬂuctuations during the last 80 years in relation to world temperature change.

Interannual and Decadal Cycles in River

Interannual variability in the Uruguay basin.

Long range streamﬂow and world continental runoﬀ ﬂuctuations since the beginning of this century.

Multi-taper method of analysis of periodicities in hydrologic data.

On the interannual variability of precipitation and runoﬀ in the MacKenzie drainage basin. Clim Dynam

Spatial and temporal runoﬀ oscillation analysis of the main rivers of the world during the 19th20th centuries.

The impact of decadal ﬂuctuations in mean precipitation and temperature on runoﬀ—a sensitivity study over the United States. Climatic Changes

The inﬂuence of ENSO in the ﬂows of the upper Parana river of South America over the past 100 years.

The temporal climatic variability in the Rio de la Plata basin displayed by the river discharges. Climatic Change

Wavelet analysis of rainfall-runoﬀ variability isolating climatic from anthropogenic patterns. Envrion Modell Softw

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