Biospheric Aspects of the Hydrological Cycle

Biospheric Aspects of the Hydrological Cycle
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  B iospheric A spects of the H ydrological C ycle A Core Project of the I NTERNATIONAL   G EOSPHERE - B IOSPHERE   P ROGRAMME  (IGBP)Number 8, December 2000 by Lelys I. Guenni, Universidad Simón Bolívar, Caracas,Venezuela; member of the BAHC Scientific Steering Committee The BAHC research agenda has been highlightingthe role of the terrestrial biosphere in the climatesystem and the hydrological cycle. This newsletter isfeaturing some of the BAHC activities in SouthAmerica. A prominent example of an integratedresearch initiative that addresses most of the BAHCgoals is the Large-Scale Biosphere-AtmosphereExperiment in Amazonia (LBA), which is now wellmoving into its implementation phase. Some initialresults of the first LBA mesoscale campaign alreadyshow the potential impacts of the land cover on rain-fall convective processes at different locations in theAmazon Basin. These results were presented in thelast issue of BAHC News. At a larger scale, it ishypothesized that effects of the Amazonian de-forestation on the Earth's climate system might becomparable in magnitude with El Niño events.In an effort to integrate Latin American researchactivities of several disciplines into the BAHCagenda, a workshop took place in April 2000 at theSimón Bolívar University in Caracas, Venezuela withthe local support of the Special Programmes Officeand the Scientific Research Council of the university.Researchers from Bolivia, Brazil, Colombia,Ecuador, Peru and Venezuela met with scientistsfrom the BAHC Scientific Steering Committee(SSC) and the Science Panel of the InternationalSatellite Land Surface Climatology Project(ISLSCP) of the Global Energy and Water CycleExperiment (GEWEX), as well as local scientists,students and policy-makers, in order to exchangeideas and to communicate ongoing activities.In this issue of BAHC News we present some re-sults of this workshop and other contributions, whichenrich and complement the discussion about the roleof the biosphere in the climate system in this region.The article on page 3 emphasizes the role of bothphases of ENSO (El Niño/La Niña) in the inter-annual rainfall variability and river discharge in trop-ical South America, and the role of the land cover inmodulating soil water fluctuations associated withthis inter-annual variability. Examples are presentedfor several characteristic types of land cover inColombia. The Role of the Biosphere in the Climate System: The Tropical South American CaseContents The Role of the Biosphere in the Climate System:The Tropical South American Case..........................1Editorial Information..................................................2ENSO-Related Variability of River Discharges andSoil Moisture in Colombia.........................................3Understanding the Inter-annual Variability of theHydrological Cycle: Trends at Regional andLocal Scales.............................................................6Large-Scale Hydrometric Integration Studiesover the LBA Domain................................................9Large-Scale Hydrological and GeochemicalResearch in Amazonia............................................11Fires and Land Cover Dynamics in Amazonia........13Towards an Improved Understanding ofAmazonian Carbon Balance...................................15The Large-Scale Biosphere-AtmosphereExperiment in Amazonia: What's new?..................18Research in the Amazon Headwaters: AndeanAmazon Rivers Analysis and Management(AARAM).................................................................20Global Change Research and Capacity-Building inthe Americas: The Inter-American Institutefor Global Change Research (IAI)..........................21Integrated Water Resources Management at RiverBasin Level: The Role of the Organization of theAmerican States (OAS)..........................................22Meetings Calendar 2001.........................................23  BAHC News No. 8, December 2000 2 The complexity of the inter-annual variability of rainfall in theregion suggest that the modulatingrole of ENSO might have shiftedbetween different years (see p. 6).The need to quantify the temporaland spatial variability of the com-ponents of the hydrological cycleis paramount in order to under-stand how all the different factorsin the climate system interact.A new international hydro-meteorological initiative, LBA-HydroNET, discussed on page 9,has the central goal to establish alarge-scale data repository forcomputing high-resolution waterbalance elements in support of LBA and other research cam-paigns. LBA-HydroNET is target-ed at data-intensive analyses toimprove our understanding of therole humans play in altering thewater cycle.An equally important databasehas been developed within theHiBAm project (AmazonianHydrology), which combines run-off with information on riverinetransport of sediments and otherconstituents in the Amazon and itstributaries (see p. 11)As a very important distur-bance in Amazonia, human-induced as well as natural fires,play a significant role in modify-ing ecosystem structure, atmos-pheric composition and water,carbon and nutrient cycles. Theimportance of fire for land coverdynamics is discussed on page 13.The Amazon rainforest exertsa key influence on the globalcarbon cycle. Recent studies andLBA measurements suggest that0.4-1.0 Gt C yr -1  may be seques-tered by forests in this region.Changes in the Amazonian carbonbalance, resulting from changes inclimate and land use, may have asignificant impact on globalclimate. Land use changes due todeforestation in the Amazon aretherefore of direct relevance todiscussions about climate changescience and in formulation of climate change policies (see p. 15). A summary of new and futureactivities under LBA is presentedon page 18. LBA as an integrativeactivity is broadening its scope toprovide a framework for a numberof national and regional researchactivities.At the regional scale, theAARAM project, focusing on theAmazonian headwaters, is pre-sented on page 20. This project,which is supported by the Inter-American Institute for GlobalChange Research (IAI), contri-butes to a better understanding of the downstream effects of landuse changes and land degradation, e.g. , on water availability.The IAI presents its view onpage 21 as a very important actorin funding multinational researchprojects bridging the gap betweenthe science community and thepolicy community.The Organization of AmericanStates (OAS), as presented onpage 22, provides an importantlink to the international conven-tions. It has a particular focus onintegrated water resources man-agement within its sustainable de-velopment and environment unit,and through its role within theInter-American Water ResourcesNetwork (IWRN).These global change researchactivities in the Americas help tobetter understand the Earth systemand the role of humans in causingchanges and at the same timebeing affected by these changes.Better communication is neededbetween the international and theregional scientific communitiesand policy-makers and otherstakeholders, who are using andmanaging the resources of theregion. Scientists need to work ina cooperative and interdiscipli-nary mode, building the scientificcapacity needed in order to under-stand all the biophysical andsocio-economic processes in-volved and therefore contribute tosustainable land use planning, en-vironmental protection, economicdevelopment and the welfare of the inhabitants.I do hope that this Newsletterprovides a comprehensive view of the important activities, playersand institutions that are con-tributing to a better understandingof the role of the biosphere and allits human-induced changes in theclimate system. Contact: Lelys I. Guenni, Edited by the  BAHC International Project Office Coordinator: Holger Hoff  Editorial Board: John Gash, Holger Hoff, Pavel Kabat, Andrew Pitman Technical Editor: Wilhelmine Seelig Newsletter requests and change of address information should be sent to: BAHC International Project Office Potsdam Institute for Climate Impact Research, P.O. Box 60 12 03, D-14412 Potsdam, Germanyphone: (+49-331) 288-2543, fax: (+49-331) 288-2547e-mail:  ~ bahc Editorial Information  BAHC News No. 8, December 2000 3 by Germán Poveda, Universidad Nacionalde Colombia, Medellín; and Álvaro Jaramillo, Centro Nacional de Investiga-ciones del Café, Chinchiná, Colombia Inter-annual variability of thehydrological cycle in tropicalSouth America is predominantlyinfluenced by both phases of El Niño - Southern Oscillation(ENSO) events (see Aceituno1988; Ropelewski and Halpert1996). With minor regional ex-ceptions in timing and amplitude,tropical South America histori-cally has exhibited negativeanomalies in rainfall and riverdischarges during the warm phaseof ENSO (El Niño), and positiveanomalies during the cold phase(La Niña). Both large-scaleforcing and land surface hydro-logy play a key role on ENSO dy-namics over the region (Marengoand Hastenrath 1993; Poveda andMesa 1997 and 2000). During ElNiño events, anomalous Hadleycells develop with descendingmotion over northern SouthAmerica (Rasmusson and Mo1993), which prevent deepconvection, thus contributing toprecipitation deficits. As a result,the centre of convection of theInter-Tropical Convergence Zone(ITCZ) over the tropical Americasis displaced south-west from itsnormal position (Pulwarty andDiaz 1993). During El Niñoevents, there are decreases inwater vapour advection into theregion as a result of the weak-ening of the low-level westerly’CHOCO’ jet ( Chorro del Occi-dente Colombiano , or westernColombian jet) that normallypenetrates from the Pacific Oceaninland Colombia (Poveda andMesa 2000). Such weakenedwinds are a response to di-minished temperature gradientsbetween Colombia and the coldtongue of sea water off thePeruvian coast. In addition, thenumber of tropical easterly wavesdecrease throughout the tropicalNorth Atlantic and the CaribbeanSea (Gray and Sheaffer 1992),also contributing to precipitationdeficits. Precipitation deficits during ElNiño events are closely associatedwith negative anomalies in riverdischarges throughout Colombia.During El Niño events river dis-charge decrease and converselyincrease during La Niña. Figure 1shows frequency histograms of average monthly discharges of theMagdalena River at Puerto Berrio,for ’normal’, El Niño and La Niñayears. Not only the two first statis-tical moments, but the wholeprobability density function(PDF) of average monthly dis-charge, and extreme hydrological ENSO-Related Variability of River Discharges and Soil Moisture in Colombia Figure 1. Frequency histograms of average monthly discharge of the MagdalenaRiver at Puerto Berrio (central Colombia), during normal years (top), El Niñoyears (centre), and La Niña years (bottom). m = mean, sd = standard deviation.Recording period is 1936-1998. Notice that the mean and standard deviationdiminish during El Niño and increase during La Niña, as compared to normal years  BAHC News No. 8, December 2000 4 events are affected by ENSO, in-cluding extreme droughts duringEl Niño and floods during LaNiña. Observations show thatENSO effects on river dischargeoccur progressively later for riverstowards the east in Colombia andnorthern South America. Also, theimpacts of La Niña are morepronounced than those of El Niño(Poveda and Mesa 1997). Evi-dence of ENSO impact on the sea-sonal cycle of precipitation andriver discharges have been pre-sented in Poveda et al.  (1999 and2000).As ENSO affects the seasonalcycle of precipitation and riverdischarges, it is important toinvestigate the effects on soilmoisture and evapotranspirationin Colombia. The influence of ENSO on evapotranspiration overthe region has been investigatedby Poveda et al. (2000) using theNormalized Difference VegetationIndex (NDVI), which is relatedwith plant activity as a surrogatefor evapotranspiration in the re-gion. Results (not shown here)confirm diminished plant activityand NDVI values during El Niñoevents in Colombia. In an attempt to investigate thedynamics of soil moisture at apoint under different vegetationcovers, volumetric soil moisture(VSM) content was measured as10-day averages for the periodMarch 1997 through August 1999.During that period, the tropicalPacific Ocean experienced astrong El Niño (warm phase)followed by a strong La Niña(cold phase). The soil moisturewas measured by time-domainreflectivity (TDR) probes at threeexperimental sites, at the National Center for Coffee Research( Centro Nacional de Investiga-ciones del Café - CENICAFÉ,5º00'N, 75º36'W, 1425 m height),in the coffee-growing region of Colombia on the western slope of the central mountain range of theAndes. The experimental siteswere located under three differentvegetation covers: (i) coffee treessurrounded by taller trees of  guamo  (  Inga edulis mart), here-after referred to as shaded coffee,(ii) a secondary forest in whichdiverse native species are re-emerging between large trees thatremained from the primary forest,and (iii) coffee trees with nosurrounding vegetation (sunlightcoffee). The three sites werelocated within a distance of 200 mto each other. Volumetric soilmoisture contents were measuredfor forest and shade coffee atdepths of 20 cm and 40 cm (here-after referred to as VSM-20 andVSM-40), and at 20 cm and 35 cmfor sunlight coffee (VSM-35). Thelandscape includes moderatelysteep to steep slopes with deepsoils derived from volcanic ashes.The soils are structurally stablewith moderate permeability, lowbulk density and substantialorganic matter content. Rainfallmeasurements from two raingauges near the experimental siteswere averaged.Figure 2 shows the time evolu-tion of the volumetric soilmoisture measured at 40 cm depth(35 cm for sunlight coffee). Forthese volcanic soils, volumetricsoil moisture becomes saturatedand reaches upper limits around50-52%, usually after intensestorms. These extreme soil mois-ture levels last for very short timesdue to coarse soil textures, highpercolation rates, low bulk densityand high organic matter content(Saxton et al.  1996). VSM-40reaches upper limits on the orderof 45% for shade coffee and forestand 40% for sunlight coffee.Typically, the average forest VSMis larger at 40 cm (25%-46%) thanat 20 cm (18%-42%). for shadecoffee, VSM at the two depths issimilar (ranging between 25-48%). VSM-20 (not shown here)takes upper limits on the order of 45% for shade coffee, 40% forforest and 30% for sunlightcoffee. For sunlight coffee, aver-age soil moisture at 35 cm depth(8%-40%) is larger than at the 20-cm depth (8%-30%), except dur-ing the driest spells of the annualcycle when, enhanced by El Niño,VSM-20 and VSM-35 decline tosimilar values (around 8%), indi-cating a lower capacity of thissoil-land cover type to retain   15   25   35   45         A     p        r   -      9 r9       J     u J      n   - n       9      7 9       A     u  u      g   - g       9      7 9       O     c Oc       t   - t       9      7 9       D     e De      c   - c       9      7 9       F     e Fe       b   - b       9      8 9       A     p        r   -      9 r9       J     u J      n   - n       9      8 9       A     u  u      g   - g       9      8 9       O     c Oc       t   - t       9      8 9       D     e De      c   - c       9      8 9       F     e Fe       b   - b       9      9 9       A     p        r   -      9 r9       J     u J      n   - n       9      9 9      V  o   l  u  m  e   t  r   i  c  s  o   i   l  m  o   i  s   t  u  r  e   (   %   ) Vumecsmosue%) Forest orest Shade coffee hade coffee Sunlight coffee unlight coffee 40 cm 0 cm Date ate Figure 2. Time evolution of measured 10-day average volumetric soil moisture incentral Colombia at 40 cm depth, beneath different land cover types - shade coffee,forest and sunlight coffee (35 cm) - during the period March 1997 to August 1999     15   25   35   45         A     p        r   -      9 r9       J     u J      n   - n       9      7 9       A     u  u      g   - g       9      7 9       O     c Oc       t   - t       9      7 9       D     e De      c   - c       9      7 9       F     e Fe       b   - b       9      8 9       A     p        r   -      9 r9       J     u J      n   - n       9      8 9       A     u  u      g   - g       9      8 9       O     c Oc       t   - t       9      8 9       D     e De      c   - c       9      8 9       F     e Fe       b   - b       9      9 9       A     p        r   -      9 r9       J     u J      n   - n       9      9 9      V  o   l  u  m  e   t  r   i  c  s  o   i   l  m  o   i  s   t  u  r  e   (   %   ) Vumecsmosue%) Forest orest Shade coffee hade coffee Sunlight coffee unlight coffee 40 cm 0 cm Date ate  BAHC News No. 8, December 2000 5 water in the soil column. The factthat VSM is higher at larger depthfor forest and sunlight coffee canbe explained by higher evapora-tion rates near the surface, whichcan be more easily controlled byradiative processes in shadecoffee.All of the series exhibitremarkable declines during July-September 1997 and duringDecember 1997-March 1998, as aresult of diminished rainfall. Thedriest spells correspond to thewell-known decrease in seasonalrainfall, though they are enhancedby the physical mechanisms thatoperate in the region during ElNiño, including feedback in theland-atmosphere system (Povedaand Mesa 1997). Rainfall alsodecreased during the periodDec./Jan./Feb. 1997-98 as a resultof the aforementioned mecha-nisms, and during Dec./Jan./Feb.1998-99 there were positive pre-cipitation anomalies associatedwith the strong La Niña event.The lag-correlation coefficientbetween 10-day rainfall averagesand the first principal componentof VSM-20 during the VSMmeasurement period, reaches amaximum value of 0.53 (99%confidence level, P = 0.01) for a20-day lag with precipitation lead-ing VSM. Thus, tropical rainfalland soil moisture variability at an-nual and inter-annual time scalesare highly and quicklyintertwined.The skewness of the proba-bility density functions (PDF) forVSM-20 and VSM-40 (Fig. 3)differ from land cover to landcover. Soil moisture at the sun-light coffee site has a higherskewness coefficient (-1.52 forVSM-20 and -1.36 for VMS-40)than the other land covers. Notethe uni-modal PDF for VSM atthe sunlight coffee site contrastingwith the multi-modal PDFs forforest and shaded coffee, whichmay be associated with higherintermittency of the sunlightcoffee soils, which appear to dryup more rapidly than the othersoils. Soil moisture at sunlightcoffee shows higher intermitten-cy (estimated through the kurtosisof the series, see Poveda et al. 2000) than soil moisture at forestand shadow coffee. As long as thethree sites are subject to the sameprecipitation regime (frequent andintense showers), the higher inter-mittency of soil moisture atsunlight coffee indicates more ex-tremes on the tails of the distribu-tion than a Gaussian distribution,thus exhibiting more sudden shiftsfrom very wet to very dry soils,which reflect a shorter water-holding capacity of the sunlightcoffee soil, thus making it difficultto smooth out precipitationdeficits during El Niño. These(uni or multi-) modalities indicatepreferred states of trajectories of the dynamical system attractor inphase space. For soils beneathforest or shade coffee, multi-modality and lower skewness re-flect longer visits to several aver-age states, far enough from thetails of the distribution. This canbe explained in terms of longerresidence times of water into thesoil, i.e. , soils with greater regula-tion capacity. For sunlight coffee,the coexistence of unimodalityand higher skewness reflect onesingle preferred state, and shortand sudden visits to the tails of thedistribution, in this case to mini-mum extreme values triggered byEl Niño. Root depths, evapo-transpiration rates and localenergy and water balances playfundamental roles in defining thelesser degree of intermittency of soil moisture at forest and shadedcoffee sites. Therefore, our resultssuggest that multi-modality of thesoil moisture PDF may be asso-ciated with the role of the soil-vegetation-land surface hydrologysystem, when subject to a similarprecipitation regime. Thus, wehypothesize that the coupling be-tween the vegetation-soil systemand land cover strongly modulateshydrological (space- ) time varia-bility in the tropics, and that theeffects of El Niño-related dryspells might be ameliorated vialand cover and land use.Showing evidence of differentdegrees of intermittency depend-ing on the land cover-vegetation-soil system is relevant to under- 0  10 0 15   20 0 25   30 0 35   -3 3 -2 2 -1  0 2   Forest 20 orest 0 Forest 40 orest 40 Shade coffee 20 hade coffee 0 Shade coffee 40 hade coffee 40 Sun coffee 20 un coffee 0 Sun coffee 40 un coffee 40 Standard deviations tandard deviations    F  r  e  q  u  e  n  c  y   (   %   ) Fe%) Figure 3. Estimated probability density functions (PDF) for volumetric soilmoisture at 20 cm and 35 cm depth for sunlight coffee, and for 20 cm and 40 cmdepth for forest and shaded coffee in central Colombia
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