15 GLOBAL PRIMARY PRODUCTION AND EVAPOTRANSPIRATION by Steven W. Running and Maosheng Zhao

DATA PRODUCTS Moderate Resolution Imaging Spectroradiometer Terrestrial primary production provides the energy to (MODIS) sensor, these variables are calculated maintain the structure and functions of ecosystems, every eight days in near real time at 1 km resolution. and supplies goods (e.g. food, fuel, wood and fi bre) MODIS GPP, NPP and ET data are available at the for human society. Gross primary production (GPP) EOS data gateway (see link below). is the amount of carbon fi xed by photosynthesis, To correct for contamination in the global and net primary production (NPP) is the amount of refl ectance data due to severe cloudiness or aerosols carbon converted into biomass after subtracting in the near real time products, these datasets are the cost of plant respiration. The water loss through reprocessed at the end of each year to build more

exchange of trace gas CO2 by leaf stomata during stable, permanent datasets. These end-of-year photosynthesis plus evaporation from soil and versions of MODIS GPP, NPP and ET are available at plants is evapotranspiration (ET). ET computes the NTSG, University of Montana (see link below). water lost by a land surface, so it is consequently a component of the water balance in a region, and RESULTS FOR 2000–2006 is therefore relevant for drought monitoring and Figure 1 shows the seven-year average MODIS water management, providing an assessment of NPP for vegetated land on earth at 1 km spatial the water potentially available for human society. resolution. Forests, especially tropical forests, have Under current, unprecedented, global environmental high NPP values, while savannahs, shrubs and change, monitoring variations in GPP, NPP and ET grasses have low NPP due to the limitations of water is signifi cant in tracking degradation in ecosystem availability or short growing season in high latitude services. Based on observations from the NASA or altitude areas. These remotely sensed NPP data T R PLANE OU F O

S N O ERVATI S B O TRIAL S Figure 3: Relationship of annual total GPP TERRESTRIAL OBSERVATIONS OF OUR PLANET TERRESTRIAL OBSERVATIONS TERRE Figure 2: Spatial pattern of 1-km MODIS NPP anomalies versus annual total ET in 2001 from 2000 to 2005

Page 40 Issues for maintaining global continuity of terrestrial primary production and evapotranspiration

Figure 1: Spatial pattern of 1-km mean MODIS NPP from 2000 to 2006

provide information on the geographic variability of with meteorological conditions. Figure 3 illustrates agricultural crops and renewable natural resources. this strong relationship between MODIS GPP and However, our food availability is largely infl uenced ET. Generally, high GPP corresponds with high ET, by the year-to-year variations and long-term trend and vice versa, although exact correspondence (decrease or increase) in NPP. Climatic fl uctuations is not expected because climate variables drive and land cover change (fi re, deforestation, these processes in somewhat different ways. This urbanization, etc.) are responsible for the changes in correlation can also give some information on NPP. Figure 2 shows the anomalies of MODIS NPP water use effi ciency (WUE), i.e. the ratio of GPP to

from 2000 to 2006. In most cases, the regional scale ET, which is about 3.2 g C/mm H2O. Both daily GPP reductions in NPP are caused by severe droughts. and ET from MODIS are validated and refi ned with In 2000, drought occurred in China and the southern the measurements from FluxNet, a global network USA. In 2002, severe drought occurred in Australia of nearly 300 eddy fl ux towers, located in different and over a large part of the USA. In 2003, the heat continents and climatic zones, and continuing to wave in Europe led to drought, and lower than expand. average NPP. In 2005, the Amazon experienced the worst drought in 100 years, making water availability CONTINUITY OF THE DATA the leading limiting factor for plant growth instead of These data may be a critical component of future solar radiation. terrestrial carbon credit calculations, and particularly provide an independent measure of carbon sinks RELATIONSHIP OF PRIMARY PRODUCTION AND at national levels. The next-generation Kyoto EVAPOTRANSPIRATION agreement may rely on these types of information Plant transpiration and photosynthesis are strongly to verify claimed carbon credits. However, the coupled because stomata are the pathway for current satellite source of this data, the NASA Earth

absorbing CO2 and releasing water vapour by Observing System, is nearing the end of its lifetime, transpiration. MODIS ET is calculated based on a and the replacement U.S. National Polar Orbiting mechanistic model, the Penman-Monteith equation, Environmental Satellite System (NPOESS) has been combined with remotely sensed surface albedo, delayed. A more international basis for generating

vegetation cover and canopy leaf area, coupled this information therefore needs to be devised. AND EVAPOTRANSPIRATION PRIMARY PRODUCTION GLOBAL

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RELATED LINKS: EOS data gateway: http://edcimswww.cr.usgs.gov/pub/imswelcome I NTSG - University of Montana: www.ntsg.umt.edu