Continuous long-term measurements of soil-plant-atmosphere variables at a forest site

It is a major challenge in modem science to decrease the uncertainty in pre dictions of global climate change. One of the largest uncertainties in pres ent-day global climate models resides with the understanding of processes i n the soil-vegetation-atmosphere-transfer (SVAT) system. Continuous, long-t erm data are needed in order to correctly quantify balances of water, energ y and CO2 in this system and to correctly model it. It is the objective of this paper to demonstrate how a combined system of existing sensor, compute r, and network technologies could be set up to provide continuous and relia ble long-term SVAT-process data from a forested site under almost all envir onmental conditions. The Central Tower Site (CTS) system was set up in 1993-1994 in a 25 m high boreal forest growing on a highly heterogeneous till soil with a high conte nt of stones and blocks. It has successfully monitored relevant states and fluxes in the system, such as atmospheric fluxes of momentum, heat, water v apour and CO2, atmospheric profiles of temperature, water vapour, CO2, shor t-and long-wave radiation, heat storage in soil and trees, sap-dow and a va riety of ecophysiological properties, soil-water contents and tensions, and groundwater levels, rainfall and throughfall. System uptime has been more than 90% for most of its components during the first 5 years of operation. Results from the first 5 years of operation include e.g., budgets for energ y, water and CO2, information on important but rarely occurring events such as evaporation from snow-covered canopies, and reactions of the forest to extreme drought. The carbon budget shows that the forest may be a sink of c arbon although it is still growing. The completeness of the data has made i t possible to test the internal consistency of SVAT models. The pioneering set-up at the CTS has been adopted by a large number of SVAT-monitoring sit es around the world. Questions concerning tower maintenance, long-term cali bration plans, maintenance of sensors and data-collection system, and conti nuous development of the computer network to keep it up to date are, howeve r, only partly of interest as a research project in itself. It is thus diff icult to get it funded from usual research-funding agencies. The full value of data generated by the CTS system can best be appreciated after a decade or more of continuous operation. Main uses of the data would be to evaluate how SVAT models handle the natural variability of climate c onditions, quantification of water. carbon and energy budgets during variou s weather conditions, rind development of new parameterisation schemes in g lobal and regional climate models. :(C) 1999 Elsevier Science B.V. All righ ts reserved.