Continuous long-term measurements of soil-plant-atmosphere variables at anagricultural 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 to correctly quantify balances of water, energy and CO2 in this system and to correctly model them. It is the objective of this pa per to demonstrate how a combined system of existing sensor, computer, and network technologies could be set up to provide continuous and reliable lon g-term SVAT-process data from an agricultural site under almost all weather conditions. A long-term climate-monitoring system within the framework of NOPEX was set up in 1993-1994 at the Marsta Meteorological Observatory (MMO). It is situ ated in a flat agricultural area where annual crops are cultivated on a hea vy clay soil. It has successfully monitored relevant states and fluxes in t he system, such as atmospheric fluxes of momentum, heat, water vapour and C O2, atmospheric profiles of wind speed, direction, and temperature, short- and long-wave radiation, soil temperature, soil-water contents, groundwater levels, and rainfall and snow depth. 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 has proven MMO to be an ideal s ite for intercomparison and intercalibration of radiometers and fast turbul ence sensors, and for evaluation of other sensors, e.g., rain gauges. The l ong time series of radiation data have been valuable to establish numerical limits for a set of quality-control flags. MMO has served as a boundary-la yer research station and results from NOPEX campaigns show how the dimensio nless wind gradient depends not only on the traditional stability parameter z/L but also on the height of the convective boundary layer. Measurements at the observatory grounds and a neighbouring field show a considerable var iability in surface properties, which must be accounted for when assessing budgets of heat and other scalars. Questions concerning long-term calibrati on plans, maintenance of sensors and data-collection system, and continuous development of the computer network to keep it up to date are, however, on ly partly of interest as a research project in itself. It is thus difficult to get it funded from usual research-funding agencies. The full value of data generated by the: MMO system can best be appreciated after a decade or more of continuous operation. Main uses of the data woul d be to evaluate how SVAT models handle the natural variability of climate conditions, quantification of water, carbon and energy budgets during vario us weather conditions, and development of new parameterisation schemes in g lobal and regional climate models. (C) 1999 Elsevier Science B.V. All right s reserved.