INCONSISTENCIES IN NET-RADIATION ESTIMATES FROM USE OF SEVERAL MODELSOF INSTRUMENTS IN A DESERT ENVIRONMENT

Studies of surface energy and water balance generally require an accur ate estimate of net radiation and its spatial distribution. A project quantifying both short term and seasonal water use of shrub and grass vegetation in the Jornada Experimental Range in New Mexico prompted a study to compare net radiation observations using two types of net rad iometers currently being used in research. A set of 12 REBS net radiom eters were compared with each other and one Swissteco, over wet and dr y surfaces in an arid landscape under clear skies. The set of REBS exh ibited significant differences in output over both surfaces. However, they could be cross calibrated to yield values within 10 W m(-2), on a verage. There was also a significant bias between the REBS and Swisste co over a dry surface, but not over a wet one. The two makes of instru ment could be made to agree under the dry conditions by using regressi on or autoregrsssion techniques. However, the resulting equations woul d induce bias for the wet surface condition. Thus, it is not possible to cross calibrate these two makes of radiometer over the range of env ironmental conditions observed. This result indicates that determinati on of spatial distribution of net radiation over a variable surface sh ould be made with identical instruments which have been cross calibrat ed. The need still exists for development of a radiometer and calibrat ion procedures which will produce accurate and consistent measurements over a range of surface conditions. (C) 1998 Elsevier Science B.V.