ERROR ANALYSIS IN ESTIMATING SOIL-WATER BALANCE OF IRRIGATED FIELDS DURING THE EFEDA EXPERIMENT .1. LOCAL STANDPOINT

To associate a confidence interval with the results of a soil water ba lance, an estimate of the errors involved in the measurements and in t he calculations is required. Volumetric water content, water storage a nd actual evapotranspiration were calculated from daily neutron scatte ring and tensiometer measurements, and the instrument, calibration and vertical integration components of their variance were compared from a local standpoint. In contrast to foregoing studies, water content va lues at different depths were not considered to be necessarily indepen dent of one another, introducing supplementary covariance terms in the calculations. As an illustration, results are presented for two irrig ated maize fields from the EFEDA experiment conducted in Spain in June 1991. The standard deviation of the total error was less than the sta ndard deviation of either water storage or computed evapotranspiration between measurements.