ERROR ANALYSIS IN ESTIMATING SOIL-WATER BALANCE OF IRRIGATED FIELDS DURING THE EFEDA EXPERIMENT .2. SPATIAL STANDPOINT

In Part 1 of this paper, an error analysis for estimating the variance of volumetric water content, soil water storage and actual evapotranspiration from a local standpoint was presented. This second part extends the analysis to the case of spatial average values corresponding to a field scale. For a low number of sampling points, the location component related to the spatial variability is the major component of the total variance of the foregoing variables. Results are presented for the two maize fields described in Part 1 and comparison is made with values of evapotranspiration provided by atmospheric latent heat flux measurements for one of them. Finally, the determination of the calibration, instrument, integration and location components of the variance of a given variable leads to an estimation of the number of sampling points required to estimate field average values with a prescribed degree of precision. This number varies significantly depending on the variable considered - water content, water storage or evapotranspiration - and proved to be different for the two fields.