Sampling of soil moisture fields and related errors: implications to the optimal sampling design

Adequate knowledge of soil moisture storage as well as evaporation and tran spiration at the land surface is essential to the understanding and predict ion of the reciprocal influences between land surface processes and weather and climate. Traditional techniques for soil moisture measurements are gro und-based, but space-based sampling is becoming available due to recent imp rovement of remote sensing techniques. A fundamental question regarding the soil moisture observation is to estimate the sampling error for a given sa mpling scheme [G.R. North, S. Nakamoto, J Atmos. Ocean Tech. 6 (1989) 955-9 92; G. Kim, J.B. Valdes, G.R. North, C. Yoo, J. Hydrol., submitted]. In thi s study we provide the formalism for estimating the sampling errors for the cases of ground-based sensors and space-based sensors used both separately and together. For the study a model for soil moisture dynamics by D. Entek habi, I. Rodriguez-Iturbe [AJv. Water Res. 17 (1994) 35-45] is introduced a nd an example application is given to the Little Washita basin using the Wa shita '92 soil moisture data. As a result of the study we found that the gr ound-based sensor network is ineffective for large or continental scale obs ervation, but should be limited to a small-scale intensive observation such as for a preliminary study. (C) 2001 Published by Elsevier Science Ltd.