Indirect (geophysical) methods offer great promise for characterizing and monitoring subsurface hydrologic states and processes.  In particular, the nondestructive, noninvasive nature of many geophysical methods can offer previously impossible views into the subsurface.  However, there are significant challenges to making quantitative use of geophysical data.  Like other measurement types, the uncertainty of geophysical measurements must be described.  Unlike direct measurements, measurement uncertainties for indirect methods must also include consideration of the effects of nonunique calibrations.  Finally, noninvasive methods require consideration of their distributed measurement sensitivity.  Examples of the impacts of these additional error sources will be presented.  Then, a method to incorporate these uncertainties in assessments of likely data worth will be described.