Time domain reflectometry (TDR) has become an accepted standard method for measuring the volumetric water content of porous media. Although the method is rapid and nondestructive, it is invasive. TDR probes present an impediment to flow. As a result, there will be a build up of water above the rods and a decrease in water content beneath the rods. The extent of water content change depends on the size, number and spacing of the rods, the capillary length of the soil, and the applied flux. We present a coupled numerical analysis of water flow and TDR response during steady-state unsaturated flow. The results demonstrate that typical TDR probe designs do not lead to a measurable error in measured water content. However, analysis of TDR probes that are attached to larger objects (e.g. access tubes) shows that significant measurement errors can occur. The approach to coupled modeling of water flow and instrument response presented can be applied to examine the effects of the invasive nature of many indirect methods (e.g. tensiometers, heat pulse probes, flux plates) on hydrologic interpretations based on these measurements.