Of the many ways to deduce plant water uptake, the soil water balance is the most frequently used, with plant water uptake determined as the residual of other terms in the equation. Extensive work with weighing lysimeters and soil water sensing methods allows us to contrast the effectiveness of and problems with these methods for determining the water balance. In particular, the soil water sensing methods are limited by un-quantified vertical and lateral fluxes, issues of measurement scale vs. plant water uptake scale, and issues of accuracy and precision. In addition to review of methods to date, recent work on field variability of soil water balance with several modern sensors will be discussed.