The main function of runway pavement grooving is to facilitate surface runoff drainage, thereby maintaining the wet-weather skid resistance of runway pavement at a sufficiently high level for safe aircraft operations. Runway grooves must be well maintained so that their groove depths are sufficient to achieve good drainage to serve their intended functions. One possible way to assess the effectiveness of runway grooves is to conduct an outflow test to measure the flow rate. Theoretically flow rate can serve as an indirect measure of the drainage effectiveness of a grooved pavement. The concept of outflow meter test was first examined and found that the conventional outflow meter test is not suitable because of the much higher flow rates encountered in the case of grooved runway pavements. A modified outflow meter test is proposed in this paper. To assess the applicability of the proposed outflow meter test, a series of laboratory experiments was conducted to determine the repeatability of the test, and a finite-element model was developed to simulate the flow measuring process and validated against the experimental results. A relationship was established between the flow rate and the average groove depth of the grooved pavement tested. This relationship would offer a quantitative assessment of the drainage effectiveness of the grooves of the runway pavement tested.