In this paper, we present the Dichotomized Cache Management (DCM) scheme for Web caches. The motivation of the DCM scheme is discovered by observing the Web reference behavior from the viewpoint of Belady's optimal replacement algorithm. The observation shows that 1) separate allocation of cache space for temporal locality and reference popularity better approximates the optimal algorithm, and 2) the contribution of temporal locality and reference popularity on the performance of caching is dependent on the cache size. With these observations, we devise the DCM scheme that provides a robust framework for on-line detection and allocation of cache space based on the marginal contribution of temporal locality and reference popularity. Trace-driven simulations with actual Web cache logs show that DCM outperforms existing schemes for various performance measures for a wide range of cache configurations.