State-to-state rate coefficients for ortho-H_2_O:He and para-H_2_O:He inelastic collisions in the 20-120K thermal range are investigated by means of an improved experimental procedure. This procedure is based on the use of a kinetic master equation (MEQ) which describes the evolution of populations of H_2_O rotational levels along a supersonic jet of H_2_O highly diluted in helium. The MEQ is expressed in terms of experimental observables and rate coefficients for H_2_O:He inelastic collisions. The primary experimental observables are the local number density and the populations of the rotational energy levels of H_2_O, quantities which are determined along the jet with unprecedented accuracy by means of Raman spectroscopy with high space resolution. Sets of rate coefficients from the literature and from present close-coupling calculations using two different potential energy surfaces (PESs) have been tested against the experiment. The Green et al. (VI/59) rate coefficients are up to 50% too low compared to the experiment, while most rates calculated here from the Hodges et al. (2002JChPh.116.1397H) PES and the Patkowski et al. (2002, JMoSt TEOCHEM, 591, 231) PES are much closer to the experimental values. Experimental rates with an estimated accuracy on the order of 10% have been obtained for ortho-H_2_O:He and para-H_2_O:He inelastic collisions between 20 and 120K by scaling and averaging the theoretical rates to the experiment.