Multiple cost-sharing rules often coexist in seemingly identical environments. We use shared irrigation costs as a context for examining the extent to which the structural environment explains cost-sharing rule selection. We find that environmental factors affecting irrigation costs and benefits, as well as those determining irrigator distribution along the ditch, have impressive explanatory power. These results present some of the first formal empirical analysis of the manner in which structural features influence the actual cost-sharing choices of economic agents. Moreover, our results also suggest that previous axiomatic studies are reflective of real-world cost-sharing behavior.