A Lagrangian model is constructed for a surface column of initial height h(0) that propagates at an average speed u and is subject to excess (i.e. net) evaporation of q m/year. It is shown that these parameters combine to form an Evaporation Length, L=uh(0)/q, that provides an estimate for the distance the column must travel before evaporating completely. While these changes in the surface water level due to evaporation are compensated for by entrainment of water into the overall column, the changes in either near surface salinity or isotopic compositions are retained and can be measured. Observations of surface salinity and isotopic compositions of d\^{18}O and dD along 1000 km to 3500 km long transects are used to estimate values of L in the Red Sea, Mediterranean Sea, Indian Ocean and Gulf Stream. The variations of salinity, d\^{18}O and dD in all four basins are linear. As anticipated, the estimated value of L is greatest in the slowly moving Red Sea, which is subject to an arid environment, and is smallest in the fast-moving Gulf Stream.