EVALUATION AND GENERALIZATION OF 13 MASS-TRANSFER EQUATIONS FOR DETERMINING FREE WATER EVAPORATION V. P. SINGH 1 AND C.-Y. XU 2 1 Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, LA 70803±6405, USA 2 Division of Hydrology, Uppsala University, Norbyva Ègen 18B, S-75236 Uppsala, Sweden ABSTRACT Thirteen equations based on the mass-transfer method for determining free water evaporation were expressed in seven generalized equations. These seven equations were then compared with pan evaporation at four climatological stations in north-western Ontario, Canada. The comparisons were based on monthly evaporation. Equations were compared by calibrating them on the entire data sets as well as by calibrating on part of the data and then verifying them on the remainder of the data. The results of comparison showed that all equations were in reasonable agreement with observed evaporation, and that the eect of wind velocity on monthly evaporation was marginal. However, when an equation with parameters obtained at one site was applied to compute evaporation at another site, the computed evaporation was not in good agreement with observed values. # 1997 by John Wiley & Sons, Ltd. Hydrological Processes, vol. 11, 311±323 (1997). (No. of Figures: 6 No. of Tables: 3 No. of Refs: 39) KEY WORDS evaporation; free water surface; mass-transfer method; north-western Ontario INTRODUCTION Evaporation estimates are needed in a wide array of problems in hydrology, agronomy, forestry and land resources planning, such as water balance computation, irrigation management, river ¯ow forecasting, investigation of lake chemistry, ecosystem modelling, etc. Of all the components of the hydrological cycle, evaporation is perhaps the most dicult to estimate owing to complex interactions between the components of the land±plant±atmosphere system. There exists a multitude of methods for measurement and estimation of evaporation. Overviews of many of these methods are found in numerous reviews, (see, for example, Brutsaert, 1982; Singh, 1989; Morton, 1990, 1994). Very recently, Panu and Nguyen (1994) evaluated four methods of estimating mean areal evaporation in north-western Ontario, Canada; whereas Winter et al. (1995) compared 11 equations for determining evaporation for a small lake in the north-central United States. Morton (1990) presented a critique of evaporation measurement methods. The methods for determining evaporation can be grouped into several categories, including: (i) empirical (e.g. Kohler et al., 1995), (ii) water budget (e.g. Guitjens, 1982), (iii) energy budget (e.g. Fritschen, 1966), (iv) mass transfer (e.g. Harbeck, 1962), (v) combination (e.g. Penman, 1948) and (vi) measurement (e.g. Young, 1947). Empirical methods relate either pan evaporation, actual lake evaporation or lysimeter measurements to meteorological factors using regression analyses. The weakness of these empirical methods is that they have a limited range of applicability because (i) their variables may not be easily measurable in other places, or some existing data may not be utilized; (ii) they are usually accurate only in a limited range, for their model structure may be only partially correct; and (iii) it is dicult to compare one method with another due to CCC 0885±6087/97/030311±13 Received 18 September 1995 # 1997 by John Wiley & Sons, Ltd. Accepted 4 January 1996 HYDROLOGICAL PROCESSES, VOL. 11, 311±323 (1997) * Corresponding author.