Abstract: To simulate annual streamflow accurately, a set of annual streamflow simulation methods were designed based on the structures of four monthly water balance models, namely, the abcd model, Thornthwaite's Water Balance Model (TWBM), Vandewiele's Water Balance Model (VWBM), and Dynamic Water Balance Model (DWBM). The main design ideas are described as follows: the accumulated observed precipitation and potential evapotranspiration in L-month (i.e., the divisor of 12) were taken as inputs, the accumulated runoff of the corresponding length of time was firstly simulated by using the selected monthly model, and the annual streamflow was subsequently calculated by the sum of the corresponding simulated runoff at different time intervals (i.e., 12/L), respectively. The developed methods were applied to 200 MOPEX (Model Parameter Estimation Experiment) catchments, and results manifest that the annual streamflow simulation methods based on all four models could obtain good performances for annual runoff simulations in most MOPEX catchments, and the abcd model outperformed the other three models. With the increase of L, the accuracy of annual streamflow simulation methods based on all four models decreased, and the stability of DWBM model outperformed the other three models. The research findings offer a new idea of thinking for studying hydrological processes and laws at the long hydrological time-scale.

Key words: monthly water balance model, mid-long-term streamflow simulation, hydrological time-scale, time aggregation, MOPEX catchments