A Review of the Relationships Between the Hydrological Components of the Water Balance in the Assessment of Rainfall Losses

In arid and semi-arid regions, a major share of rainfall never reaches streams or groundwater. This part which is known as rainfall loss can play a decisive role in river flow or groundwater recharge. Rainfall losses are used in many hydrological models as one of the basic forecasting components. In this study, the components of the hydrological losses including interception, evapotranspiration, infiltration, and soil moisture have been reviewed, as well as the characteristics of the mentioned components, general concepts, conducted studies, and also the comparison of the useful calculation methods of these components of hydrological losses was reviewed.  The review of the studies shows that due to the complex and stochastic nature of the rainfall process and the many factors affecting it, rainfall losses have not yet been comprehensively understood and in many simulations, it is enough to simplify the systems by considering infiltration as a loss function.  As a result, in practical applications, fixed values for losses are often used and this simplification leads to a sort of inaccurate runoff simulation. Therefore, firstly, it is recommended to pay attention to the time and place format in the existing rainfall loss models and even their concepts and secondly, more consideration should be paid to the interrelationships among components,  because, for example, in the daily time scale, a component such as transpiration may become less important than in long-term time periods, or in an urban watershed due to human manipulation, the loss processes may not be similar to natural watersheds.