This question is quite hard to answer. I would say there is no accurate method or model for calculating the amount of runoff generated from an ungauged watershed, as each watershed has its own unique hydrological features. I do not want to recommend a parameter regionalization technique. The role (and hydrological meaning) of a parameter is determined based on the model structure and its relationship with other parameters directly and indirectly associated with the parameter. In addition, models and methods have different structures and employ unique simulation or estimation strategies. Thus, the original (or intended) meaning and role of a parameter can be interpreted and used differently in models and methods. It would be the best to start monitoring right now, but I know often it is not practical. In this case, you may want to collect remotely sensed data, such as soil moisture, to calibrate your model or try to find any indirect observations and measurements such as flow mark or lines on stream back and reservoir. I hope this can help. Young

Hi Saeed,

A sound knowledge of gradients in soil and raw geomorphic surface hydrological properties (permeability, drainage class, depth to slowly permeable horizon) in conjunction with climatic data can be used. 

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Hello Saeed. It's difficult to say that there is an accurate model or method to predict runoff in a certain period of time. As you can imagine, all computation model have there weakness. So, it's not easy to deal with natural environment like water resources. try to use the most that you currently have domain.

Dear Jeeban Panthi, 

You can use regionalization method : once you calibrate available data for the near by stations  you can develop your own regional model and you will know the charactersitics of ungauged catchment and possible to predict and simulate flows for  your ungauged catchment.

for further  you can 

Sisay, E., Halefom, A., Khare, D., Singh, L. and Worku, T., 2017. Hydrological modelling of ungauged urban watershed using SWAT model. Modeling Earth Systems and Environment, pp.1-10.