Simple Water Balance Tools for Scarce Hydrology Data?

Are there any ​simple tools to ​calculate the ​basic water ​balance for a ​region despite ​the lack of ​hydrology data?​  ​

We would like ​to do ​calculations ​although we ​have very ​little data for ​rainfall/river ​discharge ​measurements ​and meteorological ​data (​evapotranspiration​)​.  ​

How can we do ​these ​calculations? ​