William, yes not rainfall that falls on the land gets into the river. Most is lost back to evaporation again. Depending on how moist your soil is the soils will also soak up a lot. You only get runoff once the soil is saturated and then it starts to flow. Or if you get a high amount of rain in one event a lot will flow, hence the reason we get floods. Predicting stream flow from rainfall is really tricky. If you have a stream flow gauging station best to use that as then tells you how much water is flowing down the river + or - depending on how good the stream flow rating curve is. But what do I know about surface water hydrology, I'm a hydrogeologist. At least you can see what's happening on the surface. Subsurface now that's an art, full of mystery and intrigue.

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William, yes not rainfall that falls on the land gets into the river. Most is lost back to evaporation again. Depending on how moist your soil is the soils will also soak up a lot. You only get runoff once the soil is saturated and then it starts to flow. Or if you get a high amount of rain in one event a lot will flow, hence the reason we get floods. Predicting stream flow from rainfall is really tricky. If you have a stream flow gauging station best to use that as then tells you how much water is flowing down the river + or - depending on how good the stream flow rating curve is. But what do I know about surface water hydrology, I'm a hydrogeologist. At least you can see what's happening on the surface. Subsurface now that's an art, full of mystery and intrigue.