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I have only few station inside the watershed so I want to use the stations outside if I can to get better results

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Yes, you can interpolate the rainfall from stations outside the watershed. However, the quality of the interpolation will depend on the distance between the stations and the watershed, the prevailing weather patterns, and to some extent the topography.

For example, it probably wouldn't be appropriate to use a station on the rainy side of a mountain divide in estimating rainfall in the rainshadow. One way to get an idea on how good you estimates are is to compare the actual rainfall at a station with the interpolated rainfall at that location, when you leave it out of the dataset.

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In short: Yes you can/should use stations outside of your watershed.

More in detail: There is well known methods in hydrology to solve the situation you are presenting.

One simple method is to build Thiessen Polygons on your map to model the incoming precipitation in your target watershed using gauge station located inside and outside your watershed.

The idea is to assign (mathematical) weights to each polygons (area covered by a station) by proportionally assigning a quantity of rain function of the area covered by the said polygon intersecting with your watershed.

Thiessen Polygons implementation example. Source: Encyclopedia of Hydrology and Water Resources

There is several other approaches to solve this problem: many examples are shown here. There is a varying level of complexity and precision for each approaches. Often, your specific case will be appropriate with certain methods only - depending on the nature of the data you have (or not) for your watershed.

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    $\begingroup$ Interesting approach. Is it the same as nearest neighbor ? $\endgroup$ – gansub May 26 at 4:41
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    $\begingroup$ @gansub yes it is an application based on the principle of nearest neighbor $\endgroup$ – Etienne Godin May 26 at 12:56

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