A field note is most useful when it resists the urge to turn an observation into a conclusion too quickly. A dry patch beside a canal, for example, is something one can record; its cause may require more information about soil, delivery timing, and recent conditions. The distinction is small on the page and important in the analysis.
Record the conditions
Write down where and when the observation was made, what was directly visible, and what was happening nearby. In agricultural or infrastructure settings, details such as weather, operating equipment, and the position of gates or channels can help a later reader understand the scene. If a detail was not checked, say so rather than filling the gap from memory.
Separate seeing from explaining
A note can preserve both an observation and a question: water is pooling near an outlet; the reason is not yet clear. Keeping those statements apart makes it easier to return with measurements, ask someone familiar with the site, or compare observations across visits. It also prevents a plausible explanation from quietly becoming a reported fact.
Let uncertainty guide the next visit
The unresolved part of a note is not wasted space. It can point to the next measurement, the person to consult, or the assumption a paper needs to test. Before leaving a site, I would mark one thing I observed, one thing I inferred, and one thing I still need to understand.