Mapping Your Farm’s Invisible Water Flow

You are standing on a substantial agricultural property. The soil looks rich, the winter sky is clear, and it is a perfect day for a farm walk. And you are almost entirely blind to how the place behaves when it rains.

That is the blind spot this discussion sets out to close. Which crossing becomes impassable, which slope is quietly losing topsoil every season, where water sits after a storm, and whether a proposed dam site has the catchment above it to fill.

It teaches you to read a contour map along the way — lines close together mean steep, a V points upstream, concentric loops mean a hill or a pan — and covers the slope units trap, where a 100 per cent slope is 45 degrees rather than 90, and confusing the two produces errors of a factor of two.

Most of the time goes on drainage: how catchment area determines whether a dam will fill, where erosion and donga formation begin, why certain paddocks underperform for reasons nobody has satisfactorily explained, and where culverts belong.

It also covers what matters locally — that summer thunderstorms make erosion a question of rainfall intensity rather than annual total, and that storing water or impeding its flow may require authorisation under the National Water Act.

And it is straight about the boundaries: the model describes terrain, not soil, and it is not a hydrological study. It gives an engineer the foundation to work from rather than replacing one.