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There are several ways to determine how much (and so when) to water:

• Soil moisture probe
• Irrigation scheduler
• Rooting in the soil

I'm convinced that for the experienced gardener the last is the most effective, as well as the least expensive, so long as the gardener gets it close to right.

I did try a soil moisture probe, not one of the cheap ones, but one advertised as a professional moisture probe. When I saw it giving very low readings (6% - 12%) in saturated soil, I complained to the manufacture, who replied that, oh yes, it doesn't work in sandy soil. I proved that it doesn't work but a tiny bit better in clay soil. So I returned that. I'm told by experts at WSU Extension that there are accurate sensors, but I didn't find any in my price range.

There are a couple of approaches to this, but the one I tried is the Irrigation Scheduler from WSU Extension.

Simply put, the Irrigation Scheduler (IS) looks at weather conditions at a nearby weather station (the closest to me is in Woodinville), considers type of soil, rainfall, irrigation, temperature, humidity (an so evaporation and transpiration) and the growing stage of a specific variety and charts the likely moisture needs. IS allows separate calculations for each crop.

A chart shows whether calculated soil moisture is between full saturation or field capacity (about 50%) and severe wilt (about 25%). These values vary depending on the type of soil.

For my crops when the moisture calculation was between the wilting point and first stress lines (above the Empty/dead line [dark brown] and below the First stress line [red]), the soil was too moist and some plants objected, the Farthest North melons having a tendency to split open.

With experience and good judgment about the varying moisture requirements of a particular variety I can tell within a reasonable margin of error when the soil needs water. This is well before the leaves wilt and the stems droop.

Another guess is just how much to water. When in doubt I make several passes and stop when water is standing on top of the soil. I guess that would be called flooding it. It works well enough for the sandy soil in my Fall City garden. But this method is still hit or miss when plants are small and needing only a little moisture and when they are mature at the peak of harvest and needing a lot more.