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How big the bucket is
How much water is put into the bucket
How water is taken out of the bucket
In many ways, our the job of irrigation is to keep crops in the “Available water” goldilocks zone. Our job is to understand a specific bucket and to help a farmer keep their bucket in its' goldilocks zone.
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This video does a great job explaining the concept of water absorption versus run-off. It also starts to delve into how the presence of crops influences these processes. Topography affects water flow - in other words, where water sheds and where it collects. Soil texture and organic matter affect the total soil water holding capacity as well as actual plant available water (which are two different things!), so all three are working simultaneously to affect water and crop variability. The table below approximates the amount of available water by soil type. The gas tank metaphor used in this article is also helpful in understanding why different soil types have different amounts of available water.
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Viewing this diagram, we can understand why loamy soils - with their relatively wide range of available water are so valued in agriculture while sandy soils are not.
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