Slide 11: Intercept: dependent on soil water holding capacity; co-varies with the rain use efficiency
It turns out that the intercept of the grass production vs rainfall curve is approximately linearly related to the slope (RUE): in other words, you actually only need one constant to characterise this relation, since you can calculate the other from it.
The higher the rain use efficiency, the lower the intercept value, following an approximately straight line relationship. Higher intercept values mean that more rain is required for a given amount of primary production in comparison to lower values. In other words, the significance of this graph is that the more efficiently water is used, the less rainfall is required, and efficiency of use is covariant with soil water holding capacity.