As RSU (or stock numbers and productivity) decrease, N removed to water decreases as leaching from urine patch (Leaching - urine patches) decreases.
However there is a point that background loss starts to increase, such that at very low stocking rates, background N leaching (Leaching -other) can be high. As RSU is increased from a low RSU (for example, < 1 SU/ha) there is a parabolic response in N leached from root zone (nutrient budget) and total N lost (farm details report).
The point of change is when the rate of release of N from the soil organic pool, which is based on soil water and climate regimes, exceeds plant N uptake which is derived from stock numbers. If Leaching - other starts to exceed Leaching - urine patches and there is no N inputs such as fertiliser or effluent, then this condition may be occurring.